Polymer-Based Organic Batteries - Chemical Reviews (ACS

Review pubs.acs.org/CR

Polymer-Based Organic Batteries Simon Muench,†,‡ Andreas Wild,†,‡ Christian Friebe,†,‡ Bernhard Haü pler,†,‡ Tobias Janoschka,†,‡ and Ulrich S. Schubert*,†,‡ †

Chem. Rev. 2016.116:9438-9484. Downloaded from pubs.acs.org by OPEN UNIV OF HONG KONG on 01/23/19. For personal use only.

Laboratory of Organic and Macromolecular Chemistry (IOMC), Friedrich Schiller University Jena, Humboldtstr. 10, 07743 Jena, Germany ‡ Center for Energy and Environmental Chemistry Jena (CEEC Jena), Friedrich Schiller University Jena, Philosophenweg 7a, 07743 Jena, Germany ABSTRACT: The storage of electric energy is of ever growing importance for our modern, technology-based society, and novel battery systems are in the focus of research. The substitution of conventional metals as redox-active material by organic materials offers a promising alternative for the next generation of rechargeable batteries since these organic batteries are excelling in charging speed and cycling stability. This review provides a comprehensive overview of these systems and discusses the numerous classes of organic, polymer-based active materials as well as auxiliary components of the battery, like additives or electrolytes. Moreover, a definition of important cell characteristics and an introduction to selected characterization techniques is provided, completed by the discussion of potential socio-economic impacts.

CONTENTS 1. Introduction 2. Characteristics of Batteries 3. General Setup 3.1. Battery Types 3.2. Housing 4. Active Materials 4.1. Conjugated Polymers 4.1.1. Poly(pyrrole) 4.1.2. Poly(thiophene) 4.1.3. Poly(aniline) 4.1.4. Other Conjugated Polymer Systems 4.1.5. Inorganic Composites 4.1.6. Enhancements by Morphology 4.2. Nonconjugated Conventional Redox-Active Polymers 4.2.1. Carbonyl Compounds 4.2.2. Organosulfur Compounds 4.2.3. Other Redox-Active Units 4.3. Organic Radical Polymers 4.3.1. 2,2,6,6-Tetramethylpiperidinyl-N-oxyl (TEMPO) 4.3.2. Other Nitroxide Radicals 4.3.3. Galvinoxyl 5. Nonactive Electrode Components 5.1. Conductive Additives 5.1.1. Carbon Particles 5.1.2. Mesoporous Carbon 5.1.3. Vapor-Grown Carbon Fibers (VGCF) 5.1.4. Graphene 5.1.5. Carbon Nanotubes (CNT) 5.1.6. Composite-Electrode Processing © 2016 American Chemical Society

5.2. Binder 5.3. Current Collectors 6. Electrolytes 6.1. Solvents 6.2. Salts 7. Characterization Methods 7.1. Preliminary Characterization of Active Compounds 7.1.1. Voltammetric Methods 7.1.2. Electrochemical Impedance Spectroscopy (EIS) 7.1.3. Spectroscopic, Spectroelectrochemical, And Surface Characterization Methods 7.2. Device Characterization 7.2.1. Charging/Discharging Characteristics 7.2.2. Electrochemical Impedance Spectroscopy 7.2.3. Spectroscopic Methods 8. Toward Commercialization 8.1. Socio-Economic Impact 8.2. Manufacturing Techniques and Applications 9. Concluding Remarks Associated Content Special Issue Paper Author Information Corresponding Author Notes Biographies Acknowledgments

9439 9439 9441 9441 9443 9444 9445 9445 9447 9447 9448 9448 9449 9450 9450 9455 9458 9460 9460 9462 9463 9463 9463 9464 9464 9464 9465 9465 9465

9466 9466 9467 9467 9468 9469 9469 9469 9470 9470 9471 9471 9471 9472 9472 9472 9473 9473 9474 9474 9474 9474 9474 9474 9474

Received: February 16, 2016 Published: August 1, 2016 9438

DOI: 10.1021/acs.chemrev.6b00070 Chem. Rev. 2016, 116, 9438−9484

Chemical Reviews References

Review

The research on organic polymer batteries evolved with the discovery of the electronic conductivity of doped conjugated polymers, which was discovered in 1977 by Alan J. Heeger, Alan G. MacDiarmid, and Hideki Shirakawa.19 Early attempts proved the potential of this idea for organic batteries.20,21 In the following, research focused mainly on conjugated polymers, such as poly(aniline), poly(pyrrole), poly(thiophene), poly(p-phenylenes), and poly(carbazole).22 In the late 1980s, Bridgestone/ Seiko and VARTA/BASF launched commercial batteries based on poly(pyrrole) and poly(aniline), respectively. However, they were discontinued after only a short period of time.18,23 One reason for this was the inferior charging/discharging behavior of the conjugated polymers. Upon doping by either electrochemical oxidation or reduction, electric energy is stored via delocalized charge carriers within the extended π-system (section 4.1).18,24 Consequently, these materials do not possess a distinct redox potential, but it is strongly dependent on the doping level. As the degree of doping changes upon charging/discharging, the voltage of the resulting battery is not constant, representing a major problem for its application. One polymer class that possesses all advantages of organic conjugated polymers but circumvents the disadvantages, such as an incomplete doping, the sloping voltage, and an inferior life cycle, is represented by polymers with isolated redox-active units attached to an insulating backbone (sections 4.2 and 4.3).18 Due to the isolated nature of the redox-active unit, these polymers feature redox reactions at very distinct potentials and, consequently, enable the construction of organic batteries with stable charge/discharge voltages. In 2002, Nakahara et al. published the first battery using the 2,2,6,6-tetramethylpiperidinyl-N-oxyl (TEMPO) radical as a redox-active unit bound to a poly(methacrylate)-based backbone.25 In the following years, in particular, the group of Hiroyuki Nishide performed intensive research on redox polymers, before, starting around 2007, more and more groups joined.18 Nowadays, most studies focus on nonconjugated redox polymers or conjugated polymers with localized charges. This review aims to offer a comprehensive overview of polymer-based organic batteries, starting with a general introduction, including an explanation of all important terms and the different battery layouts. In the following, all components of a battery (electrode materials, electrolytes, and current collectors) are critically discussed. To enable the reader to gain an overview over the common methods of characterization of redox-active polymers in the solid state, electrodes and electrochemical cells, Section 7 provides an introduction to the applied techniques. In the end, the possible socio-economic impacts of polymer-based organic battery systems are discussed.

9474

1. INTRODUCTION The rapidly and globally increasing demand for energy results in challenges concerning not only the conversion but also the storage of electrical energy. Various types of systems are applied, depending on the required capacity and power values, including pumped-storage hydropower plants, flywheels, and electrochemical energy-storage devices.1,2 In particular batteries,3−5 fuel cells,6,7 and supercapacitors8−11 are in the focus of current research due to their key role regarding “mobile energy”, as well as the rapid development of active radio-frequency identification (aRFID) tags, integrated-circuit smart cards, mobile sensor systems, smart clothes, electric vehicles, and grid-level energy storage. In all these key technologies, the relatively poor battery performance of existing energy storage systems represents the major bottleneck. The currently most common battery systems are based on the Li-ion technology. This technology was proposed by M. S. Whittingham in 1976, commercialized by SONY in 1990, and represents the best investigated and, due to its uniquely high power density, most popular battery system today.12 However, for applications related to the Internet of Things (IoT), such as aRFID tags, sensors, smart clothes, or smart packaging, the Li-ion technology reaches its limits. The demands for such thin-film applications clearly differ from conventional batteries (e.g., consumer electronics or electromobility). Vital requirements are flexibility, absence of toxic and harmful metals, the production from abundant and, ideally, renewable resources, rapid charging, excellent cycle life, and efficient processing using roll-to-roll or similar processing techniques. There are examples of flexible Liion batteries, which, however, need the electrode materials to be converted to nanomaterials before.13−16 Furthermore, current Li-ion batteries often contain toxic metal salts such as cobalt- or nickel-based cathodes and require an elaborate overcharge protection to avoid thermal runaways. The largest natural resources of these metals are located in politically unstable regions, and the substitution with renewable resources is impossible. All these drawbacks can be circumvented by using organic materials, more precisely polymers, as active components. The transition from inorganic to organic materials is facilitated by the great variety of available redox-active organic compounds that are suitable as active materials for battery application (i.e., they feature chemically reversible redox reactions that enable energy storage). Such organic batteries possess inherent advantages over Li-ion batteries. Organic compounds allow facile tailoring of their redox properties (e.g., by the introduction of functional groups), thus enabling the synthesis of both cathode and anode materials as well as the tuning of the voltage of the battery. Films of organic polymers are known to be flexible, and the production and processing of plastics is performed on an industrial scale. As most organic materials, redox-active polymers are synthesized from fossil oil, but there are already several attempts and achievements in using renewable resources.17 In contrast to inorganic materials, whose redox reactions are based on complex intercalation mechanisms, the electrochemical behavior of organic materials is based on simple redox reactions, leading to high rate performances and long life cycles.18 In addition, the absence of heavy metals, like cobalt, lead, and nickel, enables simple and environmentally friendly disposal.

2. CHARACTERISTICS OF BATTERIES At first, some basic characteristics and key parameters of batteries need to be defined and explained. The general setup of a battery is discussed in Section 3, and the methods to determine the batteries’ characteristics are provided in Section 7. Several nomenclatures are common in battery research and some are even incorrect with respect to the used dimensions. In this review, we mostly follow the terms given by Linden and Reddy in the third Edition of the Handbook of Batteries.4 Anode: the electrode of an electrochemical cell that is oxidized during discharge is named anode (negative pole). Cathode: the electrode of an electrochemical cell that is reduced during discharge is named cathode (positive pole). 9439


Chemical Reviews

Review

Figure 1. Schematic representation of the (a) discharging and (b) charging process of a metal-based secondary battery.

The theoretical voltage and capacity of a battery are defined by the redox potentials and the molar masses of the applied cathode and anode materials. Theoretical voltage (V [V]): The theoretical voltage (cell voltage) of a battery can be calculated from the (standard) potentials of the respective electrodes as follows:4 cathode potential − anode potential = cell voltage

Espec =

(1)

Edens =

(2)

where n is the number of transferred electrons per redox reaction, F the Faraday constant, and Mw the molar mass of the structural unit. In most studies, the discussed specific capacity refers only to the limiting electrode. The specific capacity of an electrochemical cell can be calculated according to eq 3. cell

anode

cathode

ηc =

(3)

Theoretical Energy (Etheo [Wh]): An even more important parameter is the energy of the electrochemical cell. The theoretical energy is the maximum energy that can be delivered by a given system with a theoretical voltage V and a theoretical capacity Ctheo (eq 4). Etheo = C theo × V

E volume

(6)

All previously discussed parameters are theoretical values. In practice, only a fraction of these is valid in a real battery,4 but lower values are present. The majority of the losses are caused by nonreactive components like current collector, conductive additives, binder, electrolyte, separator, housing, and seals, which increase the mass/volume of the battery but not its capacity. Besides these constructionally induced losses, other factors that diminish the energy density of a battery are, for example, a partial activity/charging of the active material, the depth of discharge (DOD), and polarization effects. Consequently, the energy that is available from a battery under optimal practical conditions is usually only about 25 to 35% of the theoretical energy.4 Most cells presented in this review are optimized neither in construction nor in formulation and, therefore, result in real energy densities distinctly below 10% of the theoretically possible values. Coulombic efficiency (ηc [%]): the Coulombic efficiency is the ratio of the obtained discharging and charging capacity.

Cspec =

Cspec −1 = Cspec −1 + Cspec −1

(5)

Energy density (Edens [Wh cm−3]): The energy density is a further common parameter for the characterization of electrochemical cells, which refers to the volume of the redox-active compound and is calculated according to eq 6.

Theoretical capacity (Ctheo [Ah]): the theoretical capacity is the electric charge that can maximally be stored in the cell, if the total amount of active material contributes. The unit of the capacity is Coulomb (1 C = 1 As) or Ah (1 Ah = 3600 As). Specific capacity (Cspec [Ah g−1]): to allow the comparison of different electrode materials, the capacity of electrodes is usually provided per mass of active material. The specific capacity of a single electrode can be calculated according to eq 2. n×F n × 96485[As mol−1] = Mw M w [g mol−1] n × 26801 [m Ahg −1] = Mw

E m

Cdischarge Ccharge

(7)

C-rate: the C-rate is a measure for the charging/discharging current of an electrochemical cell.26 iapplied C= i1h (8) with i1h as the current that is necessary to completely load the cell within 1 h on the basis of the theoretical capacity. Thereby a Crate of 1C corresponds to a full charge/discharge within 1 h. Current density (j [A m−2]): the current density of a system defines the electric current per cross-sectional area. In battery

(4)

Specific energy (Espec [Wh g−1]): The specific energy is the maximum energy of a cell that can be used per mass of the capacity-limiting active material. 9440


Chemical Reviews

Review

Figure 2. Schematic representation of the processes during (a) discharging and (b) charging of a Li-ion battery.

primary or secondary cells. Primary cells are not rechargeable since their electrochemical reaction is irreversible. As a consequence, they are disposed after discharging. Due to the absent demands concerning reversibility, the diversity of materials for primary cells and also the obtained energy densities are naturally much larger than for secondary cells. Secondary cells are electrically rechargeable several times after being discharged. For that reason, both electrodes need to possess chemically reversible redox reactions. An applied current forces the electrochemical reaction to the opposite direction and restores the charged state. Depending on the reversibility of the redox process and the stability of the electrode, hundreds or thousands of charge/discharge cycles can be applied. Figure 1a depicts the processes occurring during discharging of a classical metal-based secondary cell. Both the anode and cathode material are placed in a solution of their respective metal salts. Under open-circuit conditions, at both electrodes, an equilibrium between the metal electrode and the solution is reached, resulting in the accumulation of charges at the electrode surface. When both electrodes are connected electrically by an external conductor, the potential difference between the electrodes constitutes the driving force, aspiring an equilibrium by redox reaction of the electrode materials. Electrons migrate from the anode to the cathode material. The system equilibrates and, consequently, the anode material is oxidized, forming A+ and electrons, which again migrate to the cathode material. This results in an increased electron density at the cathode, which is compensated by the reduction of C+ ions. During this process a constant flow of anions to the anode and cations to the cathode enables charge balancing. This process stops when either the anode is consumed or all C+ ions are deposited at the electrode. In an ideal system with two metal electrodes, the limiting factor for the performance of the battery is consequently the movement of the ions. For that reason, the electrolyte solution should have a low resistance. During the recharge process, a reversed current forces the anode material to be reduced and the cathode material to be oxidized. Due to markedly higher energy densities, nowadays, the majority of secondary batteries used for mobile applications are based on the Li-ion technology.16,27−31 The fundamental concept of a Li-ion battery is the same as discussed above. The

research, often porous electrode materials are used to increase the surface of a given dimension and consequently reduce the local current density for a given total operating current.4

3. GENERAL SETUP 3.1. Battery Types

In electrochemical power sources, chemical energy is converted into electrical energy. During this conversion, one component is oxidized and transfers its electrons over an external circuit to another redox-active species, which is reduced, a so-called redox reaction.5 The best-known and most simple electrochemical cell is the galvanic element. In this cell, two electrode materials with different redox potentials are placed in electrolyte solutions that are connected by a salt bridge or a porous membrane (Figure 1). Its general structure and principle mode of operation remains the same for all electrochemical cells. The three crucial components of electrochemical cells are anode, cathode, and electrolyte. The electrolyte allows the transport of ions between the electrodes and enables charge balance (section 6). For that reason, a high ionic conductivity is desirable. To avoid internal short-cuts, significant electronic conductivity is unfavorable. Although the terminology of anode and cathode is only valid for the discharge process of rechargeable batteries, negative electrodes are often regarded as anodes and positive electrodes as cathodes throughout the battery literature.4,5,22 In this review, we adopt this nomenclature. Depending on the redox potential of the individual electrodes, the output voltage of such a cell usually ranges between 1 and 3 V but does normally not exceed 5 V. Since most applications require higher voltages, usually several electrochemical cells are connected in series, leading to an addition of the output voltages of the individual cells. When cells are connected in parallel the voltage remains constant, whereas the capacity increases.5 In general, the term cell is used for an individual pair of electrodes, connected electrochemically by an electrolyte. Depending on the used cell type, some of the electrodes are additionally separated by a porous membrane. The term battery usually describes a ready-to-use product, providing the required voltage, current, shell, and seals. Depending on the reversibility of the redox reaction of the anode and cathode active material, the cells are classified as 9441


Chemical Reviews

Review

Figure 3. Schematic representation of the processes during (a) discharging and (b) charging of an all-organic battery.33

and anode. The respective electrodes are placed in an ionconducting electrolyte (section 6) and separated by a porous ionpermeable membrane (Figure 3). In contrast to metal-based electrodes, usually, the uncharged form of the organic polymer is processed. For that reason, the cathode is oxidized initially by applying a sufficient current, while the anode is reduced. The charges formed at the electrodes are compensated by electrolyte ions (Figure 3). Consequently, the limiting factors for the charging process are either complete oxidation of the cathode material, complete reduction of the anode material, or consumption of the electrolyte salt. To ensure a high energy density, the active-material contents of cathode and anode, more precisely their specific energies, should match. Common exceptions are organic materials featuring a second irreversible redox reaction at a potential close to the utilized one. In this case, the respective organic material is used in a slight excess to limit the charging by the other electrode material and to avoid irreversible overcharging. To discharge the battery, the electrodes are connected by an external load and the electrons migrate from the anode to the cathode. Consequently, the counterions are set free and migrate from the polymer matrix into the electrolyte (Figure 3b). In contrast to Li-ion cells, where the rate performance is limited by the slow diffusion during the Li+ intercalation, the limiting factors in organic batteries are the migration of electrolyte ions and the standard rate constant for the electron transfer k0 of the redox reaction.24,33,34 To ensure a high ionic conductivity and a good availability of counterions, high salt concentrations in the electrolyte are usually used. However, too high concentrations can lead to high viscosities and, consequently, to lower ion mobilities. The electron-transfer rate constant k0 of compounds commonly used as electrode materials is in the order of 10−1 cm s−1.22,24,33−35 Their high values are the most important benefit of organic batteries since they enable rapid charging and discharging of the electrode. Furthermore, due to the amorphous and swollen structure of the organic polymer electrode, a fast ion mobility during the charge/discharge process is ensured, which is reflected by apparent diffusion coefficients (Dapp) in the order of 10−8 cm2 s−1.34 In order to provide comparable test systems for organic electrode materials, the majority of new materials is not tested in an all-organic battery but in a metal (mostly lithium)-organic

anode (negative electrode) usually consists of graphite intercalating lithium atoms, and the cathode (positive electrode) is a lithium-ion-containing compound, such as Li1−xCoO2x− (Figure 2). During discharging, the lithium is oxidized at the anode and released to the electrolyte, while the electron is transferred by an external conductor to the cathode (Li1−xCoO2x−). To compensate charges, a Li+ ion migrates to the cathode and forms LiCoO2. Again, the charging process works vice versa, and Li+ is reduced and intercalated into graphite, while the cathode material is reoxidized. The utilization of graphite or similar intercalation electrodes presents a significant progress in Li-ion battery development, since the formation of metallic lithium and, thus, the associated dendrite formation is suppressed due to the separation and immobilization of the lithium atoms in the graphite lattice. As outlined in the introduction, two major advantages of organic batteries in comparison to Li-ion batteries are the excellent rate performance and a very long cycle life. This can be explained by the superior charge/discharge mechanism. In inorganic intercalation electrodes, the intercalation of Li+ ions, which is accompanied by a transformation of the lattice and the layered structure, results in slow kinetics and heat generation during the charge/discharge process.32 These kinetic problems limit the usage of classical Li-ion batteries in high-power applications. In contrast, organic batteries allow, due to an inherently simple and fast redox process that avoids extensive structural alterations, an excellent rate performance. As for metal-based secondary batteries and Li-intercalation batteries, also organic batteries consist of two electrodes with different redox potentials. In principle, all organic molecules that undergo reversible redox reactions are suitable candidates for electrode materials. In contrast to inorganic materials, where the redox-process is related to the valence charge of the metal, the redox reaction of organic materials is based on the change of the state of charge of the redox-active organic molecule.24 Depending on whether they undergo chemically reversible oxidation or reduction, the organic compounds can be classified into three different groups: p-type organics are oxidized during charging, yielding cations, whereas n-type organics are reduced to form anions, and b-type organics can be both oxidized and reduced. Consequently, in an all-organic battery, an organic n-type material would be applied as anode and a p-type material as cathode, whereas b-type materials can be used as both cathode 9442


Chemical Reviews

Review

Figure 4. Schematic representation of the processes during (a) discharging and (b) charging of a metal−organic battery.

battery (Figure 4). In this case, the metal serves as anode and both p- and n-type organic materials are tested as cathode. When p-type organic materials are used as cathodes, the working procedure is similar to an all-organic battery. Most importantly, the cation of the electrolyte salt should correspond to the applied metal. In an initial charging step, electrolyte salt cations (e.g., Li+) are reduced and deposited at the anode. In the case of a lithium salt, also graphite can be used to enable an intercalation similar to Li-ion batteries. The p-type material is simultaneously oxidized. During discharging, the organic material is reduced and Li is oxidized to form Li+. In the case of n-type organic materials, the general setup is the same. However, due to the reverse nature of the redox reaction of n-type materials, the battery is already charged after assembly. When cathode and anode are connected electrically, the n-type organic material (cathode) is reduced and the metal anode oxidized.

