Nanostructured Conjugated Polymers: Toward High Performance

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Nanostructured Conjugated Polymers: Toward High Performance Organic Electrodes for Rechargeable Batteries Jian Xie, Peiyang Gu, and Qichun Zhang ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.7b00494 • Publication Date (Web): 04 Aug 2017 Downloaded from http://pubs.acs.org on August 7, 2017

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ACS Energy Letters

Nanostructured Conjugated Polymers: Toward High Performance Organic Electrodes for Rechargeable Batteries Jian Xie,a Peiyang Gu,a Qichun Zhang*a,b a

School of Materials Science and Engineering, Nanyang Technological University, Singapore,

639798, Singapore. b

Division of Chemistry and Biological Chemistry, School of Physical and Mathematical

Sciences, Nanyang Technological University, Singapore 637371, Singapore

AUTHOR INFORMATION Corresponding Author *Email: [email protected]

ABSTRACT. Nanostructured conjugated polymers are a type of electrochemically active organic material with a high electronic conductivity, good electrochemical stability and superior flexibility. Owing to these advantages, increasing research interest has been recently focused on the utilization of nanostructured conjugated polymers as green and renewable resources for potential high-performance energy storage and conversion devices. This perspective reviews the current situation and the state-of-the-art developments on nanostructured conjugated polymers 1 ACS Paragon Plus Environment

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including their one- & two- dimensional (1D & 2D) structures as well as their three-dimensional (3D) composite forms as electrode materials for rechargeable batteries. In particular, the fundamentals, challenges and possible directions to improve the performance of the current systems are included in this perspective. With sufficient depth and advanced technologies devoted to the exploration of the electrochemistry and mechanism, nanostructured conjugated polymer-based electrodes will be of great scientific interests for future practical applications.

TOC GRAPHICS

The design and development of high-performance, light-weight, flexible and environmentfriendly rechargeable batteries have recently attracted tremendous attentions.1 As one result of these rigorous demands, strong efforts have been conducted to develop promising organic-based electrode materials.2 Earlier approaches on organic electrode materials have been focused on organosulfide, thioether and nitroxyl radical compounds.3 Although some of them were demonstrated to show noteworthy electrochemical performances, there still remain many challenges to develop high energy-density and long cycle life organic electrode materials. For 2 ACS Paragon Plus Environment

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examples, organosulfides and thioethers suffer from serious dissolution problems as well as slow reaction kinetics.4-5 Nitroxyl radical compounds often possess low theoretical capacity values due to their large molecular weight (per repeating unit) and limited reaction electrons involved.5 The advantages of conjugated polymers in comparison to inorganic electrode materials (e.g. LiCoO2,6 Li4Ti5O12,7 etc.) are the excellent electronic conductivity and superior energy storage due to the extended π-conjugation and tunable property through the modification of organic structures as well as introduction of different functional groups or redox moieties.8 More importantly, redox-active conjugated polymers can allow a simple and fast redox process, thus the serious structural alterations that often encountered with inorganic materials can be successfully avoided.9 Due to these superiorities, remarkable rate performance and ultra-long cycle life rechargeable batteries can be potentially achieved on the basis of conjugated polymers. Conjugated polymers have been extensively explored on anion-based battery designs in combination with negative metal electrodes (e.g. lithium (Li), sodium (Na), etc.).10 Nowadays, the most popular rechargeable battery systems are relied on metal-ion technology (especially Liion batteries) for their high energy and power density. Successful developments of organic materials as either positive or negative electrodes on rechargeable Li-ion, Na-ion and even dualion batteries have been demonstrated to show great potential.11-12 Therefore, this perspective will mainly focus on these battery systems. With further explorations of conjugated polymers on novel battery designs, future all-organic batteries or all-solid based flexible batteries with high performance can be obtained. Since nanostructured conjugated polymers exhibit accessible redox states, high surface area and fast kinetics, they are also potential high power-density candidates for electrochemical capacitor (e.g. pseudocapactor & supercapacitor) applications. As

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comprehensive reviews and excellent works on electrochemical capacitors (including 1D & 2D organic materials) have been reported,13-15 we will not cover these in detail here. Since this perspective intends to highlight the representative developments on nanostructured conjugated polymers, we here will present the background of conjugated polymers and review the latest achievements on the state-of-the-art contributions of nanostructured conjugated polymers. Moreover, the challenges and recommendations with regard to improving the performance of nanostructured conjugated polymer electrodes are also discussed. Research on conjugated polymers has become an emerging topic since the first discovery of the conductive polymer polyacetylene (PAc) in the 1970s.16-17 Typical examples of conductive polymers including polyaniline (PANI), polypyrrole (PPy), polythiophene (PTh), poly(3,4ethylenedioxythiophene) (PEDOT), polyparaphenylene (PPP), and polyindole (PI) have been extensively investigated in the past two decades as pure active electrode materials for rechargeable batteries.10 However, due to their low doping availability during the electrochemical reactions, rechargeable batteries based on pure conventional conductive polymer electrodes have already reached the bottleneck of their performance. In order to address this issue, several effective routes have been suggested to improve the electrochemical performance of conjugate polymers.