Figure 5. Schematic representation of a coin cell.

3.2. Housing

As mentioned above, the charge/discharge performance of most organic electrode materials is initially tested versus a lithium metal anode. The reasons are the simple one-electron redox reaction, the low redox potential (−3.04 V vs SHE) of lithium, which enables the potential usage of plenty of organic materials as cathode, the small ion radius of Li+, which, in case of n-type organics, easily migrates to the reduced polymer site, the stability of the lithium anode, and the large number of available techniques known from the Li-ion technology. The most commonly used housing for a scientific test cell is the coin cell (Figure 5). In these cells, low quantities of active material can be easily tested. The cathode is separated from the anode by a porous membrane, and the electrodes are pressed together by a spring. When water- and/or air-sensitive materials are handled (e.g., lithium, sodium, and LiPF6), the coin cells are assembled in a glovebox. The finished and sealed cells can be measured and used under air. This simple experimental setup enables rapid and reproducible manufacturing of prototype batteries under inert atmosphere. The electrode setup used in a so-called Swagelok cell (Figure 6) is in general similar to that of coin cells. The main differences are the reusability and the straightforward cell disassembly, enabling the investigation of the electrodes after charge/ discharge cycling by, for example, SEM to visualize structural

Figure 6. Schematic representation of a Swagelok cell.

changes. Moreover, the application of a reusable system is of economic and environmental interest. When, besides the basic charge/discharge behavior, further information about the electrochemical process is required, a more sophisticated housing is used. The Swagelok T-cell is a three-electrode cell and enables the usage of a separate reference electrode (Figure 7). This reference allows a detailed study of the processes occurring at the working electrode and counter electrode and is, therefore, usually used in initial studies of academic research. A more often applied housing is the pouch cell. This cell design was established in 1995 and represents a simple, flexible, and 9443


Chemical Reviews

Review

Figure 9. Schematic representation of a cylindrical cell.

Figure 7. Schematic representation of a Swagelok T-cell.

lightweight battery design.36−39 In contrast to the abovementioned systems, this cell achieves more than 90% packaging efficiency, which is the highest among all battery housings. To assemble a pouch cell, several layers of cathode and anode material are stacked in a sandwich design (Figure 8). Similar to

reactions. Yet, a compromise between structure optimization and synthetic effort is necessary. Simple and straightforward synthetic routes and cheap raw materials are desirable with regard to a possible application. The solubility in the electrolyte represents a further challenge in the course of the development of organic electrode materials. Dissolved active material can shuttle between the electrodes, which leads to the self-discharge of the battery and affects significantly the cycling stability, in particular, for small molecules. The incorporation of the redox-active unit into a polymeric backbone is a synthetic approach used to overcome this issue. The substances are classified as p-type (P+/P), n-type (N/N−), and b-type (B+/B/B−), depending on whether they can be electrochemically oxidized (p-doped), reduced (n-doped), or both. Usually, p-type organics possess higher redox potentials than n-type organics. In general, materials of every class can be utilized in an anode or a cathode. Whether it acts as a positive or negative electrode depends on its redox potential and on that of the particular counter electrode. A large potential difference between the electrodes is desirable as it leads to a high energy density. If both applied electrodes are of the same type, it must be considered during the assembly of a cell that one electrode has to be in the oxidized state while the other one is in the reduced state. Organic compounds are more often suitable as cathode than as anode material, their redox potentials range roughly between 2 and 4 V vs Li+/Li. The redox properties of the organic materials are mainly determined by their electroactive moiety. Different reaction mechanisms of the compound classes cause huge differences in reaction kinetics. While the redox reaction of inorganics is related to a valence change of transition metals or elemental substances, in organics, a change of the state of charge of the electroactive organic group occurs. Mechanisms that contain bond breaking/formation reveal slow kinetics, while other redox reactions like those of stable radicals reveal high rate performances, positively affecting the power density. Furthermore, large differences between structures regarding the shape of the charge/discharge plateaus and the average charge/discharge voltages occur. In this chapter, the great variety of polymeric active electrode materials is discussed, classified into conjugated polymers,

Figure 8. Schematic representation of a pouch cell.

other layouts, a separator foil prevents a direct contact between the electrodes. Subsequently, metal (usually copper for the anode and aluminum for the cathode) current collectors are welded together to form the electrodes and the complete stack is sealed by a thin aluminized plastic back. A rather common commercial housing for batteries is the cylindrical cell. This setup features both an easy manufacturing and a good mechanical stability. Anode and cathode electrodes are separated by a porous membrane, wound, and placed in a case (Figure 9). However, in organic battery research, there are no examples of cylindrical cells published so far in the literature.

4. ACTIVE MATERIALS The basic requirement for a substance to be utilized in a battery is a reversible electrochemical redox reaction. The great structural diversity and synthetic tailorability of organic substances allow adjusting the redox potential or increasing the capacity, either by lowering the molar mass or by enabling multielectron redox 9444


Chemical Reviews

Review

reduction, which make them also favorable for utilization in highpower applications such as supercapacitors.51−55 The circumstantial review of Novák et al. covers the literature about redox-active polymers for batteries until 1997 and provides detailed information on the electrochemical properties and synthetic routes of the numerous conjugated polymers.22 More recently, the progress on flexible polymer-based energy storage systems, in particular supercapacitors and secondary batteries, was summarized by Nyholm et al.47 Further, deeper insights into conjugated polymers for energy storage and their electrochemistry are provided by several recent reviews.22,23,43,56−59 Therefore, the following sections of this review highlight interesting recent examples and focuses in particular on the utilization of poly(indole), poly(pyrrole), poly(thiophene), and poly(aniline) as active material. An overview over the described active materials and their properties is provided in Table 1. 4.1.1. Poly(pyrrole). One of the most intensively studied conjugated polymers for energy storage applications is poly(pyrrole) (PPy). It is mostly utilized as cathode material. Nevertheless, already in 1986, a battery with PPy as both anode and cathode in an acetonitrile-based electrolyte was developed.60 More recently, Nyström et al. presented an all-polymer, “paper-based” battery fabricated from poly(pyrrole) and reinforced with cellulose nanofibers. By oxidizing pyrrole on the surface of cellulose, an electrode material with a complex pore structure was obtained, which can be used as both anode and cathode material, yielding a pole-less battery with a sloping discharge voltage of up to 1 V. This poly(pyrrole) cell was charged with up to 320C, still maintaining about 70% material functionality (25 Ah kg−1) and operates simultaneously as rechargeable battery and capacitor due to the large surface area of the electrodes.51 In addition, poly(pyrrole) was used as an anode material to manufacture an aqueous Li-ion battery in conjunction with a LiCoO2 cathode.61 This approach treats the common problem of poor cycling stabilities of aqueous Li-ion cells.62 The battery demonstrated an average output voltage of 0.85 V and a capacity of 47 Ah kg−1, while showing no significant decrease in cycle performance during the first 120 charge/discharge cycles. With the long-term aim of fabricating paper-based, flexible, and bendable rechargeable batteries, a free-standing thin-film poly(pyrrole) electrode was produced and subsequently used to assemble a poly(pyrrole)/lithium battery.63 The benefit of a freestanding film is the decreased risk of cracking and peeling of the polymer film from the current collector, which leads to a superior cycling stability. In addition, stretchable and “buckle-structured” poly(pyrrole) electrodes were presented in 2011 by Wang et al. focusing on directly implantable medical devices/batteries.64 Thereby, a poly(styrene-b-isobutylene-b-styrene) film sputtercoated with gold was used as flexible and stable current collector (substrate). Studies on the electrochemical properties of a poly(pyrrole)poly(imide) composite revealed that the material stores charges by means of doping/dedoping in the electric double layer.65 The material appeared to be more suitable for the use in supercapacitors than in rechargeable batteries, since the ultracapacitance of the material increases upon repeated charging/discharging of the electrode. A composite with inorganic compounds was prepared by doping PPy with Fe(CN)64− anions.66 Upon employing the material in a battery versus a metallic lithium anode, a capacity enhancement of 300% compared to undoped PPy was observed. Furthermore, the cycling stability as well as the charge and

nonconjugated conventional redox-active polymers, and radical compounds. 4.1. Conjugated Polymers

In the 1980s, conductive polymers were extolled as promising materials for the next generation of environmentally benign and efficient batteries. Their interesting electronic properties originate in the overlap of adjacent π-orbitals, resulting in a band structure analogous to inorganic semiconductors.40 Their conductivity is greatly affected by the doping, which occurs upon charging and creates cations and anions that are delocalized along the polymer backbone.41,42 Although the intrinsic conductivity of these materials represents one of the major advantages, due to the inherent connection of their redox centers to the current collector, it results in a great drawback as well. Since the charged centers are not separated electronically and strongly affect each other, the redox potentials of conjugated polymers depend on the doping level and gradually change upon charging/discharging (Figure 10). This results in a sloping cell voltage, which limits the range of application.

Figure 10. Comparison of the discharging behavior (cell voltage) of batteries based on conductive polymers and polymers with distinct redox units. Batteries based on conductive polymers often show a sloping curve (i.e., a varying cell voltage), while distinct redox units lead to a voltage plateau (i.e., to stable cell voltage). Reprinted with permission from ref 18. Copyright 2012 John Wiley & Sons.

Another issue is represented by the limited achievable degree of doping. While, ideally, one charge should be stored per repeating unit, conjugated polymers typically reach reversible doping levels of at most 0.3 to 0.5, causing significantly decreased capacities.11,43 High levels of charge destabilize the polymer matrix through undesired interchain interactions or lead to a destruction of the polymer chains.44,45 Highly charged species also react more likely with electrolyte components and/or oxygen.28,44,46 Though, the stability of charged species can be improved by utilization in composites with materials like graphene or carbon nanotubes.47,48 Bridgestone-Seiko and VARTA/BASF launched commercial batteries in the late 1980s, based on poly(pyrrole) and poly(aniline), respectively.49,50 Since these batteries suffered from the mentioned drawbacks and demonstrated lower energy densities than conventional products, their production was discontinued after only five years on the market.23 Despite these drawbacks, the interest in conjugated polymers as active material in the development of electrochemical energy storage is still present, not least because of the high rates of oxidation and 9445


Chemical Reviews

Review

Table 1. Overview of Conjugated Polymers Applied as Active Material in Secondary Batteries

9446


Chemical Reviews

Review

Table 1. continued a

The symbols indicate the trend of the discharge voltage: sloping (s) and stable plateau with one (p) or two (pp) steps. bCurrent densities up to 6000 A m−2. cSignificant drop of discharge capacity during first cycles then stable capacity. dIncrease of discharge capacity during first cycles. eMore supercapacitor character than battery character.

redox reaction but also by addressing the migration of the counterions. A block copolymer of PT and poly(ethylene oxide) (PEO) was synthesized, which segregated into domains of ionconductive PEO and electronically conductive PT segments.74,75 The copolymer, possessing both ionic and electronic conductivity, was employed in a solid-state Li-ion battery, though not as redox-active material but in a composite with LiFePO4.76 Analogous studies were conducted for a P3HT-PEO block copolymer.77 4.1.3. Poly(aniline). The first known prototype of a polymer battery in 1968 consisted of poly(aniline) (PANI) pellet electrodes with different redox states, a specific capacity of 13 Ah kg−1 and a specific energy of 3 Wh kg−1.78 Although its redox reactions are more complex than those of, for example, poly(pyrrole) or poly(thiophene), PANI is still in the focus of recent research, in particular with respect to composite materials containing inorganic redox-active compounds (section 4.1.5). It features several redox states, including the completely reduced leucoemeraldine, intermediate emeraldine, and the completely oxidized pernigraniline. Some of these states include protonation−deprotonation equilibria, complicating the electrochemistry of PANI, which is therefore pH-dependent. While the protonated emeraldine salt is conductive, all the other redox states of PANI are nonconductive.47 Recently, a derivative [i.e., poly(2,5-dihydroxyaniline)] was synthesized by chemical demethylation of poly(2,5-dimethoxyaniline) and applied in a lithium-organic setup.79 This structure hybridizes the redox properties of quinone groups, which are part of the polymer backbone, with the intrinsic conductivity of polyaniline. The electrochemical performance strongly depends on the degree of demethylation. While a low degree of methylation leads to higher specific capacities, the cycling stability decreases significantly. The best characteristics for a battery application, in particular regarding the cycling stability, were achieved at a rather low but not exactly determined amount of free hydroxyl groups. Here, the discharge capacity increased during the first 20 cycles to 120 Ah kg−1 of which 100 Ah kg−1 sustained after 100 cycles at 0.1C. A significant improvement in terms of initial discharge capacity of a poly(aniline) battery was achieved by Zhang et al. (980 Ah kg−1).80 The authors prepared a poly(aniline)-poly(sulfide) by substituting hydrogen atoms of PANI with sulfur. However, due to degradation of S−S-bonds, the capacity rapidly decreases after the first discharging run, stabilizing at about 403 Ah kg−1 after 20 cycles. The utilization of electroactive disulfide groups is commonly employed in order to improve the charge-storage capacity of a conductive polymer. Since the major amount of the capacity of this material arises from the reversible redox reaction of the disulfide groups, this class of material is discussed in more detail in section 4.2.2.1. Another hybrid material was prepared via grafting technique from diisocyanate-modified graphite oxide and a copolymer of 5amino-1,4-dihydrobenzo[d]-1′,2′-dithiadiene and aniline.81 The polymer intercalates in the two-dimensional lamellar structure of the graphite oxide, increasing the electrochemical stability as well as the conductivity of the material. The material features two redox sites: the oxidizable S−S-bond and the doping of

discharge rates were improved. In accordance with the authors, these effects are not only attributed to the electrochemical contributions of PPy and Fe(CN)64−, but the polymer was additionally activated by the redox-active Fe(CN)64−/3− anions. It was postulated that the Fermi level of Fe(CN)64− is close to that of the polymer and it therefore acts as a charge-transfer mediator between polymer and electrolyte. Another approach toward an improvement by inorganic components is the modification of PPy by covalently anchoring ferrocene groups to the polymer. In a copolymer with 50% functionalized repeating units, a flattening of the charge/ discharge slope could be achieved.67 A short voltage plateau near 3.5 V vs Li+/Li was assigned to the ferrocenium/ferrocene redox couple. The specific capacity increased to 65 Ah kg−1 compared to 20 Ah kg−1 for pure PPy under equal conditions. 4.1.2. Poly(thiophene). Poly(thiophene)s (PT) have been of interest for electrochemists for decades. The first battery with poly(thiophene) as an active material was described in 1983.68 Recently, poly(3′-styryl-4,4″-didecyloxyterthiophene) with a maximum capacity of 45 Ah kg−1 and poly(4,4″-didecyloxyterthiophene) with maximum capacity of 95 Ah kg−1 were used as anode material in combination with a poly(pyrrole) cathode.69 The polymers were electrodeposited on stainless steel and Ni/ Cu-coated nonwoven polyester fabric, respectively. The first substrate led to stable electrodes, while the capacity of the latter decreased rapidly. A solvent-free preparation of FeCl 3 -supported poly(thiophene) was described by Tang et al.70 Even with a short polymerization time of 20 min, sufficient yields were achieved at room temperature. The obtained polymer shows structural characteristics of the thiophene and tetrahydrothiophene repeating units and reveals a capacity of 400 Ah kg−1 and more. Since the tetrahydrothiophene units cannot form polarons and the discharge voltage as well as the capacity are similar to thiolane polymers, the authors suggest that the redox process involves thioether groups that form isolated thioether cations upon oxidation. Recently, Suga et al. described the vapor-phase polymerization of 3,4-ethylenedioxythiophene in the presence of poly(TEMPOfunctionalized acrylamide) or poly(galvinoxylstyrene) (section 4.3.3) in order to obtain a composite for the production of a flexible, metal-free battery.71 As an example for the incorporation of additional redox-active centers into a conjugated polymer, poly(thiophene) was polymer-analogously functionalized with TEMPO units by click reaction and subsequently electrodeposited onto ITO or carbon substrates.72 However, a decreased capacity compared to nonfunctionalized poly(thiophene) was obtained. This effect was attributed to a partial cleavage/oxidation of the PT backbone facilitated by the oxidative properties of the TEMPO moieties. In a later publication, a different group functionalized PT with TEMPO via esterification, yielding a polymer that is stable upon reversible charging and discharging with only 23% loss of capacity after 50 cycles and a specific capacity of 79 Ah kg−1 (87% of the theoretical value).73 In further studies, an approach was investigated to improve the performance of the electrode material not only in terms of the 9447


Chemical Reviews

Review

kg−1 for the n-doping reaction. However, in an all-organic battery setup, the performance drops significantly. Recently, attention turned to poly(indole)s as electroactive material in batteries. These materials were prepared by electropolymerization90 or conventional chemical polymerization using, for example, ferric chloride.91 They show good thermal stability in comparison to poly(aniline), poly(pyrrole), and poly(thiophene),92 as well as high intrinsic conductivities.90 In combination with a lithium anode, a stable and robust battery was fabricated revealing 80% of its initial capacity after 30000 charge/discharge cycles (80 to 70 Ah kg−1 at 10 to 103 A m−2). Similar to other conductive polymers, poly(indole) features a change in its conductivity depending on its redox state. The conductivity decreases from 2.0 S cm−1 to 1.5 × 10−1 S cm−1 upon discharging, showing a sloping discharging curve from 3 to 2 V.91 A first lithium/oligo-indole battery was reported by Kim et al., providing no clear discharging plateau as well but rather a gradual decrease of the output voltage from 4 to 2.5 V.93 The initial discharge capacity quickly decreases from 70 to about 55 Ah kg−1 after 50 cycles. In addition, poly(5-nitroindole) and poly(aniline) were employed in one battery as active materials. The system achieves a specific charge of 79 to 65 Ah kg−1 at current densities of 10 to 103 A m−2. Noteworthy is the rather flat discharging curve with a plateau at about 1.1 V.94 The excellent cycling stability makes poly(indole)s an interesting subject for further research. It will be of predominate importance to increase the materials doping capability and its charge-storage capacity to ensure competitiveness with conventional systems. 4.1.5. Inorganic Composites. Lately, much effort was invested in the combination of conjugated polymers with inorganic redox-active compounds. The utilized redox activity of these materials often arises from the inorganic part of the composite, and the polymer only features supporting functions. Therefore, only a selection of interesting results is discussed. For a more detailed discussion of hybrid organic/inorganic materials, the reader is referred to reviews that were published recently by Li and co-workers as well as Muñoz-Rojas et al.95,96 Poly(pyrrole) and poly(aniline) are often used as additives to improve the capacity and/or rate capability of (lithium) batteries. Metal(oxide)/polymer hybrid electrodes are formed by dispersing metal(oxide) particles in a polymer matrix or a monomer solution, which is subsequently polymerized. An electrodeposited carbon-coated LiFePO4/poly(pyrrole) composite electrode, for example, requires no additional carbon or binder and eliminates parts of a conventional battery’s dead weight, improving its capacity and rate capability.97 A poly(pyrrole)/vanadium oxide nanotube composite, synthesized by hydrothermal treatment and subsequent oxidative polymerization of pyrrole, revealed an enhanced electronic conductivity and improved structural flexibility leading to a four times higher capacity (159 Ah kg−1 after 20 cycles) with respect to conventional vanadium oxide nanotube electrodes and improved cycling stability. However, the electrode suffers from severe deterioration, indicated by a 30% capacity loss within only 20 charge/discharge cycles.98 Better results were obtained by Kim et al., who prepared films by electrochemical polymerization, with the V2O5 particles acting as oxidative polymerization catalyst and an embedded particle component in the actual composite electrode. The capacity decreases from initial 497 to 221 Ah kg−1 after 48 cycles.99 Also reticulated vitreous carbon was used as substrate for a composite material with poly(pyrrole). The authors claim that this “three-dimensional” electrode achieves higher capacities (95

poly(aniline). Although no battery has been assembled yet, the composite might represent a promising approach to new electrode materials. Similar to the previously mentioned poly(pyrrole)-based material, a ferrocene-containing poly(aniline) was synthesized.82 Copolymers with different ratios of ferrocene-functionalized and pristine aniline units were examined as cathode material against a lithium foil anode. The cycling stability as well as the discharge plateau, which occurs at the characteristic redox potential of ferrocene, continuously improve with an increasing ferrocene ratio. Concurrently, the specific capacity decreases since the molar mass of the additional substituent and spacer is significantly higher. A poly(aniline)/ferrocene homopolymer reveals a specific capacity of 36 Ah kg−1 and a loss of 8% over 30 cycles, compared to 108 Ah kg−1 and 24% loss of pristine PANI. To address the stability at high potentials, a poly(aniline):polyacid complex, namely poly(2-acrylamido-2-methyl-1-propanesulfonic acid) (PAAMPSA), with a theoretical capacity of 294 Ah kg−1 was prepared, which revealed electrochemical stability up to 4.5 V vs Li+/Li.83 After a preconditioning step, the fully reduced and oxidized species are both reversibly accessible. A Li-polymer sandwich cell showed superior cycling stability compared to pristine PANI. Other approaches to improve the stability of conjugated polymers at high voltages comprise the introduction of electron-rich fused rings into the conjugated backbone, like in poly(dithieno[3,2b:2′,3′-d]pyrrole),84 poly(3,4-ethylenedithiothiophene),85 and poly(benzo[1,2-b;4,5-b′]dithiophene).86 An alternative use of conductive polymers was, for example, presented by Liu et al.87 By tempering a mixture of poly(aniline) and phosphoric acid at 700 °C, nitrogen-containing activated carbons were prepared. This intercalation matrix suffers from low Coulombic efficiency but achieves up to 2200 Ah kg−1 (100 A kg−1) for the first discharge capacity. The high capacity occurring at the first cycle is attributed to irreversible corrosion processes, such as the formation of a solid electrolyte interface (SEI), which are common for amorphous carbonaceous materials. Typical and reversible capacities appear to be in the range of 750 Ah kg−1. 4.1.4. Other Conjugated Polymer Systems. In 1981, an all-polymeric solid-state battery based on poly(acetylene) for anode and cathode and a PEO-based electrolyte was presented.20 However, in the last years, this class of polymers was only rarely investigated in terms of energy storage. Recently, a TEMPOsubstituted poly(acetylene) was reported.88 A rechargeable Libattery revealed an initial specific discharge capacity of 103 Ah kg−1, but only 66% are retained after 40 charge/discharge cycles at 0.3C. At higher discharge rates, the capacity decreases but remains nearly constant upon cycling. The Coulombic efficiency of the cell continually improves during cycling and reaches a value of 92% after 40 cycles. A discharge plateau at 3.6 V vs Li+/Li is close to the characteristic redox potential of the TEMPO moieties, indicating that the electrochemical activity arises from the radical functionalities. The bipolar poly(para-phenylene) can act as both cathode and anode active material, as recently shown by Yang et al.89 The large potential gap of 3.0 V between the p- and n-type reaction of the polymer makes it interesting for an application in all-organic batteries with the same anode and cathode material. In batteries versus Li, the material displays good performance in the anode as well as in the cathode. In this setup, Coulombic efficiencies up to 98% and a good reversibility are reached. The specific capacity amounts to 80 Ah kg−1 for the p-doping and remarkable 350 Ah 9448


Chemical Reviews

Review

Ah kg−1) than conventional electrodes prepared from twodimensional substrates.100 The idea of a three-dimensional microbattery was also picked up by Min et al.101 The electrode was prepared in a carbon-microelectromechanical systems (CMEMS) microfabrication process, in which poly(pyrrole) was deposited on an array of carbon rods. The authors demonstrated that the electrode intercalates lithium with better gravimetric capacity than a conventional film. Poly(aniline)/LiNi0.8Co0.2O2 nanocomposites feature an improved specific capacity (160 Ah kg−1), specific energy (594 Wh kg−1), and specific power (18 W kg−1).102 In another case, a nickel-foam-supported NiO/poly(aniline) revealed less polarization and the cycling performance improved by 20% in comparison to conventional NiO electrodes (520 Ah kg−1 after 50 cycles) due to an enhanced electrical contact ensured by the addition of the conductive polymer.103 Very similar results were achieved with a NiO/poly(3,4-ethylenedioxythiophene) (PEDOT) electrode.104 In addition, a MoO3/poly(aniline) composite was prepared by oxidative polymerization with ammonium peroxydisulfate, which performs at an initial discharge capacity of 285 Ah kg−1.105 On a poly(aniline) electrode, grown by means of potentiodynamic polymerization, V2O5 was deposited electrochemically from a VOSO4 solution. The poly(aniline)/V2O5 composite electrode possesses an initial discharge capacity of 460 Ah kg−1 at 0.2C. At a current rate of 1C, the capacity drops to 272 Ah kg−1, remaining stable for at least 50 cycles (only 3.4% loss). Indicated by several experiments, this system stores electric energy due to both lithium intercalation/deintercalation and the redox reaction of poly(aniline).106 Furthermore, a zinc-poly(aniline)/Fe3O4 battery revealing a likewise improved capacity was reported.107 The widely applied electrode material LiFePO4 was also used to produce poly(aniline) composite electrodes. It was proven that the polymer not only functions as a conductive matrix and binder but also as an additional host for lithium ion intercalation. At 0.2C, a capacity of 165 Ah kg−1 was obtained dropping by 25% to 123 Ah kg−1 at 10C. Nevertheless, flat discharging curves were obtained accompanied by a good cycling stability.108 With a similar intent, LiFePO4/PEDOT composites were prepared. PEDOT-coated phosphate particles show a considerably decreased charge transfer and solid electrolyte interface film resistance since the conductive polymer bridge-links the individual particles.109 PEDOT was also used for the fabrication of cer(III)-ion/PEDOT-doped montmorillonite clay nanocomposites, which can be employed as electrode material in batteries or as antistatic coating.110 Interestingly, the cer-iondoped clay matrix impregnated with a 3,4-ethylenedioxythiophene monomer induced a spontaneous polymerization yielding a PEDOT-interspersed montmorillonite clay composite with about 6% polymer content. Aniline and pyrrole were combined and polymerized using cetyltrimethylammonium chloride as a template to form poly(pyrrole-co-aniline) nanofibers.111 By subsequent coheating of the polymer and sublimed sulfur, a nanosulfur/poly(pyrroleco-aniline) electrode was prepared. The porous morphology, increased electronic conductivity, and large specific area of the composite lead to an improvement of the specific capacity of a lithium/composite test battery to practical 866 Ah kg−1 (after 40 cycles). Subsequently, the cycling performance and polarization behavior were improved. The superior cycling stability with respect to sulfur/lithium batteries is attributed to an adsorption effect of the porous polymer matrix preventing the diffusion of polysulfide anions into the electrolyte.

Sulfur/poly(pyrrole) composites have attracted the interest of Liang et al., who revealed that polymers of granular morphology are inferior to poly(pyrrole) of tubular morphology.112 The coating of sulfur particles with poly(thiophene), which functions as a conducting additive and adsorbing agent for the electrolyte, leads to sulfur/poly(thiophene) core/shell particles.113 At the optimized ratio of 72% sulfur and 18% poly(thiophene), an initial discharge capacity of 1119 Ah kg−1 (100 A kg−1) was obtained. A reversible capacity of 830 Ah kg−1 after 80 cycles were achieved with this system. The authors demonstrated as well how pore size and shell thickness affect the battery performance. 4.1.6. Enhancements by Morphology. The implications of different morphologies were studied by means of poly(aniline) electrodes (Figure 11). In combination with a zinc anode, a

Figure 11. SEM micrographs of (a, c) poly(aniline) revealing a spongy and porous morphology and (d) a polymer having a denser and less porous spongelike morphology. This is attributed to the formation of an aniline/o-PDA copolymer or composite. (b) In contrast, poly(aniline), revealing a granular (i.e., pebblelike) structure. Reprinted with permission from ref 114. Copyright 2009 Elsevier.

spongelike and pebblelike poly(aniline) cathode was produced by electrodeposition on glassy carbon substrates at different pH values, revealing a strong dependence of the capacity on the polymer film morphology.114 The discharge behavior is dominated by a strongly sloping discharging curve in both cases, while the capacity of the sponge-like films is almost twice as high compared to the pebble-like films (81 Ah kg−1 at 1C). The authors attribute this behavior to the improved kinetics within the porous structures of the sponge, which facilitates the diffusion of ions. A similar effect was observed for poly(thiophene). A slow oxidative polymerization at low temperatures yields porous particles with an improved electrochemical performance.115 Using this method, an increase of the specific capacity compared to other pristine poly(thiophene) electrodes was achieved.116−118 The problem of a sloping discharging curve also occurs for a poly(aniline-co-m-aminophenol)/zinc battery. Nevertheless, the material displays an improved performance at increased current densities with respect to poly(aniline)/zinc batteries and a capacity of 94 Ah kg−1 at 5 mA cm−2.119 Besides the influence of the polymer morphologies, the capacity can be further increased by optimizing the electrolyte.120,121 If the charging potential is limited to 0.35 V, the addition of citrate to a chloride-saltcontaining electrolyte leads to an energy increases due to a shift 9449


Chemical Reviews

Review

of the doping-potential limit by about 30 mV. This increased the potential window for the doping process and, thus, the amount of stored charge. In conclusion, conducting polymers are applied as anode and cathode material in the course of the development of rechargeable batteries. High rates of oxidation and reduction enable their utilization in high-power applications.51−55 However, low capacities owing to low attainable doping degrees represent a major drawback.53,63,122 Further shortcomings are a limited postsynthesis processability due to their poor solubility, infusibility, and mechanical brittleness. An armoring of the conducting polymers with cellulose fibers to improve the cycling stability and mechanical robustness represents a promising solution.47 Extensive research is performed on hybrid materials employing conducting polymers as supplement for conventional inorganic electrodes. New composites excel in improved charge-storage capacity and charging/discharging rate. Nevertheless, the cycling stability and lifetime of conjugated-polymerbased electrodes must be increased considerably to be applied in competitive battery systems. By fusing the key benefits of nonconjugated redox-active compounds with the conductivity of conjugated polymers, a new class of high-performance materials may emerge in future battery research.

system upon reduction as the main driving force. Commonly, several of these effects are found in the utilized structures. A circumstantial review about carbonyls for battery applications including polymers as well as small molecules was recently published by Häupler et al.135 An overview over the subsequently discussed carbonyl-based active materials and their properties is provided in Table 2. 4.2.1.1. Quinones. The electrochemical properties of quinone and anthraquinone polymers were reported by Manecke and Storck several decades ago.136,137 Although Alt et al. have already suggested the use of quinone compounds in the early 1970s,126 not much battery-related research occurred afterward. Only in the course of the increasing interest in alternative active electrode materials during the past decade, several new quinone- and anthraquinone-containing polymers were studied. Poly(anthraquinonylsulfide), which was prepared by the polycondensation of 1,5-dichloranthraquinone with sodium sulfide (Phillips method), proved to have a reversible redox reaction at 2.33 V (vs Li+/Li), similar to anthraquinone.138 Its good cycling performance and capacity of 185 Ah kg−1 (at 50 mA g−1) made it an interesting candidate for more advanced research in the field. A follow-up study systematically investigated the factors influencing the performance of batteries containing this active material.139 The substitution pattern was varied by shifting the second sulfide bond from the 5- to the 8-position; however, the new polymer revealed a lower capacity (130 Ah kg−1 after 20 cycles) but remains quite stable over 200 cycles. The authors ascribed this effect to a lower conductivity of the polymer. Different binders, namely PVdF, CleviosP (PEDOT:PSS), and the lithium salt of carboxymethyl cellulose were tested as well. While PVdF shows the best results at low charging rates, at 5C, the composites containing the ionically and electronically conductive CleviosP reveals slightly higher capacities. As electrolytes, mixtures of organic carbonates on the one hand and organic ethers on the other hand were tested. Cyclic voltammetry in organic carbonates revealed an irreversible oxidation leading to a capacity drop upon cycling. In organic ethers, however, the redox system seems to be fully chemically reversible. In a subsequent study, the rate capability was optimized by the fabrication of the composite materials by in situ polymerization in the presence of functionalized graphene.140 The polymercoated graphene sheets obtained by this method were applied as cathode active material in Li-organic batteries, leading to an initial capacity of 177 Ah kg−1 at a charging speed of 2C and a narrow voltage plateau at 2.2 V. At 100C, still a capacity of about 100 Ah kg−1 was reached. In a further study, Deng and coworkers presented an all-organic battery with poly(anthraquinonylsulfide) as anode and poly(triphenylamine) as cathode material.141 The battery displayed an average discharge voltage of 1.8 V with a rather sloping charge/discharge plateau dropping by over 1 V. The battery was charged at a rate of 8C, revealing an initial capacity of 185 Ah kg−1 with a moderate loss after 500 cycles (158 Ah kg−1). A similar approach was employed for the preparation of poly(2,5-dihydroxyl-1,4-benzoquinonyl sulfide).142 Noteworthy is a strong and irreversible capacity loss after the first charge/ discharge cycle, which was ascribed to an irreversible doping with lithium ions. Afterward, the capacity declined from 228 to 184 Ah kg−1 upon 100 cycles at a rate of 0.05C. An improvement regarding the cycling stability and charging speed was achieved by the application of the lithium salt of the polymer.143 The resulting lithium-organic batteries revealed an initial capacity of

4.2. Nonconjugated Conventional Redox-Active Polymers

Since conjugated polymers possess significant disadvantages, like a sloping redox potential, and offer only a limited number of active moieties, the researchers’ focus shifted to other classes of redox-active polymers during the past decade. These materials are, in general, based on a nonconductive backbone bearing electroactive pendant groups, which determine the redox behavior of the material and lead to a distinct redox potential due to the localized redox sites. Besides the organic radicals, which gained attention at the beginning of this millennium (section 4.3), many other conventional electrochemically active compounds were studied in the last years. An overview of previous studies was given by Novák et al. in their comprehensive review.22 As redox polymers permit the access to a wide range of material properties and redox potentials, they promise to be a valuable alternative to the established, mainly inorganic, electrode materials. In particular, since there is a lack of n-type radicals that are stable and can be used as anode material in organic batteries, the larger part of conventional redox polymers provide suitable anode materials for the construction of fully organic batteries.124,125 The following chapter covers the development of redox polymers used for battery application during the last years. 4.2.1. Carbonyl Compounds. At first, carbonyl compounds were studied only sporadically in the 1970s and 80s126−134 in the context of energy storage but fairly intensively during the last ten years. As it is a common organic functionality, chemists can choose from a variety of synthetic means for the design of new active materials. The suitable structures display oxidative ability and undergo reversible reductions. Extended conjugated carbonyl systems can be reduced under formation of multivalent anions. With dependence on the substituents, the charge stabilization is based on different mechanisms. Vicinal carbonyls employ stable enolates upon reduction, while aromatic carbonyl groups disperse the negative charge over the delocalized system. Quinoidic structures benefit from the formation of an aromatic 9450


Chemical Reviews

Review

Table 2. Overview of Carbonyl-Based Redox-Active Polymers Applied as Active Material in Secondary Batteries

9451


Chemical Reviews

Review

Table 2. continued

a

The symbols indicate the trend of the discharge voltage: sloping (s) and stable plateau with one (p) or two (pp) steps. bSignificant drop of discharge capacity during first cycles, then stable capacity. cIncrease of the discharge capacity during first cycles. dCurrent densities up to 6000 A m−2.