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Scheme 1. A brief illustration towards high performance conjugated polymer electrodes Scheme 1 shows a brief illustration of the key parameters and effective strategies to achieve high performance conjugated polymer batteries. Of all the strategies, one is to employ nanostructured conductive polymers.18 On the basis of the advantages of nanostructures such as the enlarged surface area, enhanced electrochemical reactivity, and shortened pathway for charge transportation, nanostructured conductive polymers deliver improved performance as the active electrode materials.19 Continuing on with this strategy, conductive polymer nancomposites in the form of hydrogels, combining the advantages of organic and inorganic materials have been developed.20 More recently, a new type of heterocyclic conjugated polymer representatives includes

poly(benzobisimidazobenzophenanthroline)

(BBL)

and

dihydropyrazino[2,3g]quinoxaline-2,3,8-triyl-7-(2H)-ylidene-7,8-dimethylidene)

poly(1,6(PQL),

with

ladder-like heterocyclic structures, have been demonstrated to show good performance as the active electrode materials for rechargeable batteries.21-22 Importantly, by extending these organic compounds from one-dimension (1D) to two-dimensional (2D) structures, conjugated microporous polymers (CMPs), covalent organic frameworks (COFs) and their redox-active structures have been developed to show more promising results as electrode materials for rechargeable batteries.23-29 To summarize the above-mentioned strategies as well as the representative demonstrations, the structures of nanostructured conjugated polymers that have been successfully employed as electrode materials for rechargeable batteries are listed in Figure 1, which will be further discussed in the following sections. To be noted that, the theoretical capacity values and number of electrons (per repeating unit) in Figure 1 are calculated based on their structures alone (before doping) with regard to the highest electron transportation capability from the literature studies.


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The key properties and performances of typical electrode materials including commerciallyavailable inorganic materials and representative organic materials are compared and listed in table 1.

Figure 1. Structures and theoretical capacity calculations of typical conjugated polymers, including conventional conductive polymers and novel conjugated polymers. Table 1. Comparison table includes the key properties of outstanding inorganic and organic electrode materials. Types

Electrode Material (electrolyte doping species)

Nanostru ctures (diameter in nm)

Electronic Conductiv ity (S cm-1)

Initial Reversible Capacity (mAh g-1)

Working voltage (V)

Cyclability (mAh g-1)

Commercial Inorganic Materials

LiCoO26

-

8.5 x 10-6

141

4.3-3.6

137 after 100 cycles

at 0.1 C

Li4Ti5O127

-

10-13

150

2.0-1.2

145 after 250 cycles

73 mAh g-1 at 3.2 A g-1

Organosulfide

PDMcT4

-

10-7

240

3.8-1.8

10 after 10 cycles

Rate Capability (at maximum current density)

at 20 mA g-1


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Thioether

PTBDT5

-

-

~240

4.2-1.4

~560 after 20 cycles ( large polarization observed)

Nitroxyl Radical

PTMA (Li+ & PF6-)30

-

-

77

4.0-3.0

~68 after 500 cycles

At 0.1 mA cm-2

Conventional Conductive Polymers

PANI (ClO4& Li+)31

50-100

7.1

75.7

3.9-2.0

75.7 after 80 cycles

at 20 mA g-1

Bulk forms

5 x 10-1

~170

4.25-2.5


at 1 C

PI (PF6-)32

50-100

1.9 x 10-3

~73

4.0-2.0

55.0 after 100 cycles

at 0.1 mA

PPy (SO42-)33

200-400

-

52.2

1.5-0.1


at 0.1 C

PPP (PF6- & Li+)34

Irregular shapes

-

80 (p-doping)

4.6-3.0

70 after 100 cycles

at 40 mA g-1

400 (n-doping)