268 Ah kg−1 at 1.75C, of which 241 Ah kg−1 remains after 1500 charge/discharge cycles. Furthermore, the polymer shows a high rate capability up to 18.6C with an only minor decrease in capacity. Quinone units were also incorporated into the polymer backbone by copolymerization with formaldehyde, as shown for poly(2,5-dihydroxy-1,4-benzoquinone-3,6-methylene).144 By

condensing 2,5-dihydroxy-1,4-benzoquinone with formaldehyde, an electroactive polymer was obtained. Although the redox sites are located in the backbone of the polymer, they are not conjugated, ensuring distinct redox potentials. The electrochemical properties of a respective electrode were improved by admixing poly(3,4-ethylenedioxythiophene) (PEDOT).145 9452


Chemical Reviews

Review

Instead, a higher charging current causes a significant enlargement of the voltage gap between charge/discharge voltage. Anthraquinone-bearing polymers for battery application were also prepared by postpolymerization functionalization.125 The condensation of poly(4-chloromethylstyrene) with anthraquinone-2-carboxylic acid yields an electroactive polymer showing the potential to be an efficient electrode material with two wellseparated one-electron reductions at −0.79 and −1.19 V (vs AgCl/Ag) and fast electrode kinetics. Under aqueous, strongly basic conditions, only a single, reversible two-electron redox signal remains, attributed to the reduction of the anthraquinone groups to the corresponding dianion, stabilized by the basic electrolyte. Carbon/polymer composite electrodes revealed a high rate capability, tested up to 75C, and a very flat discharge curve at about 0.9 V (vs AgCl/Ag). Another material was synthesized by the polymer-analogous reaction of poly(methacryloyl chloride) with amine-functionalized pyrene-4,5,9,10-tetraone.152 A drawback of this approach is the incomplete functionalization, which obligates the quenching of the remaining acid chloride groups with methanol. The pyrene tetraone polymer undergoes two two-electron redox reactions between 3 and 2 V vs Li+/Li. The lithium-organic battery exhibited an initial capacity of 231 Ah kg−1 at the rate of 1C and, consequently, a two-stage discharge plateau at 2.9 and 2.2 V. With 83% of the initial capacity remaining after 500 cycles, the polymer reveals a good cycling stability. Additionally, the system was capable of cycling at higher rates of 30C with only minor capacity loss toward 207 Ah kg−1. Noteworthy is the utilization of a special ionic-liquid-like electrolyte consisting of equimolar amounts of triglyme and LiTFSI, wherefore all experiments were performed at 45 °C. A coordination polymer of quinone and lithium (Scheme 1) was prepared by dehydration of Li2(2,5-dihydroxy-1,4-benzo-

A similar formaldehyde-based polymer consists of 1,4,5,8tetrahydroxynaphthalene as redox-active unit and trioxane as a cross-linking agent.146 Composite electrodes were prepared with 87% SuperS carbon additive. The charge/discharge plateaus reveals a rather slopy shape and emerges at 3.5 V for charging and 2.5 V for discharging. At a charging speed of 0.04C, the initial capacity of 60 Ah kg−1 further decreases to 50 Ah kg−1 after seven cycles. Amino-naphtho- and anthraquinones are the basis for quinone-polymers with conjugated backbones. Namely, poly(5-amino-1,4-naphthoquinone) was prepared chemically and electrochemically.147 Although the polymer consists of a redoxactive poly(aniline) backbone, the redox chemistry of the quinone system is not influenced since the PANI redox reaction occurs at a higher potential. A battery with a LiMn2O4 counter electrode exhibited an average cell potential of 2.6 V and an initial capacity of 290 Ah kg−1 at a charging/discharging speed of 0.07C, which corresponds to the theoretical specific capacity. During 17 cycles, the capacity decreases to about 200 Ah kg−1 (30% loss). Poly(5-amino-1,4-dihydroxy-anthraquinone), polymerized by oxidative polymerization technique, was also utilized as cathode material in a lithium-organic battery.148 In theory, two waves of one-electron redox reactions lead to a theoretical capacity of 420 Ah kg−1. In the lithium-organic battery, 129 Ah kg−1 at 0.9C were reached, while the charge/discharge curves reveal a slopy shape between 2.0 and 3.5 V. According to the authors, the low capacity is attributed to the dissolution of the active polymer in the electrolyte. Quinones were also applied as pendant groups at polymer backbones. An innovative polymer/air battery employing poly(2-vinylanthraquinone) as anode and manganese dioxide as oxygen reduction catalyst as cathode was presented.149 The material was prepared by a free radical polymerization technique, which is remarkable since quinones are known to scavenge radicals and, therefore, inhibit radical polymerizations. In a basic aqueous KOH electrolyte (pH 12), which allows the swelling of the otherwise insoluble polymer, it possesses one reversible twoelectron redox reaction at −0.82 V vs AgCl/Ag. The electrode, consisting of a thin, drop-casted polymer layer on a current collector, reveals high rate performance up to 300C (charging in 12 s). The initial specific capacity of 214 Ah kg−1 deceases by only 7% over 500 charge/discharge cycles. In a more recent example of quinone/air batteries, a poly(norbornene) substituted with two anthraquinone pendant groups per repeating unit was applied together with a MnO2based oxygen reduction cathode and a 10 M NaOH aqueous electrolyte.150 The battery revealed a discharge voltage of 0.7 V and 99% Coulombic efficiency. The specific capacity of 205 Ah kg−1 remains almost stable over 300 cycles. Although the cathode works under open-air conditions, the anode half-cell of these systems must be kept oxygen-free to avoid unfavorable side reactions of O2 at the anthraquinone layer electrode. A polymer with quinoidic structure with condensed thiophene rings and polyvinyl backbone, capable of reversible reduction, was utilized in a Li-organic battery.151 The specific capacity of 219 Ah kg−1 was reached with a composite electrode with only 10% active material content, corresponding to a full material activity, but only 50% of the capacity remains after 100 cycles. Furthermore, the influence of active material content and charging speed was investigated in detail. While an increase of the ratio of active material leads to the expected effect on the specific capacity, it is mostly independent from the charging speed.

Scheme 1. Schematic Representation of a Quinone/LithiumIon Coordination Polymer Used for Lithium-Ion Batteries.153

quinone) × 2H2O.153 The Li−O-bridged two-dimensional layered electrode material permits the utilization as a lithium insertion electrode. At about 1.7 V, the reduction of the carbonyl groups and insertion of lithium ions is observed. The Li-bridged structure stays intact upon charging and discharging; however, the polymer exhibits a rather flat discharge curve that reveals slight steps due to structural changes within the material. A rearrangement of the coordination structure during the cycling process causes a gradual capacity loss. Hence, the cycling stability is the main drawback for this type of coordination polymer electrodes. 4.2.1.2. Polyimides. The research on polyimides represents a relatively young field in the development of redox-active polymers for electrochemical energy storage. In general, polyimides are prepared via a polycondensation reaction of dianhydrides with diamines. The amide functionalities undergo two one-electron reduction steps, one fully reversible reduction and a second one, which leads to the decomposition of the redoxactive structure. Thus, only the first redox stage can be utilized in a battery application. 9453


Chemical Reviews

Review

applied in a lithium-organic battery.160 Despite its high molar mass, the good material activity leads to a capacity of 115 Ah kg−1, of which 85 Ah kg−1 remains after 300 cycles. The perylenebased polycondensates with ethylenediamine, hydrazine, and urea were investigated in a lithium-organic system.161 Notably, the urea-linked derivative exhibits a capacity of 80 Ah kg−1 in the first cycle, which continuously increases to about 130 Ah kg−1 within 50 cycles. The polymer obtained from perylene-3,4,9,10tetracarboxylic dianhydride and ethylenediamine was further investigated to optimize the material activity and, thus, the capacity in a sodium-organic battery.162 Here, a specific capacity of 126 Ah kg−1 is achieved, of which 88% remains after 5000 cycles (at 0.8C), revealing an excellent cycling stability. The condensation polymerization of tris-functionalized amines with aromatic dianhydrides leads to highly cross-linked polyimide networks. Here, the structural design significantly influences the electrochemical behavior. In a recent study, a polyimide of 1,3,5-tris(4-aminophenyl)-benzene with different anhydrides was prepared and utilized in Li-ion batteries.163 Among the studied polymers, the network synthesized with 1,4,5,8-naphthalenetetracarboxylic dianhydride exhibits the highest initial capacity with 103 Ah kg−1 but with a capacity loss of over 30% during 30 cycles. The polyimide obtained with perylene-3,4,9,10-tetracarboxylic dianhydride, in contrast, revealed a lower initial capacity of 78 Ah kg−1 but displayed much higher stability upon cycling. An alternative polyamide structure, beside the polycondensates, is based on tetraketopiperazines.164 Several nonpolymeric materials were studied revealing significantly fading capacities due to the dissolution of the small molecules. To overcome this problem, oligomers of N,N′-diallyl-2,3,5,6-tetraketopiperazine were prepared applying acyclic diene metathesis. A lithiumorganic battery with this material applied as cathode exhibited an initial capacity of 165 Ah kg−1 with a loss of 39% during ten cycles at 0.25C. Interestingly, the presence of nitrogen atoms in the structure of this molecule does not effect a significant influence on the redox potential compared to C6O6-type oxocarbon-based materials. Polyimides represent a promising class of redox-active electrode materials for energy storage applications. The characteristics are mainly determined by the aromatic unit. The π−π stacking of perylene units, for instance, induces a semiconducting behavior, lowering the amount of necessary conductive additive. Nevertheless, the electrochemical performance still strongly depends on the interaction with the latter. The best results in terms of capacity and lifetime could be obtained by in situ polycondensation in the presence of the carbon additive. Hence, a composite electrode with only 3% carbon nanotubes could be achieved, instead of the typical 60 to 80%.160 However, a significant drawback of polyimide systems is the lack of protection against deep discharging (in a cathode application) or overcharging (anode application) as the second reduction step of the diimides is irreversible and leads to the decomposition of the active material. 4.2.1.3. Carbonyl Derivatives. Functionalized hydroquinones, namely 2,5-di-tert-butyl-1,4-dialkoxybenzene, a wellknown redox shuttle, were tethered to a polymer and used in lithium-ion batteries.165 These alkoxybenzenes reveal a different redox mechanism than the discussed quinones and undergo a one-electron oxidation. They contain bulky tert-butyl groups, which are mandatory for the stabilization of the formed radical cation. By copolymerization of methacrylic acid 3-(2,5-di-tertbutyl-4-methoxy-phenoxy)-propyl ester and the cross-linker

In 2010, two groups simultaneously began the investigation of polyimide systems for energy storage applications. Nishide et al. investigated composite materials containing polyimides, obtained by the polycondensation of 1,4-phenylenediamine with pyromellitic dianhydride or 4,4′-oxidiphthalic anhydride.154 The materials were examined in half-cell tests conducted at low Crates. The initial specific capacities of 95 and 78 Ah kg−1 for the pyromellitimido-based and oxo-bridged compounds, respectively, decreased by one-third after 10 cycles. In addition, a cell of the oxo-bridged compound with a PTMA counter electrode was prepared, but only cyclovoltammetric measurements were presented. At the same time, Song et al. worked on polyimides and published further electrochemical studies regarding the performance in lithium-organic batteries.155 The condensation products of the different combinations of pyromellitic dianhydride and 1,4,5,8-naphthalenetetracarboxylic dianhydride with 1,4-phenylenediamine and 1,2-ethanediamine were utilized as active electrode materials and revealed output voltages in the range from 2.0 to 2.5 V vs Li+/Li. The performance at different charge/ discharge rates up to 0.5C was investigated. Specific capacities of around 200 Ah kg−1 were achieved with Coulombic efficiencies close to 100% and a moderate loss after 100 cycles. In general, the naphthalene diimides reveal higher potentials due to their larger delocalized system, which leads to an easier reducibility. The same research group was able to increase the rate capability by utilizing nanocomposites synthesized through polycondensation of 1,4,5,8-naphthalenetetracarboxylic dianhydride and ethylenediamine in situ in the presence of functionalized graphene sheets.140 At a charging rate of 10C, a capacity of 135 Ah kg−1 was obtained. A significant drawback of this material is a large voltage gap of 0.9 V between the charge and discharge plateaus. A polyimide obtained from pyromellitic dianhydride and ethylenediamine was synthesized in situ onto a three-dimensional graphene network.156 In combination with this conductive additive, a specific capacity of 123 Ah kg−1 at only 0.5C is obtained, which decreases to 101 Ah kg−1 after 150 cycles. This performance improves when the same polymer is synthesized in the presence of single-walled carbon nanotubes.157 At a rate of 20C, the specific capacity still amounts to 158 Ah kg−1. More material is active at lower rates (0.5C), where 206 Ah kg−1 (175 Ah kg−1 after 200 cycles) and a slightly higher cell voltage is achieved. The polyimide from the condensation reaction of 1,4,5,8naphthalenetetracarboxylic dianhydride and ethylenediamine was furthermore applied in a sodium-organic battery.158 The specific capacity of 140 Ah kg−1 at 1C decreases by only 10% during 500 cycles. The cell potential amounts to 2.0 V. Upon charging at a rate of 30C, a capacity of 80 Ah kg−1 was reached. The polymer electrode was also coupled with a sodiumintercalation electrode and utilized as anode material. A polymer of 1,4,5,8-naphthalenetetracarboxylic dianhydride and hydrazine was applied as anode material in aqueous rechargeable lithiumand sodium-ion batteries.159 The polymer composite was used as anode versus LiCoO2 and NaVPO4F cathodes. The Li-ion battery exhibits an initial capacity of 70 Ah kg−1 at 2C (56 Ah kg−1 in the 200th cycle) and a cell potential of 1.17 V. The Na-ion battery displays poorer energy-storage properties due to the limiting performance of the NaVPO4F cathode. A perylene-based polyimide synthesized from perylene3,4,9,10-tetracarboxylic dianhydride and ethylenediamine was polymerized in situ in the presence of carbon nanotubes and 9454


Chemical Reviews

Review

disulfides and thioethers. An overview over the discussed active electrode materials and their properties is provided in Table 3. 4.2.2.1. Disulfides. Disulfide polymers employ a reversible two-electron oxidation of the disulfide bond to the corresponding thiolate anion, which is used for electrochemical charge storage. A general problem of the early disulfide-based polymers was the poor cycling stability, as in particular the interpolymer disulfide bonds suffered from poor rebonding efficiency throughout the charge/discharge process. For this reason, current research is focused on molecular structure enabling better rebonding for an improved cycling stability. In addition, disulfide polymers suffer from a slow reaction rate at room temperature. This effect is attributed to the redox mechanism, which includes breaking and formation of the disulfide bonds at charging and discharging, instead of a fast change in the state of charge. Intramolecular self-catalysis, which is affected by a conjugated, conductive, and electroactive polymeric backbone, significantly benefits the performance of a disulfide polymer as intensively studied in the case of 2,5dimercapto-1,3,4-thiadiazole.174−176 For this reason, all disulfide polymers feature a conjugated polymer backbone. Nevertheless, they are discussed in this section of the review because the disulfide functionality predominantly determines the properties of this material. Poly(2,5-dimercapto-1,3,4-thiadiazole) is an electrode material with a high theoretical capacity of 362 Ah kg−1 but suffers from a low redox reaction rate constant and, hence, a low power density. Increased energy densities could be observed in combination with the electrocatalyst poly(pyrrole).177 The poly(pyrrole), produced by potentiostatic polymerization, additionally acts as a protective layer, limiting the dissolution of the reduced species in the electrolyte solution, which otherwise represents a serious drawback since the polymer backbone disintegrates upon doping (and reverse upon dedoping). Thus, a protective poly(pyrrole) layer was deposited on a layer of the disulfide polymer, fabricating a bilayered nanoelectrode with a capacity of 320 Ah kg−1. An adduct of poly(2,5-dimercapto-1,3,4thiadiazole) and poly(aniline), suggested by Wang et al., was intercalated into graphite oxide by hydrogen bonding between the amino and thio groups of the polymers and the carboxyl groups of the graphite oxide.178 PANI and PDMcT form an electron donor−acceptor adduct, improving the electrochemical properties due to the increased conductivity. An approach to overcome the poor intrinsic conductivity (0.84 S cm−1) of poly(2,5-dimercapto-1,3,4-thiadiazole) was described by Jin et al. The authors mixed the polymer with sulfonated graphene and applied the composite in a lithium/polymer battery, which exhibited a rather flat discharge curve.179 Although the initial discharging capacity of 268 Ah kg−1 was very promising, it rapidly decreased to 124 Ah kg−1 after only 10 cycles. Similar problems were described by Kiya et al., who used a poly(2,5-dimercapto-1,3,4-thiadiazole)-PEDOT composite.180 Electrostatic interactions between the polymer and PEDOT:PSS may help to reduce diffusion of the charged species into the electrolyte, improving the performance of the battery. In this context, Abruña et al. investigated the electrocatalytic activity of several conducting polymers on a 2,5-dimercapto1,3,5-thiadiazole model compound.174−176 In particular, poly(3,4-ethylenedioxyselenophene)-film-modified electrodes proved to have high heterogeneous charge-transfer rate constants. The work showed a good overlap of the conductivity window with the redox potential of the model compound. It emerged that interactions of the heteroatoms of the conducting

ethylene glycol dimethacrylate, an insoluble redox polymer network (4.0 V vs Li+/Li) with reasonable cycling stability and an initial discharge capacity of 52 Ah kg−1 at 1C was obtained. But, since the oxidation potential is close to the potential that cleaves of the linkage to the backbone and induces charge shuttling,166 overcharging represents a critical issue. Interesting alternative structures were prepared by substitution of the carbonyl function of poly(2-vinylanthraquinone). In 9,10-di(1,3-dithiol-2-ylidene)-9,10-dihydroanthracene (exTTF), a π-extended tetrathiafulvalene system replaces the quinoidic substructure and induces the ability of a two-electron oxidation.167 A vinyl-functionalized polymer was utilized in a Li-organic battery, exhibiting a specific capacity of 108 Ah kg−1. After 20 cycles, the capacity drops by 24% and remains stable for 230 further cycles. Poly(2-vinyl-11,11,12,12-tetracyano-9,10-anthraquinonedimethane) (poly(TCAQ)) is likewise able to store two electrons per repeating unit. 168 The dicyanomethane groups are introduced at the carbonyl sites by a Knoevenagel reaction to the 2-vinylanthraquinone monomer, which is afterward polymerized utilizing radical polymerization technique. Interestingly, the polymer features one two-electron redox reaction, which is ascribed to a conformational change during the first reduction step. Thus, the second reduction wave is more favorable and occurs at the same potential. In a battery with a lithium metal anode, the material exhibits a discharging voltage of 2.25 V (charging at 3.05 V) and a specific capacity of 156 Ah kg−1 in the first cycle at 1C. An excellent cycling stability leads to a capacity loss of only 12% after 500 charge/discharge cycles at a consistently high Coulombic efficiency of 99%. In a similar approach, a modified anthraquinone with an Ncyanoimide replacing one keto group was utilized.169 The functionalization yields an isomeric mixture, which could be used without isolation, due to the similar electrochemical properties of the isomers, involving two-electron storage. The polymerization was conducted by a free radical polymerization of an attached vinyl group. Lithium-organic cells reveal a discharge voltage of 2.28 V and an initial discharge capacity of 130 Ah kg−1. High Coulombic efficiencies and an acceptable capacity loss of 15% after 100 cycles at 5C are achieved. Even though the polymeric approach is clearly the main object in organic battery research and exclusive content of this review, “small molecules”, for example, carboxylic acids, used for Li- and Na-ion batteries are mentioned as well since several studies have been conducted on certain structures with sufficient insolubility.135 4.2.2. Organosulfur Compounds. Sulfur, one of the most abundant elements on earth, is produced as a mere waste product on a million-tons scale from fossil resources. Years of research on its redox chemistry, its simple accessibility, and environmental advantages in comparison to heavy metals established the basis for sulfur-containing compounds to become candidates for modern battery electrodes. Numerous sulfur-based battery systems were developed over the last decades, with sodium/ sulfur batteries leading the way.170 Furthermore, organic electrode materials based on organosulfur compounds revealed to be versatile systems as briefly summarized in recent reviews.22,171 In general, the poor electrode stability of soluble “small molecule”-based materials, such as 2,5-dimercapto-[3,4thiadiazole], is the major disadvantage.172 For this reason, research was expanded to polymer-bound organosulfur compounds.173 The substance classes applied as active material cover 9455


Chemical Reviews

Review

Table 3. Overview of Organosulfur-Based Redox-Active Polymers Applied as Active Material in Secondary Batteries

a The symbols indicate the trend of the discharge voltage: sloping (s) and stable plateau with one (p) or two (pp) steps. bSignificant drop of discharge capacity during first cycles, then stable capacity. cIncrease of the discharge capacity during first cycles. dIncrease of the discharge capacity at later cycles after an initial decrease. eVoltage decrease upon repeated charging/discharging.

9456


Chemical Reviews

Review

comparatively low with respect to other disulfide polymers, it represents a promising material for battery systems due to its excellent cycling stability and rate capability. At a current rate of 10C (25 A m−2), still 80% of the initial discharge capacity are achieved. Due to an increase of the electronic conductivity of the polymer film upon cycling, the capacity increases slightly during the first ten cycles before it stabilizes at about 110 Ah kg−1. Notably, the disulfide bridges appear to be not electroactive within the used potential window from 3.2 to 4.4 V (vs Li+/ Li).192 Since the disulfide bond stays intact upon charging/ discharging of the electrode, a good cycling stability is obtained, compared to conventional disulfide-based polymers, which show a high capacity loss of up to 70% within only a few charge/ discharge cycles. Disulfide-based polymers represent a group of active materials revealing high specific capacities but often also inferior cycling stabilities. In general, polymers that contain disulfide groups as substituents, not as part of the backbone, are favorable, as the polymer backbone stays intact upon charging and, therefore, they remain less soluble. 4.2.2.2. Thioethers. Zhang et al. introduced thioethers as a novel class of electroactive compounds into the field of battery development. Poly(2-phenyl-1,3-dithiolane) and its doubly functionalized counterpart poly[1,4-di(1,3-dithiolan-2-yl)benzene] are the first examples proposed to store electric energy by oxidation of a thioether to a thioether dication (Scheme 2).193

polymer with the model compound are important for efficient electrocatalysis and, hence, battery performance. Recently, a poly(thiocyanogen)/lithium battery, which stores electric energy by reversible oxidation of the polymers S−Sbonds, was introduced.181 Prepared from the inexpensive NH4SCN by constant-current electrolysis and a subsequent autocatalytic polymerization process, the polymer demonstrates a fairly low and strongly sloping discharge voltage between 1.6 and 0 V and capacities in the range from 200 to 275 Ah kg−1 at 0.2C in a Li-organic battery setup. A further approach deals with poly(paraphenylene)s, namely poly(5,8-dihydro-1H,4H-2,3,6,7-tetrathiaanthracene), representing a promising class of disulfide polymers. Since this material exhibits two disulfide functionalities per repeating unit, it possesses a high theoretical capacity of 471 Ah kg−1.182 Under practical conditions, 422 Ah kg−1 are reached, rapidly declining by 60% after 40 cycles. Poly[bis(2-aminophenyloxy)disulfide] almost reaches its theoretical charge-storage capacity of 261 Ah kg−1 based on a formal three-electron redox reaction, with 0.8 electrons assigned to the backbone and two to the disulfide group.183 However, the capacity decreases by 70% within only seven charge/discharge cycles, which is, according to the authors, attributed to either impurities or a low polymerization degree. A copolymer of 5-amino-1,4-dihydrobenzo[d]-1′,2′-dithiadiene and aniline revealed the hybrid properties of a redox and a conductive polymer. Although the discharge curve was strongly sloping, a small plateau, attributed to the reversible oxidation/ reduction of the disulfide bonds, was observed.184 The conductive backbone is considered to function as an electrocatalyst, improving the redox reaction rate. Nevertheless, the copolymer proved to possess inferior conductivity with respect to the poly(aniline) homopolymer since the extent of the conjugated system of the backbone is limited. On the other hand, the capacity of the material was improved by the addition of electroactive disulfides, an extra “energy storage”. As the authors only provided the data of one charge/discharge cycle, no conclusion on the cycling stability of the copolymer can be drawn. The polymer poly(anthra[1′,9′,8′-b,c,d,e][4′,10′,5′b′,c′,d′,e′]bis-[1,6,6a(6a-SIV)-trithia]pentalene), prepared by oxidative polymerization with ferric chloride, was studied rather extensively185−188 but showed only poor cycling stability in organic-carbonate-containing electrolytes.189 However, the lifetime was greatly improved by employing an inert, ether-based dioxolane/dimethoxyethane electrolyte solvent, which leads to a decrease of the redox potential. Further studies suggest 12,12diethyl-2,5,8-trioxa-12-silatetradecane as a suitable electrolyte since it shows a high boiling and flash point as well as an extended stable potential range.185 A peculiar redox oligomer was prepared by the reaction of sodium acetylide, liquid ammonia, and sulfur, followed by a spontaneous oligomerization.190,191 The formed polyene oligosulfide is oxidized by air to yield electroactive disulfides. Furthermore, upon treatment with hot benzene, the formation of oligo(thienothiophene) is observed. The material revealed a high initial discharge capacity of 400 to 750 Ah kg−1, depending on its sulfur content and the preparation method, which rapidly drops by about 70% due to partial dissolution of the polysulfides in the electrode. Exhibiting a surprisingly robust redox system, poly(tetrathionaphthalene) shows virtually no capacity decrease within 180 charge/discharge cycles.173 Although its capacity is

Scheme 2. Schematic Representation of the Proposed Mechanism of the Charge/Discharge Process of Thioether Compounds

The battery performance of systems based on thioether polymers is superior with respect to disulfide polymers because the charge/ discharge process is not accompanied by bond cleavage. Therefore, thioether-based polymers may emerge as a new class of stable materials. That the charge storage in these kinds of polymers is not only attributed to the conjugated backbone is demonstrated by studies on the aliphatic polymers poly[methanetetryl-tetra(thiomethylene)] and poly(2,4-dithiopentanylene).194 The first polymer reveals a very high initial discharge capacity of 504 Ah kg−1, but it rapidly declines to 200 Ah kg−1. In contrast, the latter possesses a lower but stable capacity of about 100 Ah kg−1 over 50 cycles. A general problem of these materials is their rather lowvoltage efficiency. In addition, a decrease of the discharge voltage over cycling was observed. Both disadvantages were attributed to the lack of intrinsic conductivity of the saturated polymer. With a practical capacity of about 300 Ah kg−1 and a discharge voltage between 2.7 and 2.0 V (vs Li+/Li), the insoluble poly(ethene-1,1,2,2-tetrathiol), made from tetrachloroethylene and sodium sulfide, is a more encouraging example.195 PEDOT functionalized with 5,5-bis(methylthio)-2,2-bithiophene was presented, which features two reversible one-electron redox reactions at potentials of 3.8 and 4.0 V (vs Li+/Li) and is supposed to be used in a redox capacitor setup.196 Besides PEDOT and the previously mentioned poly(2-phenyl-1,3dithiolane), other conductive polymers functionalized with electroactive thioether functionalities were investigated. Poly(tetrahydrobenzodithiophene) shows an increased electric 9457


Chemical Reviews

Review

similar capacity of 117 Ah kg−1 after 50 cycles was achieved. The cell featured good cycling stability but a remarkably poor Coulombic efficiency in the first cycle, which was explained by an irreversible side reaction, namely the coupling of reactive adjacent carbazole radical cations upon oxidation.206−208 Furthermore, the authors utilized the polymer as redox-active binder in composites with LiFePO4.205 Higher current densities were reached with PEDOT:PSS as conductive additive.209 Charge/discharge rates up to 200C were applied for composite electrodes with carbon black and 1 wt % PEDOT:PSS additive, which additionally replaced the conventional PVdF binder. Nevertheless, despite the good cycling stability, the cell potential, which is comparable to conventional positive electrode materials like LiFePO4, and the wide commercial availability, carbazoles did not establish as active cathode material, essentially because of the rigorous slope in discharge potential. 4.2.3.2. Triphenylamine. The first application of poly(triphenylamine) as active electrode material was presented by Yang et al.210 A highly cross-linked poly(triphenylamine)backbone polymer, which facilitates conjugation upon oxidation and thus stabilizes the p-doped redox centers, was used. The polymer was prepared by oxidative polymerization with ferric chloride, yielding particles with a porous structure, which permits effective penetration of the electrolyte and ion transfer. The good electrolyte penetration and the excellent electronic conductivity of about 1.0 S cm−1 in the doped state enable high-rate charging/ discharging processes. At an average discharge voltage of 3.8 V and a capacity of 91 Ah kg−1, the lithium/poly(triphenylamine) battery loses only 8% of its original capacity after 1000 cycles at 20C. However, the self-discharge of 10% within 2 weeks represents a worrying issue. Slight capacity enhancements were achieved by the utilization of poly[tris(thienylphenyl)amine] derivatives, which form a polymeric structure that is analogous to poly(triphenylamine).211 Polymers with one bridging thiophene unit per phenyl ring feature a discharge cell potential in the range from 4.2 to 3.5 V vs Li+/Li, which is comparable to poly(triphenylamine). However, both triphenylamine and thiophene moieties undergo a reversible redox reaction and contribute to the capacity, with the thiophene unit being oxidized at slightly higher potential. The triphenylamine and thiophene groups store 94 and 35 Ah kg−1, respectively, determined by two separated discharge plateaus with different slopes; 91% of the initial capacity is retained after 50 cycles. Recently, a p-dopable poly(4-cyano triphenylamine) was utilized as electrode material in an organic battery.212 The introduction of an electron-withdrawing cyano group slightly increases the redox potential of the oxidation. The average discharge cell potential amounts 3.9 V versus a lithium anode. At a current density of 40 A kg−1, a specific capacity of 80 Ah kg−1 is reached, of which 74% remains upon increasing the charge/ discharge rate 10-fold, accompanied by an only minor capacity loss during continuous cycling. Besides their use as electrode material, poly(triphenylamine)s proved to be a promising class of reversible, self-actuating overcharge protection materials for Liion batteries.213,214 4.2.3.3. Viologen. In contrast to other redox polymers, the ground state of viologen materials features a double-positively charged cation. Upon charging, it is reduced within two oneelectron redox reactions to form a radical cation and, finally, an uncharged species. In general, the anionic reduced states of redox polymers are nucleophiles and (in an aqueous milieu) strong

conductivity compared to poly(2-phenyl-1,3-dithiolane) because its thioether bonds are coplanar with respect to the poly(phenylene) backbone.197 The average discharge capacity is given as 560 Ah kg−1. Nevertheless, a strong variation of the capacity, including an initial increase and subsequent decrease, was observed; this is a common feature of other thioether polymers, too. If the sulfur atom is directly linked to a poly(phenylene) backbone, as in poly(1,2,4,5-tetrakis(propylthio)benzene), rather well-defined charge/discharge plateaus are obtained at about 2.4 V (vs Li+/Li).198 Yet, the cycling performance of the material needs to be improved since a 50% capacity loss occurs within only 20 cycles. Recently, a thiathrene-functionalized norbornene was reported.199 Thianthrenes are known to undergo two distinct oxidation steps toward a dication and were suggested as overcharge protection materials for Li-ion batteries.200 The first redox reaction of the thianthrene was recently utilized in a Li-organic battery with a discharging plateau at a voltage of 4.0 V vs Li+/Li. However, the discharge capacity, which rises to 63 Ah kg−1 during the first 14 charge/discharge cycles, subsequently decreases rapidly to about 20 Ah kg−1 in the 100th cycle. In comparison, the mentioned exTTF-containing anthracene derivative (section 4.2.1.3) reveals longer life times due to the stabilization of the formed positive charge by the enlarged delocalized structure. In summary, thioether-based polymers represent a new and promising class of electroactive materials revealing a competitive charge-storage capacity. A drawback, yet to overcome, is their strong capacity and voltage fading upon repeated cycling. 4.2.3. Other Redox-Active Units. Besides carbonyl- and sulfur-based redox-active materials, several further nonconjugated polymers bearing functionalities like carbazole, triphenylamine, viologen, and organometallic ferrocene-based compounds were investigated in the context of charge storage. An overview over the discussed active electrode materials and their properties is provided in Table 4. 4.2.3.1. Carbazole. Carbazoles are utilized for electrochemical energy storage mostly in the form of conjugated poly(carbazole)s201 or as carbazole units attached to an aliphatic backbone, like in poly(N-vinyl carbazole), which was examined and published by Shirota et al.202 Despite the aliphatic backbone, limited intrinsic conductivity is observed in these polymers due to electronic pathways formed by the π-stacked carbazole moieties.203 Poly(N-vinyl carbazole) was utilized in composites with single- and multiwalled carbon nanotubes in batteries versus metal lithium foil,204 for which it was polymerized electrochemically on carbon nanotube films. In rechargeable lithium batteries, the electrodes reveal specific discharge capacities of 45 and 115 Ah kg−1 (compared to the theoretical value of 139 Ah kg−1) during the 20th cycle for the composites with single- and multiwalled carbon nanotubes, respectively. Besides the superior capacity, multiwalled nanotubes also facilitate higher cycling stabilities, even though only at slow charging rates (10 A kg−1). However, the major drawback of this battery is the significantly sloping discharge voltage from 3.5 to 1.0 V, comparable to conjugated polymers, caused by the electronic interaction of the stacked carbazole units. Further investigations on poly(N-vinyl carbazole) in lithiumorganic batteries were conducted with classical composite electrodes containing 50 wt % active polymer.205 The battery was tested, allowing a higher maximum cell voltage, and revealed a still sloping but more applicable discharge plateau between 4.6 and 3.4 V vs Li+/Li. Despite the differing applied voltage range, a 9458


Chemical Reviews

Review

Table 4. Overview of Other Redox-Active Polymers Applied as Active Material in Secondary Batteries

a The symbols indicate the trend of the discharge voltage: sloping (s) and stable plateau with one (p) or two (pp) steps. bIncrease of the discharge capacity during first cycles.

capable of high-rate charging and discharging. The specific capacity of 165 Ah kg−1 at 60C decreases merely to 110 Ah kg−1 at a rate of very high 1200C. At lower rates, 80% of the capacity remains after 2000 cycles, making this a promising all-organic system. Viologen was also bound to a poly(pyrrole) backbone via an alkyl linker at the pyrrole N.216 The all-organic test cell consisted of an electrodeposited polymer film as anode and PPy doped (electrostatically bound) with 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (PPy[ABTS]) as cathode. The discharge voltage is 1.0 V and 70% of the initial capacity of 16 Ah kg−1 (theoretical 17 Ah kg−1 for both electrodes combined) is retained after 100 cycles. The authors furthermore examined viologen that was electrostatically bound to poly(pyrrole), which revealed inferior cycle life because of the dissolution of active units and, due to a low viologen-to-pyrrole ratio (1:10), much lower capacities. 4.2.3.4. Ferrocene. An alternative to the discussed organic macromolecules are polymers with organometallic groups like

bases and are, therefore, likely to undergo side reactions leading to a capacity loss. The reduced viologen, however, overcomes this issue. The first application of a viologen-based polymer in a battery was as anode material in an all-organic battery with a TEMPOradical-based cathode.124 It features an output voltage of 1.2 V and a Coulombic efficiency of 95%. In addition, it reveals high cycling stability, with 80% of the initial capacity retained after 2000 cycles. But, since the TEMPO-based electrode was in the focus of this study and further values are related to this half-cell, no detailed information about the viologen-based electrode was provided. The n-type material viologen is furthermore utilized as anode active material in all-organic batteries (e.g., in a poly(tripyridiniomesitylene) vs poly(TEMPO acrylamide) battery with water-based electrolyte).215 The two redox steps of viologen enable the high capacity of the cross-linked poly(viologen) film but lead to two distinct charge/discharge plateaus at 1.5 and 1.0 V. Due to the applied thin active polymer film, the battery is 9459


Chemical Reviews

Review

electrode material was presented by Nakahara et al. in 2002.25 TEMPO became the gold standard in organic battery material research as it reveals excellent electrochemical properties (i.e., favorable stability; the bare TEMPO radical survives for more than a year in an aprotic solution containing 1 M LiPF6)234 and fast redox kinetics. Notably, TEMPO radical polymers are known to feature solidstate electrical conductivity alongside the polymer chains, depending on the redox state of the radical moieties.235 The electron transfer processes in polymers that bear TEMPO units and in composites thereof were investigated in several studies.236,237 As for most electron transfers, the mechanism includes two steps.238 The first is the heterogeneous electron transfer from the current collector to the radical component, and the second step is the homogeneous charge transfer among the radical moieties. The standard rate constant for the first step was investigated using TEMPO-containing structures deposited on a platinum surface and was determined to be ca. 10−1 cm s−1,234 which is comparable to the redox reaction of transition metal ions.224,239 The homogeneous step is driven by concentration gradients leading to a self-exchange between neighboring radicals with a bimolecular rate constant of 1.8 × 105 M−1 s−1.237 The rapid electron transfer is facilitated by only negligible changes of the conformation of the molecule and enables a Nernstian adsorbate-like behavior in polymer layers from 10 to 100 nm thickness. Hence, thicker polymer layers and a higher content of active material in the composite are enabled. The fast kinetics, furthermore, result in a small gap between charge and discharge voltages. The most common polymer that bears TEMPO units is poly(2,2,6,6-tetramethyl-4-piperinidyl-N-oxyl methacrylate) (PTMA).25,223,239−241 Radical polymerization techniques are not suitable to polymerize monomers that contain stable radicals. Thus, the prevailed synthesis method is the free radical polymerization of the nonradical precursor monomer 2,2,6,6tetramethylpiperidine methacrylate, followed by chemical oxidation of the piperidine units, yielding the radicals. The polymer-analogous oxidation is critical as it should proceed quantitatively. An alternative polymerization technique is the group-transfer polymerization (GTP) of the TEMPO methacrylate.241 Usually, PTMA is utilized as composite cathode in Liorganic batteries and leads to a discharge cell voltage of 3.5 V with a small gap of only 100 mV between charging and discharging.242,243 Specific capacities of 110 Ah kg−1 at 1C nearly conform to the theoretical capacity of 111 Ah kg−1, which indicates an excellent material activity. PTMA also reveals a good cycling stability; after several hundred cycles, the material activity is still close to 100%. The self-discharge of PTMA systems is, as for organic batteries in general, only rarely investigated. One case is reported with a capacity loss of 38% after 1 week, which is, however, almost completely recovered after recharging.242 In this case, the self-discharge was attributed to the dissolution of radical polymers into the electrolyte, acting as charge shuttles; this effect was successfully suppressed by cross-linking the polymer chains.241 An example for a successful application of PTMA in a practical ORB is based on a combination of a PTMA/carbon cathode (50 wt % PTMA and 45 wt % VGCF, 5 wt % binder, cf. section 5) and a graphite anode that is fabricated as an Allaminated pouch cell.227 Here, the fraction of the active polymer amounts to 6.2% of the total battery weight. Another common polymer backbone with TEMPO units is poly(norbornene).233,237,244,245 The respective norbornene monomers, which contain two TEMPO moieties per unit, are

ferrocene, which is known for its excellent electrochemical properties in terms of kinetic and reversibility. Tamura et al. investigated polymers that are synthesized from the monomers vinylferrocene, ethynylferrocene, and 1,1′-dibromoferrocene as cathode active material against lithium.217 The most promising one in terms of capacity and stability was poly(vinylferrocene), which revealed a specific capacity of 105 Ah kg−1 and only 5% loss after 300 cycles. A power density of up to 4.1 kW kg−1 was obtained. The small gap between charge and discharge cell voltage indicates the rapid redox kinetic of this material. Besides the ferrocene-substituted poly(pyrrole) and poly(aniline) (section 4.1), ferrocene-substituted poly(fluorenylethynylene) in an alternating copolymer with thiophene was tested as metalorganic cathode material and revealed rather low intrinsic conductivity.218 The authors investigated the dependence of the specific capacity on the charge/discharge rate of Li-organic batteries assembled with this polymer. At slower rates (1 to 5C), capacities of 52 Ah kg−1 were reached at average discharge cell voltages from 3.4 to 3.5 V vs Li+/Li. At higher rates (10C), the capacity and output voltage dropped significantly. In all cases, the reversibility of the ferrocene’s redox reaction leads to high Coulombic efficiencies and a low capacity decay within 100 cycles. Analogously, an alternating copolymer with triphenylamine was analyzed. In comparison to the polymer with thiophene groups incorporated in the backbone, a higher specific capacity accompanied by a faster decrease upon cycling was observed. 4.3. Organic Radical Polymers

A type of organic, polymer-based batteries that gained special interest during the past decade is the “ORB”, the organic radical battery.13,25,219−229 These batteries contain active materials that consist of polymers with pendant stable organic radicals, which possess an unpaired electron in the uncharged state. The first battery that utilized this concept was published by Nakahara et al. in 2002.25 ORBs represent a very prominent subclass in the field of polymer batteries due to their superior redox chemistry, in particular their favorable kinetics. Redox reactions involving the singly occupied molecular orbital enable fast electron transfer and, thus, high rate performance. The accompanying stability problems of such systems (i.e., bond formation of the unpaired electrons), for example, by dimerization, can be overcome through the design of the molecule. In particular, a combination of resonance effects and steric hindrance is used to induce the necessary stability of the radicals. Nitroxyl, phenoxyl, and hydrazyl groups are commonly used examples for robust and stable radicals. In particular nitroxylradical-bearing structures, like 2,2,6,6-tetramethylpiperidinyl-Noxyl (TEMPO), are often applied.219,223,225 The electrochemical cells are constructed as Li-ion batteries16,29,31,230,231 or metalfree, all-organic batteries.124,232,233 A detailed review about radical polymer active materials was published recently.18 Therefore, this chapter comprises only, besides some milestones of the ORB research, the latest studies and significant breakthroughs. An overview over the described active materials and their properties is provided in Table 5. 4.3.1. 2,2,6,6-Tetramethylpiperidinyl-N-oxyl (TEMPO). With the new century, a new class of polymers, bearing stable, organic nitroxyl radicals, was established for electrochemical energy storage, which led to the development of the ORB. In particular, the widely known TEMPO free radical attracted attention and the first example of a TEMPO-based active 9460


Chemical Reviews

Review

Table 5. Overview of Redox-Active Radical Polymers Applied as Active Material in Secondary Batteries

a The symbols indicate the trend of the discharge voltage: sloping (s) and stable (p) plateau. bCapacitance of the conductive additive significantly contributed to the specific capacity. cSpecific capacity based on the total weight of active polymer on both electrodes.

layer was first spin-coated on an ITO surface and subsequently photochemically cross-linked. The films revealed similar capacity even at rates up to 50C, suppressed dissolution of active material into the electrolyte, and a high mechanical stability. A polymeric environment that facilitates the electrochemical processes in particular in protic electrolytes is represented by PTVE [poly(2,2,6,6-tetramethylpiperidinyl-N-oxyl vinyl ether)]. The hydrophilic polymer features high affinities to water-based electrolytes, which are surpassing with regard to price and sustainability but also possess high ionic conductivity. On the basis of this principle, a rechargeable battery with a PTVE layer on a glassy carbon substrate as cathode and a zinc anode was developed.247 The polymer undergoes a reversible redox reaction

polymerized by ring-opening metathesis polymerization (ROMP) using Grubbs’ catalysts. What is noteworthy is that the electrochemical properties of these polymers depend on the endo/exo configuration of the two associated TEMPO moieties.246 The capacity of the polymerized bifunctionalized norbornene with endo/endo configuration amounts to only 54 Ah kg−1, whereas the endo/exo isomer reveals 109 Ah kg−1 (full material activity) in a Li-ion coin cell. This effect was explained by the short distance between the radical substituents of only 10 Å in case of the endo/endo configuration, which is close enough for the generated positive charge to affect the oxidation of the neighboring group. To prevent dissolution of the polymer, it was also cross-linked by an azide-based cross-linker.226 A polymer 9461


Chemical Reviews

Review

one publication points out the influence of different carbon blacks with high specific surface area (Black Pearl 2000, VGCF, and acetylene black) on the capacity of PTMA composites.257 The choice of the carbon additive is crucial and leads to capacity differences of up to 25%, which are partly caused by the additional contribution of double-layer charging of the largesurface-area mesoporous carbon to the overall capacity. In common composite electrodes, the conductive additive is ideally uniformly coated by the active polymer. On the one hand, this ensures a facile electron transfer from the active polymer to the conductive additive, but, on the other hand, the polymer insulates carbon particles, which causes a poor intrinsic conductivity within the carbon fiber network and between carbon and current collector.258,259 This issue was addressed by a vertically aligned design of carbon nanotubes, grown from a stainless-steel current collector.260 The setup ensures a direct contact between the current collector and the conductive additive and prevents the formation of an insulating PTMA layer. This way, reasonable specific capacities were reached at charging rates up to 100C. A more scalable technique for the fabrication of PTMA/ carbon composites is the melt-polymerization of TEMPO methacrylate precursors in the presence of nanostructured carbons.261 The low melting point of the precursor monomer of 61 °C enables a solvent-free polymerization. The oxidation of the tetramethylpiperidine to TEMPO moieties was carried out in dichloromethane dispersion by meta-chloroperoxybenzoic acid. In contrast to coating carbon particles with a polymer film, this approach leads to a homogeneous incorporation of carbon particles into the polymer matrix on the nanoscale. The close interaction of the conductive additive with the active compound leads to shorter conduction pathways and, thus, performance improvements. This enables relatively high rates at low carbon contents (5 wt % carbon at 4C or 10 wt % at 10C charging speed) with 50% material activity. The cycling stability is, with only 15% loss after 1200 cycles, competitive. Besides the oxidation to the oxoammonium cation, the TEMPO radical can be reduced, yielding the aminoxyl anion. However, the reduction is known to be irreversible under commonly used conditions. For this reason, some efforts were made to make this step utilizable.262 The stepwise reduction of the oxoammonium cation via the free radical to the aminoxyl anion was described using promotion of the p- and n-type redox states by graphene composites.263,264 An attempt with graphene directly functionalized with TEMPO units was reported as well.265 A special approach was represented by DNA substituted with cationic amphiphilic lipids containing TEMPO radicals.229 However, the signals of the reduction are less distinct and reasons for the stabilization of the anionic state are not provided. Some of the results should be deemed mindfully. For a deeper look into TEMPO-based active electrode materials, we recommend the before mentioned reviews.18,234 4.3.2. Other Nitroxide Radicals. In 2,2,5,5-tetramethyl-2,5dihydro-1H-pyrrol-N-oxyl or the saturated form 2,2,5,5tetramethylpyrrolidin-N-oxyl (PROXYL), the nitroxide functionality is part of a five-membered ring. Compared to the TEMPO radical and its six-membered ring, the lower molar mass increases the specific capacity. An active material based on a PEO backbone with PROXYL units, linked via a methoxymethyl spacer, was investigated and compared to analogous TEMPO-bearing polymers, like PTGE.266 In a Li-organic battery, the PROXYL-functionalized polymer allows a slightly higher cell voltage of 3.6 V. At charging

in neutral and acidic electrolytes, while at pH > 8, an irreversible nucleophilic attack of hydroxide anions at the oxoammonium cations occurs.248,249 To prevent the formation of zinc hydroxide, the electrolyte additionally contained 0.1 M NH4Cl. The affinity of the polymer to the aqueous electrolyte allows a homogeneous solvation of the active polymer and a sufficient supply with counterions in polymer layers of 1 μm thickness, which enables a specific capacity of 131 Ah kg−1 at charge/discharge rates of 60C, with 65% of the initial capacity sustained after 500 cycles. The high rate performance of this polymer as active electrode material combined with an excellent cycling stability was further highlighted in half-cell tests applying charging rates up to 1200C (discharging at 60C) using thin PTVE films.250 The concept of a backbone supporting the ionic conductivity of the electrolyte was also applied in a cross-linked PTGE [poly(2,2,6,6-tetramethylpiperidinyl-N-oxyl glycidyl ether)]. The polymerization was accomplished via anionic ring-opening polymerization of 4-glycidyloxy-TEMPO with a phosphazene base initiator.251 The good swelling and ionic transport properties of the main chain, analogous to PEO, enable an efficient charge transport throughout the polymer matrix. This allows for thicker polymer layers and smaller amounts of conductive additive. In the given case, a ratio of 10 wt % SWCNT in the composite is sufficient to maintain the electrode performance enabling a high charge-storage density. An assembled battery revealed a specific capacity of 80 Ah kg−1 at 10C. However, although the polymer was cross-linked, 30% of the capacity fades during the first 200 cycles. Recent studies focused on the optimization of TEMPO-based materials through a comprehensive improvement of the whole composite system. One approach addressed the polymer morphology by applying a poly(styrene)-block-PTMA copolymer, which self-assembles into micelles with a poly(styrene) core and a PTMA corona in the used electrolyte (1 M Li-triflate in EC/DEC/DMC 1:1:1).252 This arrangement is intended to allow an easier access for counterions to the redox-active sites upon charge/discharge as the active polymer blocks are directed toward the electrolyte. However, because of the additional weight of the “dead” poly(styrene) block and, probably, a smaller contact area between active polymer and conductive additive, only a low specific capacity of 15 Ah kg−1 was reached. Another approach also concerns the accessibility of counterions. Namely, instead of balancing the positive oxoammonium cation by consuming an anion from the electrolyte, lithium-sulfonatebearing co-monomers compensate the formed charge.253,254 The key advantages are the stable lithium concentration in the electrolyte during the charge/discharge process and the only small necessary amounts of lithium salt. However, the additional molar mass of the co-monomer decreases the theoretical capacity and compensates the saving of counterion excess. The utilization of micro- and nanofibrous polymer films leads to a significant performance increase of PTMA-based electrodes.255,256 The fibers are manufactured by electrospinning of a mixture containing 50 wt % PTMA, 20 wt % binder, and 30 wt % carbon black in an acetone/NMP (1:1) solution. The special morphology of the nanofibrous films enables high rate capability and cycling stability and, in addition, prevents the material from dissolution. A Li-ion battery with this cathode material revealed a specific capacity of 111 and 109 Ah kg−1 at 10 and 50C, respectively, underlining the promising rate performance of the electrode with 50 wt % active polymer content. Systematical studies on the effect of different conductive additives and composite formulation methods are rare. However, 9462


Chemical Reviews

Review

rates of 10C, a superior capacity of about 80 Ah kg−1 was found and sustained over 100 cycles. The best theoretical capacity in an organic battery that is based on nitroxyl-radical-bearing polymers was predicted for a PEO with PROXYL groups directly bound to the backbone with 147 Ah kg−1. It is comparable to the specific capacity of lithium-intercalation electrode materials like LiCoO2 (140 Ah kg−1).16,230 However, in actual lithium-organic test cells, only half of the theoretical value is reached,267 while the cycling stability of the battery is competitive (80% remains after 1000 cycles) and a cell voltage of 3.7 V is achieved. Another approach to increase the charge density is the introduction of a second nitroxide site per repeating unit, as demonstrated in spiro-bis(nitroxide)s.268 However, prototype batteries versus lithium revealed that only the radical that is part of the six-membered ring undergoes a reversible redox reaction, and thus, the expected capacity gain was not achieved. A polymer that carries noncyclic stable nitroxide radicals is poly(nitroxylstyrene). Here, the NO-radical is stabilized by a phenyl ring, which enables delocalization of the electron density, on the one hand, and a sterically hindering tert-butyl group on the other hand. Further functionalization of the phenyl ring with electronwithdrawing CF3-groups allows for an n-type redox reaction to the aminoxyl anion.228 As a consequence, the material can, depending on the substitution pattern, undergo either p- or ntype reactions and is suitable as cathode and anode material. However, these materials were not applied in a battery, yet. Redox-active polymers that undergo both reversible oxidation and reduction can be applied as anode and cathode material and are, thus, of particular interest. One of the few examples is the nitronyl nitroxide radical. A nitronyl-nitroxide-functionalized poly(acetylene) was obtained by rhodium-catalyzed polymerization of ethynylphenyl nitronyl nitroxide.269 It revealed reversible redox processes at 0.80 V and −0.84 V vs AgCl/Ag. Although the compound was not utilized in a battery so far, the electrochemical investigations strongly suggested its viability. In chronopotentiometric half-cell tests using a nitronylnitroxide-monofunctionalized poly(norbornene), the specific discharge capacities were 59 Ah kg−1 for the p-type and 44 Ah kg−1 for the n-type reaction.270 The less reversible reduction reflects the general observation that n-type reactions of radical polymers are more susceptible to side reactions than cathode processes. A polymer containing difunctionalized norbornene units likewise displayed reversible bipolar reactions but with inferior performance. A symmetric all-organic battery with poly(nitronyl nitroxyl styrene) as anode and cathode active material was built by Suga and co-workers.232 The device showes charge/discharge plateaus at around 1.3 and −1.3 V, confirming the suitability of the active polymer for a bipolar system and enabling a “poleless” battery. The capacity is 44 Ah kg−1 at 10C based on the total weight of poly(nitronyl nitroxyl styrene) at the cathode and anode (86% of the theoretical capacity). In addition, an asymmetric battery with a poly(nitronyl nitroxyl styrene) anode and a poly(galvinoxyl styrene) cathode was investigated (section 4.3.3). 4.3.3. Galvinoxyl. A radical that is not based on nitroxide moieties is the galvinoxyl. Galvinoxyl radicals are reversibly reduced to the corresponding phenolate ion and are, although the redox potential is relatively high, utilized as n-type material in all-organic battery systems. Due to the high molar mass combined with a one-electron redox reaction, the system possesses a theoretical capacity of only 51 Ah kg−1. An allorganic radical battery composed of a poly(galvinoxyl styrene) anode and a poly(TEMPO norbornene) cathode displayed a cell

voltage of 0.66 V and a capacity of 32 Ah kg−1 (regarding the total weight of anode and cathode active polymer; 92% of the theoretical value).233 The cell is sufficiently stable upon cycling (about 75% active after 250 cycles) and reveals a good rate performance of 10C. An ORB with a poly(ethynylphenyl galvinoxyl) anode and a PTMA cathode was built with an aqueous electrolyte.271 The Coulombic efficiency of only 72% was ascribed to the latter, and the charge/discharge plateaus were only weakly defined. While PTMA works best under slightly acidic or neutral conditions, the galvinoxyl polymer shows best activity in an alkaline milieu. Therefore, the chosen neutral aqueous NaCl solution is not optimal for the cell system. A battery with a poly(nitronyl nitroxyl styrene) anode and a poly(galvinoxyl styrene) cathode reveals a storage capacity of 29 Ah kg−1 (regarding both electrodes; 91% of the theoretical value) at 0.6 V output voltage.232 The nitronyl nitroxide is reduced upon charging, incorporating cations, while the reduced galvinoxyl is reoxidized and releases the same amount of counterions. Hence, in both half cells, only neutral and negatively charged states are present and only counter cations are exchanged between electrolyte and polymer matrices, leading to a migration in a “rocking-chair” manner. Thus, a smaller amount of supporting electrolyte is necessary to maintain the redox reactions as the concentration of free counterions remains constant, independent of the state of charge of the battery. Other promising radical compounds are, for example, the oxylbased phenoxyl radical,272 the verdazyl,273,274 and Blatter’s radical.275 However, these were not yet applied in organic battery system. Although radical polymers cover a significant portion of the publications in the field of organic batteries, the number of different types of radicals is rather limited, compared to the great variety of utilized organic compounds. Nevertheless, radicals reveal excellent electrochemical properties, in particular regarding their redox kinetics. TEMPO is by far the most intensively studied moiety among the active materials for polymer-based batteries, not least because of its superior electrochemical stability.

5. NONACTIVE ELECTRODE COMPONENTS 5.1. Conductive Additives

In general, polymers (except conjugated polymers) feature low intrinsic conductivities. One approach to overcome this issue is the application of thin polymer films on current collectors. Electrodes manufactured by electropolymerization,72,215,216 surface-initialized polymerization,276,277 dropcasting,149 or spincoating226,233,237,278,279 have been successfully applied, and their electrochemical behavior were examined in detail.237 In particular, the diffusion coefficient149 and the electron exchange rate280,281 in the solid state can be calculated and enable a deeper insight into the molecular electrochemical properties of the redox-active units in the polymeric environment. However, the surface area loading of thin polymer film electrodes (full material activity up to 200 nm thickness)226 is insufficient for any application due to the ultralow capacity. Usually, this drawback is circumvented by the preparation of composite electrodes consisting of the redox-active polymer as active material and one or more conducting additive(s). Mainly (nanostructured) carbon materials are applied, which form porous electrodes whose properties are strongly dependent on both size and structure of the carbon particles and the 9463


Chemical Reviews

Review

in the range from 10 to 100 nm and a surface area between 25 and 250 m2 g−1. An overview of different carbon blacks is given in Table 6.

formulation process of the electrode. The carbon material serves as a buffer to mitigate volume changes of the active material and provides electron pathways from the redox-active units of the polymer to the current collector and vice versa. Sufficient electronic conductivity and high surface area of the carbon material are required to enable high-rate-performance electrodes and high active-material loading.282 The conductive additive needs to be in contact or is, in best case, coated with the active polymer material to enable a barrier-free electron transport between these two components and additionally forms a loose, porous but mechanically stable network that is penetrated by the electrolyte. A successful redox reaction, mandatory for the charge-storage process, requires several processes to occur: (1) an electron is transferred between the redox units in the polymer chain, (2) a redox-active unit gains/releases an electron through the conductive-additive interface from/to the current collector, and (3) it simultaneously releases/obtains an ion to/from the electrolyte, which is crucial for the charge balance.258 Redoxactive units that do not fulfill these requirements do not participate in the charge-storage process and can be considered as dead material. In contrast to anode materials for Li-ion batteries, the carbon material itself needs to be electrochemically inert in the operational window of the electrode, which is mandatory in particular for the anode, as carbon materials are known to intercalate metal ions, such as lithium or sodium, at low potentials. In general, the conductive additive is considered as inactive material, as it does not contribute to the capacity of the battery, although it severely influences the performance of the electrode. The necessary amount of the conductive additive depends on the nature of the polymer backbone and the redoxactive unit. Units with large aromatic parts, such as perylene (in perlyene-based poly(imides)160−162 or poly(anhydrides)283), or free-radical compounds, like TEMPO-based polymers,234,237 feature semiconductivity or advanced electron hopping mechanisms, respectively, and therefore, only small amounts of conductive additive are required for a high material activity. Besides the amount of conductive additive, the electrode thickness has a crucial influence on the electrochemical performance of the battery but is not optimized in many studies.284 Furthermore, detailed processing parameters and characteristics of the composite electrode, such as roughness and material loading, are often missing or insufficiently discussed. This complicates the comparison of the rate performance of electrodes between different publications, even when the same active polymer is utilized. In the following, we provide an overview over commonly used conductive additives and their influence on the battery performance. 5.1.1. Carbon Particles. Carbon particles, such as ordinary graphite, SuperP, and carbon black/acetylene black, have been utilized at the beginning of the application of polymers as active electrode materials. However, their moderate conductivity and surface area limits the electrochemical performance and active material loading to such an extent that these materials are only rarely applied in polymer-based electrodes by now. Carbon black is produced by incomplete combustion of aromatic hydrocarbons at high temperature and is made of particles fused together forming modular aggregates. In most cases, these aggregates do not break up during dispersion and processing. Five different types of carbon blacks are produced industrially and possess different physical and chemical properties: furnace black, thermal black, lamp black, channel black, and acetylene black.285,286 In general, the particles feature diameters

Table 6. Physical Properties of Selected Carbon Blacks285,286 name

particle diameter (nm)

BET (N2) surface area (m2 g−1)

Vulcan XC72 Vulcan PA90 Elftex TP Vulcan P SuperP

30 20 20 20 20

254 140 130 140 62

5.1.2. Mesoporous Carbon. Mesoporous carbon particles are spongelike porous carbon materials whose synthetically adjustable pore size ranges from 2 to 50 nm.287 Composites of mesoporous carbon and electrochemically active materials have been investigated recently as charge-storage electrode materials and improved the performance of Li-ion batteries due to the fast ion transport within the pores and the high electrical conductivity. One example for a porous conductive additive applied in polymer-based batteries is Ketjenblack (KB), a commercially available highly electro-conductive carbon black with a mesoporous structure and high surface area (800 to 2800 m2 g−1) and pore volume (4.7 cm3 g−1).262 Different methods for composite fabrication have been evaluated with PTMA as active material and can be transferred to other polymers. Simple solvent-less ball-milling of KB with PTMA as active material leads to a large decrease of the surface area and the pore volume, indicating heavily occupied pores. These PTMA-KB composite electrodes with low amounts of active material (17% PTMA) revealed superior electrochemical performance compared to similar PTMA-SuperP electrodes.262 Furthermore, the largesurface-area mesoporous carbon additive contributes to the overall capacity via excessive double-layer charging (up to 23 Ah kg−1 for BP-2000).257 However, the small pore size and the one-dimensional pore structure complicate the loading of mesoporous carbon with bulky electrochemically active materials. In addition, the diffusion of electrolyte into and within the pores is hindered. This may make them an appropriate conductive additive for small-molecule-based electrode materials but less suitable for active polymers. 5.1.3. Vapor-Grown Carbon Fibers (VGCF). Vapor-grown carbon fibers can be prepared via catalytic decomposition of a gaseous carbon source, such as methane or benzene,288 providing a potential low-cost procedure. Depending on the preparation conditions, their diameter and length can be adjusted from several tens of nanometers up to tens of microns in diameter and from several microns up to centimeters in length. These fibers reveal a highly preferred orientation of their graphitic basal planes parallel to the fiber axis, with an annular ring texture in the cross section, leading to a carbon material with excellent mechanical strength and high electrical as well as thermal conductivity but a relatively low surface area. This allows for their application as conductive supporting materials in batteries.289 The fiberlike structure represents the ideal conductive additive for the liquid−solid mixing technique (see below), enabling polymer coating of the fibers, which reinforces the mechanical stability of the electrode.290,291 Therefore, VGCF are the most frequently applied conductive additive in polymer-based composite electrodes. For instance, in optimized TEMPO9464


Chemical Reviews

Review

m2 g−1.299,300 There are four methods established to synthesize carbon nanotubes: arc-discharge,301 laser ablation,302 gas-phase catalytic growth from carbon monoxide,303 and chemical vapor deposition from hydrocarbons,304 which is the only method that has the potential for the production of large quantities. Besides the use as ordinary conductive additive, fabricated by simple mixing techniques,151,160 several more advanced approaches leading to high-performance electrodes have been evaluated. The wrapping of active polymer (PTMA) around singlewalled carbon nanotubes was accomplished by an ultrasonification/ultracentrifugation process,305 leading to a composite material with ultrahigh active polymer loading (96 wt %).306 These transparent composite electrodes, with thicknesses of 970 nm, exhibit complete material activity during the charge/ discharge process even at very high rates.307 In comparison, polymer films reveal complete material activity under the same conditions only up to a film thickness of less than 200 nm, indicating the large influence of even small amounts of nanostructured carbon conductive additive. Besides wrapping, in situ polycondensation onto SWCNT films represents a promising approach toward fully flexible high-performance polymer-based electrodes.157 During the polycondensation of an aromatic dianhydride with a diamine, nanoflakes of polymers start to grow on the SWCNT network, leading to vertically aligned channels between the flakes, which support the electrolyte transport (Figure 13). These composite electrodes

based polymer electrodes, VGCF allow both high active material loading of up to 80%290 and high rate capability of up to 50C.243 The electrode performance was further improved by the application of nanostructured additives such as carbon nanotubes or graphene. 5.1.4. Graphene. Graphene is a single-layer graphite with closely packed, conjugated hexagonal lattices and is the basic building block of all multidimensional carbon materials.292 This unique structure of graphene reveals a variety of superior properties, such as high electrical and thermal conductivities,293,294 a good transparency,295 high mechanical strength,296 good flexibility, and a huge specific surface area (up to 2600 m2 g−1).297 It can be prepared by different, low-cost processes,298 such as chemical vapor deposition, oxidation-exfoliationreduction of graphite, exfoliation of graphite via sonification of intercalation compounds, and by chemical synthesis, making it a promising conductive additive for organic polymer-based batteries. Of particular interest are three-dimensional graphene networks, which have been used to improve the electronic conductivities of composite materials. These composites are mainly manufactured by dispersion-deposition processes, where the dissolved polymer is forced into the pores by capillary and adsorption effects and adhered at the graphene surface after solvent vaporization. For instance, a PTMA/graphene composite electrode revealed an ultralong life cycle of 20000 charge/ discharge cycles at a rate of 100C.263 However, the active material loading in the electrode composite is only 10%, while the graphene loading is as high as 60%. Higher contents of active polymer clog the pores of the conductive additive. Thus, the active material in the pores is inaccessible for the electrolyte and is electrochemically inactive. Higher material loading in graphene-based electrodes was achieved using poly(anthraquinone sulfide) or poly(imide), prepared by in situ polycondensation, in the presence of the graphene sheets. This technique leads to a homogeneous distribution of high amounts of polymer up to 60 wt % and enables high-performance electrodes, which can be charged with up to 30C featuring complete material activity and only a negligible capacity loss (Figure 12).140

Figure 13. (a) Schematic representation of the preparation process of PI/SWCNT film; SEM images of (b) SWCNT film and (c) PI/SWCNT film after 4 h reaction time. Reprinted with permission from ref 157. Copyright 2014 John Wiley & Sons.

are extremely flexible, act as both electrode and current collector, and reveal superior electrochemical performance as electrode material with both high material activity and excellent rate capability. 5.1.6. Composite-Electrode Processing. Several different processing options have to be evaluated to manufacture composite electrodes. In general, the three components of the composite (active polymer, conductive additive, and binder) are present as solids and a homogeneous mixture is desired. Simple solid−solid mixing in a ball mill leads to a uniform distribution of all three components throughout the composite. This technique is preferred for spherical and porous carbon materials, as the polymer particles are attached to the surface of the conductive additive leading to a porous composite. However, this technique is strongly dependent on the particle size of the active polymer, whereas large polymer particles lead to low material activity. Liquid−solid mixing of a solution of polymer and binder in the presence of the carbon additive achieves a good distribution and a large contact area between active polymer and carbon.243 This technique is in particular suitable for fiber-like additives and

Figure 12. Schematic representation of the in situ polymerization toward poly(anthraquinone sulfide) and poly(imide) in the presence of a conductive additive (functionalized graphene sheets). Reprinted from ref 140. Copyright 2012 American Chemical Society.

5.1.5. Carbon Nanotubes (CNT). The group of carbon nanotubes is divided into two types: single-walled carbon nanotubes (SWCNT) and multiwalled carbon nanotubes (MWCNT). SWCNT basically consist of one single graphene layer wrapped to a tube, whereas MWCNT consist of two or more graphene layers with van der Waals forces binding adjacent layers and a moderate surface area in the range from 100 to 400 9465


Chemical Reviews

Review

soluble polymers, as the coated fibers form a porous conductive network upon drying, which significantly improves the electrochemical performance (Figure 14). By contrast, this technique is

stability, binding capability, and electrolyte absorption ability. PVdF is the superior binder for liquid−solid-processed composite electrodes as it reveals a similar solubility behavior as most of the active polymers and supports the formation of a porous coated carbon network243 but can also be applied in solid−solid mixing processes in its finely powdered form. However, PVdF requires the usage of toxic organic solvents such as NMP, while a water-based processing of the manufacturing of the composite electrode is preferred not only from an environmental viewpoint. There are several naturally abundant, water-soluble binders available, enabling cheap and environmentally friendly electrode processing. This, however, requires the application of nanometer-sized active polymer particles, as carbon coating does not occur if the active material is not in solution. Carboxymethylcellulose (CMC) is the most frequently applied binder for waterbased slurries and leads to a similar electrochemical performance like PVdF.290 Ionic binders, such as lithium or sodium salts of CMC139 or poly(acrylic acid) (PAA),309 were proposed to improve the rate capability by the introduction of additional ions into the composite electrode, which are readily available to ensure charge balancing. Furthermore, PEDOT:PSS was proposed as binding material because its conducting nature improves the electrochemical performance at high charge/ discharge rates.139 Besides the binders that are applied in polymer-based batteries, other interesting concepts were presented recently and may find their way into the field of organic batteries. Deriving from natural sources, purified guar gum reveals high elasticity, in contrast to the low-elastic PVdF, which, therefore, suffers from structural degradation during cycling.310,311 This may be in particular advantageous for polymers that reveal structural changes upon charging and discharging. Further research focused on ion-conductive binder materials. A poly(vinylimidazolium) nanonetwork, for instance, reveals sophisticated Li-ion conductivity and improves the performance of intercalation electrodes for Li-ion batteries compared to PVdF as well as the durability of the system.312 Imidazolium-based poly(ionic liquid)s were applied as binder in Li-ion battery cathodes as well. In combination with carbon additives, they formed very effective Li-ion- and electron-conducting pathways improving both the rate performance and the lifetime of the electrodes.313 Similarly, thiazolium-based poly(ionic liquid)s, applied with a variety of electrolyte anions, also increased the capacity of electrodes.314

Figure 14. SEM images of PTMA−carbon composite electrode made by (a) liquid−solid and (b) solid−solid mixing methods. Reprinted with permission from ref 243. Copyright 2007 Elsevier.

not suitable for spherical or porous carbon materials, as the polymer clogs the pores of or between the spherical carbon particles. However, using liquid−solid mixing technique, a number of further parameters, which arise from the drying process and need to be optimized, were generated. Inappropriate drying conditions, in particular at polymer-rich electrodes, lead to brittle or cracked composites, in particular, if the polymer reveals strong swelling in the solvent.290 For these polymers, water-based slurries represent an alternative way of processing. This technique can be considered as solid−liquid−solid mixing, and best results were obtained if carboxymethylcellulose, which is soluble in water, was applied as binder material.227,242 Another approach, which is suitable for polycondensation reactions157 and for liquid or low-melting polymers,261 but could be extended to polyaddition reactions, is the in situ polymerization. The monomers are polymerized in the presence of the conductive additive, and composites with a large accessible surface area and high electronic conductivity can be obtained. Ideally, the applied solvent is suitable for both the polymerization reaction and the dispersion of the carbon additive.140,160 Another attempt to construct porous composites is electrospinning,256 leading to three-dimensional cobweb structures with fully interconnected pores and multifibrous layers, which ease the penetration of the electrolyte.255 The thickness of the electrode represents a crucial factor for the electrochemical performance of the active polymeric material. However, the optimal thickness of a composite electrode depends on several factors such as the active polymer, the conductive additive, the electrolyte, and the manufacturing parameters of the composite electrode. It has to be evaluated for each combination and can be adjusted by the gap width of the doctor blade. In general, electrode thicknesses in the range from 100 to 300 μm are used.

5.3. Current Collectors

The current collector of an electrode plays an important role in rechargeable batteries as it transfers the electrons from the composite electrode to the electric consumer and vice versa. Thus, it has to satisfy several requirements: good electrical conductivity, mechanical strength, lightweight and low thickness, chemical and electrochemical stability in the electrolyte, and adhesiveness to the polymer-carbon composite.315 The current collector is considered, similar to the conductive additive and housing, as inactive mass and volume of the battery, which reduces the gravimetric and the volumetric energy density. Thus, thin and light materials, such as foils, are preferred. Dependent on the cell potential and the electrolyte of the battery, a passivation reaction of the current-collector surface might occur, which inhibits corrosion and/or degradation.316 In general, the setup is adopted from the Li-ion battery technology (i.e., aluminum foil serves as current collector for the positive and

5.2. Binder

With dependence on the active material loading and the conductive material, binders can be mandatory to stabilize the composite electrode mechanically and ensure a large contact area between the current collector and the carbon-polymer composite. Sometimes, if high active polymeric loading is present, the active material fulfills this task. Although, in general, only 2 to 10 wt % of a composite electrode consists of binders, they can have a crucial influence on the cell performance and life cycle.308 Fluorinated polymers such as poly(tetrafluoroethylene) (PTFE) and poly(vinylidene difluoride) (PVdF) are mostly applied as binder materials in composite electrodes of polymerbased organic batteries since they feature good electrochemical 9466


Chemical Reviews

Review

the salt to suppress the formation of ion pairs, which do not contribute to the ionic conductivity. An additional essential property of the electrolyte solvent is the electrochemical stability in the used potential window of the battery and the chemical inertness to all battery components, for example, the housing, membranes, and electrode supports, as well as all redox states of the active materials. In addition, the dissolution of the active polymers has to be prevented. Here, it has to be taken into account that the solubility, in particular of organic active materials, can change dramatically upon redox reactions. Furthermore, the solvent should promote thermal stability, remain liquid in a wide temperature range, has a high flashpoint and low toxicity, and should provide good swellability and affinity between active material and electrolyte to facilitate the exchange of counterions between electrolyte and polymer matrix. Because these diverse requirements can hardly be met by an individual solvent, mixtures of different organic liquids are usually used. The electrolytes for organic batteries are mainly adopted from common inorganic-based batteries, in particular from Li-ion systems. They represent sophisticated electrolyte systems, which are based on decades of development and designed to work over a wide potential window of over 4.0 V. In addition, new organic active materials are most often tested in Li-ion battery setups first, which makes such electrolytes the obvious choice. One of the solvents used in the discussed organic batteries is propylene carbonate (PC), a cyclic diester of carbonic acid. It reveals a high dielectric constant and thus possesses the ability to dissolve a large variety of salts.320 It undergoes reductive decomposition at newly formed lithium surfaces, upon which a protective layer is formed, preventing further electrolyte degradation.321 However, on metal lithium anodes, the surface is renewed during each cycle, leading to poor cycling stabilities. At graphitic anodes, on the other hand, PC is not able to form a stable solid−electrolyte interface (SEI). Instead, it intercalates into the carbon structure, resulting in the exfoliation of graphite layers. The behavior of ethylene carbonate (EC) differs strongly, despite the seemingly small differences of the molecular structures. It is capable of forming an effective protective layer at graphite anodes, which prevents electrolyte decomposition.320 Furthermore, it reveals an even higher dielectric constant than water. The major drawback of this compound is its high melting point of 36 °C, arising from the high molecular symmetry, which leads to a stable crystalline lattice.322,323 However, in mixtures with solvents of especially low viscosity, like organic ethers or linear organic carbonates, the melting point decreases to a sufficient value enabling the application at low temperatures. Significant differences arise for linear organic carbonates. While cyclic carbonates are polar and highly viscous, their linear counterparts are only weakly polar but rather fluid. The linear carbonates that are applied in organic batteries, namely dimethyl carbonate (DMC), diethyl carbonate (DEC), and ethylmethyl carbonate (EMC), are in particular interesting due to their low viscosities and low melting points. The increased fluidity leads to higher ionic conductivities, in particular at low temperatures.320 However, their dielectric constants are too low for application as pristine solvents, so that they are mixed with EC and/or PC at various ratios. In combination with EC, a notable synergistic effect occurs: the electrochemical stability limit increases to 5.0 V vs Li+/Li as observed in Li-ion batteries with a spinel cathode, compared to 4.0 V for DMC alone.324 Organic ethers, like the linear 1,2-dimethoxyethane (DME) or the cyclic 1,3-dioxolane (DOL), are other commonly used

copper foil for the negative electrode). However, dependent on the redox potential and the electrolyte, several other current collectors were applied and are briefly discussed in the following section. Aluminum is the material of choice for high-voltage cells because of its high electrochemical stability up to 5 V vs Li+/Li in organic, anhydrous media due to the formation of a passivation layer,316 as well as its commercial availability as thin foils with good conductivity and reasonable weight and price. At a potential of around 0.3 V vs Li+/Li, an Al-cation alloy formation process can be induced, which leads to the destruction of the current collector.317 In aqueous environment, aluminum possesses an effective corrosion protection (i.e., a dense passivation layer of aluminum oxide) in the range from pH 5 to 8 but is subject to corrosion at high or low pH values. Copper is usually applied as the negative current collector, if materials with very low potentials are applied (0 to 0.5 V vs Li+/ Li), as no alloys with cations of the electrolyte are formed. It is suitable as a current collector for the positive electrode only up to a potential of 2 V vs Li+/Li since CuO, which is formed under air, is reduced at this potential.318 In some examples, zinc foil (ca. 2.3 V vs Li+/Li) was applied as both active anode material and current collector in zinc-organic batteries with both organic and aqueous247,319 Zn-salt-containing electrolytes.124 The dendrite formation and the oxidation of zinc can be prevented by lowering the pH value of the aqueous electrolyte.247 Carbons, such as glassy carbon and graphite, have been widely used in electrochemical research as electrode material and were furthermore applied as current collector for electrochemical investigation of composite electrodes.167,168 However, carbon materials, in particular glassy carbon, are expensive, inflexible, sometimes catalytically active, and only moderately conducting and are, therefore, unlikely to be utilized in industrial applications. Indium tin oxide (ITO)-coated substrates, such as glass slides and, more importantly, PET foils, are transparent and flexible current collectors for electrochromic devices. They are electrochemically relatively stable in the potential range of cathode materials (2.5 to 4.3 V vs Li+/Li), although, at lower potentials, they may be reduced and intercalate cations from the electrolyte. Due to its high surface roughness, which leads to an improved electrical conductivity in combination with polymercarbon-based composites, ITO is the superior current collector for polymer-based electrodes.259 Furthermore, it allows the fabrication of fully transparent and flexible batteries.226

6. ELECTROLYTES The function of the electrolyte is the ionic transport of charges between the electrodes, which is crucial for the charge balance at the redox-active groups and mandatory for the charge/discharge processes. The electrolyte’s properties rely on the characteristics of both the used solvents and the conducting salts. 6.1. Solvents

The characteristics of the solvent are basically determined by two key factors: first, the viscosity, which influences the ion mobility and, thus, the ionic conductivity, and second, the ability to dissolve sufficient amounts of conducting salt. Polar solvents are necessary to dissolve the salts in sufficient concentrations by ion-dipole interactions. Hence, the relevant properties of the electrolyte solvent toward high solubilities are a high dipole moment and a high relative permittivity (dielectric constant). High dipolar moments and permittivities enable the salt solubilization by interaction with the free ions and dissociate 9467


Chemical Reviews

Review

electrolyte solvents. They are highly fluid but reveal only moderate dielectric constants and are used as additives or as part of mixed electrolytes. However, these solvents promote dendrite formation, which can lead to short circuits.320 In addition, they can possibly be oxidized within the usually applied potential window, depending on catalytic effects of the used cathode material. Nevertheless, ethers are preferentially applied for ntype organic polymers, which form strong nucleophilic anions in the charged state, which are less reactive toward this class of solvents. Other organic solvents like acetonitrile are also suitable for electrolytes. In fact, they are often applied in all-organic batteries with smaller operative windows.216,232,233 However, acetonitrile is not stable at low potentials (e.g., in the case of metallic lithium or sodium), and the application is limited. Water-based electrolytes are used whenever the conditions allow their application. Examples for aqueous electrolytes can be found in zinc-organic,107,114,119,120,247 all-organic,51,215,271 Li-intercalation,61,159 and polymer−air batteries.149 Their main advantages are the excellent polarity and ionic conductivity, environmental friendliness, and the ubiquity of the resource. On the other hand, water electrolysis limits the usable potential window to ca. 1.2 V, which is additionally strongly pH-dependent. Furthermore, the comparatively high reactivity of this protic solvent, in particular toward charged species, restricts the compatibility to many redox-active materials. There are several mechanisms that lead to the decomposition of electrolyte solvents and, consequently, to a decreased battery performance. The electrolyte oxidation or reduction at the cathode or anode, respectively, strongly depends on the catalytic properties of the electrode surface. However, for polymer-based electrodes, the degradation processes were not investigated up to now. But, since many redox-active polymers are applied in Liorganic setups, the electrolyte reduction is omnipresent due to always incomplete protective interphases on the lithium metal anodes. Another cause of decomposition reactions arises from the merely metastable state of electrolytes.325 Once the operating conditions vary (e.g., due to elevated temperature, overcharge/discharge, or trace moisture), irreversible reactions occur. In particular the latter initiates a cascade of parasitic reactions. In solutions that contain frequently applied but moisture-sensitive hexafluorophosphates (e.g., LiPF6), the hydrolysis products, like HF and phosphorus oxyfluoride (OPF3), arise. OPF3 reacts with linear carbonates to start a chain reaction releasing two equivalents of OPF3, leading to an autocatalytic reaction. In the case of cyclic carbonates, the hydrolysis products initiate the polymerization of the solvent, yielding oligoethylene oxides and CO2.326,327 In organic carbonates, trans-esterification constitutes another decomposition pathway, initialized by nucleophilic attack of other decomposition products (i.e., lithium alkoxides) and catalyzed by acidic impurities. A deterioration of the cell performance and capacity is the consequence.328 The complex nature and the diversity of decomposition reactions depend on many parameters, such as electrolyte composition, undesired impurities, battery components that can react with electrolyte components or act catalytically (current collector, graphite surfaces, etc.), and external influences like temperature.326 However, several attempts to slow down the degradation mechanisms by new additives were conducted. Lewis bases inhibit the thermal decomposition of electrolyte salts329,330 and electrophilic additives such as vinylcarbonate

(VC) suppress nucleophilic attacks at carbonate molecules.331 VC is also believed to form polymeric interphases, which offer a better protection of the anode surfaces.325 6.2. Salts

The second key component of the electrolyte is the conducting salt. To increase the ion conductivity, a good solubility and dissociation is necessary, which can be assured by choosing an appropriate combination of solvent and salt. The ionic conductivity of the electrolyte is additionally determined by the ion mobility, which, in turn, depends on the ion radius and increases with smaller sizes. Furthermore, the electrochemical stability within the potential range of the cell and the compatibility to the active electrode materials, as well as the chemical stability and inertness toward all battery components, such as electrode supports, current collectors, separator, or housing, is mandatory. If a metal-based electrode is applied, the supporting electrolyte cation has to match the metal. Besides, an ideal battery conducting salt possesses also a low molar mass and good thermal stability, is environmentally friendly, nontoxic, and features a good availability. Since lithium anodes are usually used in test cells for active polymer materials, the application of lithium salts dominates the literature. Furthermore, they are by far the most intensively investigated electrolyte salts, since they were utilized in Li-ion batteries at large scales for decades.320 With regard to organic electrodes, the ions are necessary to compensate the charges formed upon the redox processes. During the charging/discharging reactions, ions migrate from the electrolyte through the polymer matrix to the redox-active group or vice versa. Therefore, the interactions between ions and polymer are substantial, although they are more forgiving compared to inorganic intercalation electrodes. The counterion has to undergo reversible intercalation and is, preferably, monovalent to prevent complexation with adjacent redox-active groups. The minimum amount of conducting salt ions needs to exceed the number of stored charges (i.e., the number of redox sites that have to be supplied with counterions). The excess should be at least 100 to 1000 times. However, a concentration that is too high increases the viscosity and, thus, decreases the ion mobility. In general, concentrations in the range from 0.1 to 1 M are applied. Although a wide range of salts is available, only a limited number is used for most reported applications. The anions that are predominantly utilized in polymer batteries are, in the order of their increasing ion radius (i.e., decreasing ion mobility332,333), tetrafluoroborates,91,173,180 perchlorates (see refs 147, 151, 158, 168, 183, 218, 232, and 233), hexafluorophosphates (see refs 25, 141, 148, 160, 179, 199, 212, 246, and 266), trifluoromethanesulfonates (triflates), 80,111,112,252 and bis(tri¯uoromethane)sulfonimides (TFSI) (see refs 113, 139, 152, 155, 157, 186, 193, and 197). For engineering a lithium-organic battery, lithium-based electrolytes are mandatory. LiClO4 reveals high anodic stability.334 However, perchlorates are known to be thermally unstable and potentially explosive.335 LiPF6 is the most frequently applied salt in commercial Li-ion batteries but is more sensitive toward moisture. Degradation products of LiPF6 are the highly reactive HF and the Lewis acid PF5, which is able to initiate the polymerization of organic solvents. Thus, high purity grades of the used electrolytes and a moisture-free fabrication environment are required for the application of hexafluorophosphates. Lithium tetrafluoroborate, however, is thermally more stable and hydrolyzes slower compared to LiPF6. It forms more 9468


Chemical Reviews

Review

stable passivation films at aluminum current collectors, which allows higher potentials.332 In contrast, the exclusive usage of organic redox-active compounds enables the application of metal-free electrolytes. Quaternary ammonium compounds, like tetrabutylammonium (TBA) salts, are commonly utilized in this case.232 It should be taken into account that TBA salts tend to promote the dissolution of the charged polymers through the formation of ion pairs with the well soluble quaternary ammonium ion. In addition, TBA ions tend to intercalate into carbon substrates at low potentials.336 The vast number of described electrolytes is based on carbonates and ethers containing lithium conducting salts. In batteries based on conjugated polymers, mostly mixtures of organic carbonates like EC/DMC and EC/DEC with LiPF6 are applied. In addition, ether-based electrolytes are used for polythiophenes.70,111,113 Aqueous electrolytes are utilized in cells where the potential window of the applied electrodes (other conjugated polymer,51,64,94 Zn anode,107,114,119,120 or LiCoO2 cathode61) enables their application. The standard electrolyte for Li-organic batteries with carbonyl-based cathode is the ether mixture DOL/DME containing 1 M LiTFSI. But also organic carbonates with LiPF6, in particular for imides,140,155−158 and with NaPF6/NaClO4, for sodium batteries, are applied.158,162 Furthermore, for the all-organic battery of poly(anthraquinonylsulfide) versus poly(triphenylamine), DOL/ DME and NaPF6 is used,141 while a poly(vinyl anthraquinone)/air battery features an alkaline water-based electrolyte.149 The prevailing electrolyte in cells containing thioether-based active polymers is the combination of DOL/DME and LiTFSI. Only for thianthrenes, EC/DMC with LiPF6 is used.199 Disulfide-Li-batteries, however, usually contain carbonate mixtures like EC/DMC and EC/DEC with lithium salts of PF6−, ClO4−, and BF4−. Li-organic radical batteries are almost exclusively based on organic carbonates, mostly EC/DEC with LiPF6. A described Zn-TEMPO battery possesses an electrolyte containing water and ZnCl2.247 However, for all-organic batteries, acetonitrile/ TBAClO4 seems to be an appropriate match supporting both electrodes.232,233 The choice of solvent and salt always depends on the nature of the active electrode material and has to be adjusted to its demands. However, the interaction of electrolyte and active material is hardly predictable but can cause large performance differences. Nevertheless, criteria for its composition are in general not further discussed in the literature. A special challenge for the electrolyte composition are all-organic batteries, where conditions have to be found that match the requirements of both active polymer electrodes. This often represents a limiting factor for the performance of such systems, where the performance of the combined system diminishes compared to half-cell tests or Li-organic cells, in which the conditions can be optimized for the specific electrode material. Solid polymer (polar macromolecules dissolving salts) or gel electrolytes (polymer as mechanical matrix swollen with liquid electrolytes) are often proposed as alternative electrolyte systems but, with some exceptions,76,337 did not yet find their way into the field of polymer-based organic batteries. A drawback of this concept is the limited penetration of the polymer matrix by the electrolyte, restricting the supply with counterions. Furthermore, ionic liquids338,339 are suggested to act as advanced electrolyte, like in a PTMA battery where an ionic-liquid-based polymer gel electrolyte is applied to stabilize the redox system.340 However,

there are only few examples for the application of ionic liquids in organic batteries,118,152,245,341−343 mainly due to their high price and their low ionic conductivity at ambient temperatures.

7. CHARACTERIZATION METHODS 7.1. Preliminary Characterization of Active Compounds

Before a compound can be considered for the application in time-consuming and elaborate device tests, preliminary studies on battery-related characteristics are necessary. Mainly electrochemical but also spectroscopic, spectroelectrochemical, and microscopic techniques are applied to prove the suitability of a species for further tests and to gain insights into key properties and crucial processes.26 7.1.1. Voltammetric Methods. Cyclic voltammetry (CV) is used to determine redox potentials as well as to gain information about the chemical and electrochemical reversibility of involved redox reactions. The redox potential is usually stated as half-wave potential E1/2, which is approximately halfway between the two peak potentials. Differential pulse and square-wave voltammetry serve as valuable supporting techniques providing higher peak resolution and sensitivity.344 Here, E1/2 is found as the peak potential, at least in cases of electrochemically reversible reactions. CV measurements exhibit different kinds of behavior regarding reversibility. A 1:1 ratio of anodic and cathodic peak current, ip,a and ip,c, respectively, and a peak split ΔEp of 57/n mV (at 25 °C, with n as the number of transferred electrons per reaction) point toward a fully electrochemically reversible process (Figure 15a) (i.e., a diffusion-controlled processes with

Figure 15. Representative CV curves for (a) reversible, (b) quasireversible, and (c) irreversible behavior. Reprinted with permission from ref 26. Copyright 2015 John Wiley & Sons.

fast electron transfer). In contrast, for electron-transfercontrolled processes, ΔEp increases with increasing scan rate. If chemical reactions follow the electron transfer, a quasireversible behavior can occur, where the backward peak may be shifted and shows a reduced peak current (Figure 15b). If chemical consecutive reactions occur and turn out to be irreversible, the backward peak vanishes, in particular for slow scan rates (Figure 15c).345,346 9469


Chemical Reviews

Review

Figure 16. (a) Typical voltammograms obtained with a rotating disk working electrode and subsequent analysis toward k0 determination: (b) Levich plot, and (c) Tafel plot. Reprinted with permission from ref 26. Copyright 2015 John Wiley & Sons.

capacitance characteristics of the surface’s electrochemical double-layer. A qualitatively similar behavior is expected in the case of a coated electrode, where the redox-active compound is deposited directly on the electrode surface.349 The analysis of a typical related Nyquist plot allows the extraction of electrontransfer as well as diffusion characteristics. A semicircle in the high-frequency region and a straight line for small AC frequencies, which are both offset by RE, is observed (Figure 17b). The abscissa-based diameter of the semicircle represents RCT, which can be converted to the exchange current i0:

Further parameters, in particular regarding the electrode transfer kinetics, can be figured out through more detailed analysis of the CV behavior (e.g., according to the method developed by Nicholson).35,344,347 Additionally, rotating-diskelectrode (RDE) voltammetry allows for further kinetic studies, namely, the determination of both diffusion coefficients and electron-transfer rates of the redox-active species. The former can be obtained via analysis of the measured voltammetric data (Figure 16, panels a and b) through the Levich equation: ilim = 0.62nFAD2/3ν−1/6ω1/2c0

(9)

R CT =

where ilim is the limiting current, n is the number of transferred electrons per redox reaction, F is the Faraday constant, A the active electrode surface, D the diffusion coefficient, ν the kinematic viscosity of the solution, ω the rotation speed, and c0 the bulk concentration of the redox-active species.345 The subsequent fit of the obtained currents to the Butler−Volmer equation, i = i0[e

−αF / RTη

−e

(1 − α)F / RTη

]

RT nFi0

(11)

thus providing another possibility to access k0. The straight line at lower frequencies is the Warburg impedance and can be expressed via: 2 Im(Z) = Re(Z) − RE − R CT + R CT CD

⎛ k k ⎜⎜ ox + red Dred ⎝ Dox

(10)

with the exchange current i0 and the overpotential η, by means of a Tafel plot (Figure 16c), yields the exchange current, which can readily be converted to the standard heterogeneous transfer rate k0.344,348 7.1.2. Electrochemical Impedance Spectroscopy (EIS). Electrochemical impedance spectroscopy4,349 represents a versatile tool for detailed studies of the electron-transfer characteristics of active electrode materials. A dissolved redoxactive species interacting at an electrode surface can be, in general, described by the equivalent circuit that is depicted in Figure 17a, with RE as the electrolyte’s resistance and RCT as the charge(electron)-transfer resistance at the electrode surface. ZW is a so-called Warburg element, which reflects the diffusion behavior of the redox-active molecules, while CD refers to the

⎞2 ⎟⎟ ⎠

(12)

It intersects the abscissa at ⎛ k k 2 RE + R CT − R CT C D⎜⎜ ox + red D Dred ⎝ ox

⎞2 ⎟⎟ ⎠

k2

2 C D D in case of equal rate which reduces to RE + R CT − 4R CT constants and diffusion coefficients for the oxidation and reduction process and allows, thus, the determination of the diffusion coefficient D.349 7.1.3. Spectroscopic, Spectroelectrochemical, And Surface Characterization Methods. Electron spin resonance (ESR) spectroscopy is of particular importance for studies on redox-active compounds since usually at least one of the species that are involved in the underlying redox reactions is a radical.344,350−355 ESR spectroscopy can provide information about the amount of present radicals (through integration of the received signal) and their location (through the obtained magnetic-field values and the hyperfine structure of the signals) in the ground-state as well as in the oxidized/reduced molecule. For the characterization of the surface composition, in particular of solid-state electrodes, Fourier-transform IR (FTIR) as well as Raman and surface-enhanced Raman scattering (SERS) spectroscopy provide valuable information.356,357 Furthermore, FTIR-spectroelectrochemical experiments allow for a detailed investigation of electrochemical surface pro-

Figure 17. (a) Equivalent circuit for a typical electrochemically active system and (b) respective Nyquist plot. Reprinted with permission from ref 26. Copyright 2015 John Wiley & Sons. 9470


Chemical Reviews

Review

cesses,358 while in situ Raman techniques are widely applied for the analysis of electrolyte and electrode composition.359 Further improvement of the surface analysis is provided by a signal enhancement by a factor from about 1014 to 1016 for the Ramanderived SERS technique.356 Besides, solid-state NMR spectroscopy allows the study of transfer and intercalation processes where NMR-active elements are involved (e.g., 1H, 6Li, 7Li, 13C, 19 F, 31P, and 27Al).358,360,361 X-ray diffraction (XRD) and neutron diffraction enable the exact determination and location of elements within the electrode or electrode coating.358,362 To obtain detailed information about the morphology of the electrode surface, scanning probe microscopy, namely atomicforce (AFM) and scanning-tunneling microscopy (STM), is favorably used. Beside pure morphological investigations concerning the smoothness, the occurrence of defects that lead to the destruction or passivation of the electrode material, and the distribution of components of heterogeneous, phaseseparated surface assemblies, also (redox) processes can be monitored (e.g., via the volume change of the studied film upon reaction).358,363,364 To gain an even more comprehensive idea of the spatial proportion of the surface processes, the microscopic characterization can be combined with IR techniques as described above.356,357 The electrochemical quartz crystal microbalance (EQCM) allows studying the mass change of coatings during electrochemical treatment.358

Figure 18. Typical voltage−time curves for a discharge process: (a) for different redox-system behavior; (b) at different discharge currents, and (c) for different discharge modes. (d) Typical charge/discharge curve. Reprinted with permission from ref 26. Copyright 2015 John Wiley & Sons.

7.2. Device Characterization

V

law R = I ). For the latter, the current changes contrary to the voltage change to maintain a constant power supply (P = IV). While the total charge is the same for all approaches, the current changes and so does the charging behavior (e.g., charging time, Figure 18c). Furthermore, the capacity (or, with respect to the used amount of active material, the specific capacity) of the battery is an essential value; it can be determined in a straightforward manner from the observed charging time and the applied current.365 The comparison of the specific capacity to the theoretical value Ctheo allows an evaluation of the electrochemical effectiveness of the compound. The ratio of discharge and charge capacity is the Coulombic efficiency of a charge/discharge cycle (Figure 18d). Including the operating voltages gives the transferred electrical energy and the energy efficiency.4,5,365 The development of the capacity and Coulombic efficiency over several charge/discharge cycles reflects the so-called cycle life. It represents the long-term applicability of the device and is usually given as the number of cycles until a certain value of the initial capacity is achieved.5 Besides, the device possesses a shelflife time, which is described by the changing of the Coulombic efficiency dependent on the charge-storage time (i.e., the time between the charging of the battery and its discharging) and determined by the stability of the charged state.4 7.2.2. Electrochemical Impedance Spectroscopy. EIS measurements provide, in contrast to most other characterization techniques, not only characteristics of the complete battery but allow also for the study of single components, layers, and interfaces within the system. Hence, the measurements can be executed directly in the completely assembled battery or, if this is not applicable, in a model cell that exhibits all the elements of the actual cell. Consequent analysis of the received data provides separate characteristic values for the different processes within the battery and enables the detailed identification of sources of error and rate-limiting steps. But first, a suitable model, associated with a suitable equivalent circuit, has to be found, which depends on the arrangement and state of aggregation of

The examination of an assembled battery device is usually much more challenging than the determination of the single compounds’ characteristics. Most of the applicable techniques provide only data that describe the overall system; identification of single problems and sources of error demand the comparison to reference setups and experiments, and can thus be timeconsuming. 7.2.1. Charging/Discharging Characteristics. One of the most essential experiments regarding the characterization of a battery is the measurement of the charge/discharge curve (i.e., monitoring the cell voltage between a start and an end voltage during the charging or discharging of the device at a defined electric current). The voltage window must, on the one hand, be large enough to include a crucial portion of the device’s capacity and, on the other hand, small enough to preclude (irreversible) side reactions. Analysis of the resulting curves provides a couple of characteristic values. First, the shape of the curve gives a hint toward the operational stability of the device (Figure 18a). A single, pronounced plateau reflects a stable charge/discharge voltage that is based on a defined, one-step conversion of the redox-active material without disturbing side reactions or kinetic hindrance. Multiple plateaus (i.e., several operating voltages) originate in multiple redox steps. If the curve slopes significantly, a constant operating voltage throughout the charge/discharge process of the device cannot be provided.4,365 Deviations of the operating voltage from the theoretical value can furthermore stem from losses due to internal resistance and polarization effects, which are more pronounced, the higher the applied current (i.e., the C-rate) (Figure 18b). Strong polarization effects of the electrode additionally appear in the form of large differences between the charging and discharging operating voltage.4 Beside the constant-current method, charge/discharge experiments can also be executed under constant resistance or constant power.4 In the former case, the current changes with changing voltage to keep up the initial resistance (according to the Ohmic 9471


Chemical Reviews

Review

the involved layers and interfaces. In principle, there are three basic modes for organic-/polymer-based battery systems to connect the redox-active species to the electrodes: (1) a solutionbased film assembly, where a layer of electroactive material is deposited on an electrode surface contacted by an electrolyte solution to the counter electrode, (2) a pure-solid assembly (i.e., a film framed by two metal electrodes without an electrolyte phase), and (3) a system that is only solution-based (e.g., redoxflow batteries366−368). The mode of assembly determines the applicable equivalent circuit and, thus, the analysis procedure providing the desired characteristic values.26,349 Beside characterization and troubleshooting, EIS is also suitable for an in situ monitoring of the state of charge of the battery since several impedance parameters depend on the exchange current i0, which, in turn, depends on the concentration of the involved species.369−371 7.2.3. Spectroscopic Methods. Although most of the organic and polymeric species that are electrochemically active also show distinct optical properties, spectroscopic studies on complete devices are executed only rarely due to the high concentrations of the materials, which are necessary to facilitate reasonable energy densities. Thus, only model cells possessing a diluted material concentration can be used372 or elaborate, nongeneral corrections have to be developed.373

Figure 19. Development of the world battery market in billion USD. Reprinted with permission from ref 374. Copyright 2013 AIP Publishing.

of organic polymers (

Polymer-Based Organic Batteries - Chemical Reviews (ACS

Recommend Documents