3.0-0.0


at 40 mA g-1

Novel Conjugated Polymers (including 1D & 2D)

at 50 mA g-1

BBL (Li+)21

~30

-

1787

3.0-0.0

1181 after 50 cycles & 496 after 1000 cycles

461 mAh g-1 at ~5.9 A g-1

PQL (Li+)35

40-60

2.1 x 10-3

1444

3.0-0.0

1770 after 100 cycles & ~500 after 1000 cycles

303 mAh g-1 at ~9.1 A g-1

PDA (Li+)36

~500

-

1818

3.0-0.0

1414 after 580 cycles & 500 after 1024 cycles

~900 mAh g-1 at 3.2 A g-1

Microporous HATN-CMP (Li+)37

~100

-

147

4.0-1.5

91 after 50 cycles

50 mAh g-1 at 1.0 A g-1

BPOE (ClO4& Na+)23

~300

4.09 x 10-6

55 (p-doping)

4.1-1.3

~200 after 50 cycles & ~160 after 1000 cycles

~45 mAh g-1 at 5.0 A g-1

185 (n-doping)

Principles and essential parameters. When a conjugated polymer is deployed as an active electrode material in an electrochemical cell, it can accept or release ions during the electrochemical oxidation and reduction.38 Such an ion accepting or releasing movement is regarded as the polymer doping or dedoping process, wherein the electric energy is stored via the movement of the ions within the π-systems along the polymer chains.39 Moreover, doping is a reversible process that makes the conjugated polymers positively or negatively charged, enabling the energy storage applications.8 Depending on the types of ions (anions, cations or both ions) that are incorporated into the organic structures, conjugated polymers are classified into three different groups: p-, n-, and bipolar type (Equation 1 & 2, bipolar type encounters both p- & ndoping).40 7 ACS Paragon Plus Environment

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p-doping (PF6- as an anion example): (Conjugated Polymer) + PF → [(Conjugated Polymer) (PF ) ] + (1) n-doping (Li+ as a cation example): (Conjugated Polymer) + Li + → [(Li ) (Conjugated Polymer) ]

(2)

is the doping level

The theoretical capacity (Cth) indicates the maximum charges (in terms of anions and cations) that can be stored in an electrode.41 It can be derived from Equation 3 that the theoretical capacity of an electrode material can be improved from two aspects. One is through the adoption of multielectron reactions. With more active sites and electrons involved into the electrochemical reactions, the specific capacity will be increased several folds. The other is through the reduction of molecular weight. Since organic materials are often consisted of low-molecular weight atoms (e.g. C, H, O, N, and S) and the structures can be designed and tailored with multielectron activities, organic materials are highly desirable for future high capacity electrode materials.11

"#$ =

& × ( )*

=

& × +,-. /0 12345 6 )* (7 12345 )

=

& × 8-9: )*

(mAh g : ) (3)

Energy density is a parameter to measure the maximum energy that an electrode can provide. The theoretical energy (Eth) can be obtained by Equation 4. A battery with a promising energy density requires both high specific capacity (Cth) and high voltage (V). Here, the voltage of an electrochemical cell (V) represents the voltage difference between cathode (Vc) and anode (Va) (= = => − =@ (volt)). Thus in a battery fabrication, the operating voltage of cathode materials 8 ACS Paragon Plus Environment

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should be as high as possible, whereas for anode materials it should be as low as possible. Regarding power density, for those materials that can provide high capacity values at high current rates, high power density performance can be obtained enabling the application of fast charge-discharge rechargeable batteries.

A#$ = "#$ × = (Wh g : ) (4)

Early studies have already proven the capability of conductive polymers to be preferably employed as the active cathode materials, since most of them were developed in their p-doped states through oxidation polymerization.8 Jean Roncali summarized the synthesis and bandgap control of conjugated polymers.42 The most attractive advantage of conductive polymer cathodes is their relatively high operating voltage upon charging and discharging, usually ranging from 34 V.8, 43 Typically, the anions from the electrolyte solution can be continually “doped” into the polymer structures at a high voltage to achieve their semi-oxidation or oxidation state (known as charging). A variety of anion electrolyte species such as PF6 -, ClO4-, BF4- and AsF6-, etc., have been successfully incorporated into conductive polymer structures.10 However, it is essentially difficult for conductive polymers to achieve their full oxidation states due to their poor intrinsic electronic conductivity and low doping availability (at most ~ 0.3 to 0.5).40 This challenge has been widely recognized in conductive polymers. High levels of doping will result in permanent destruction of polymer chains and subsequently lead to unsatisfied electrochemical performance.44-46 Studies in the 1980s indicated that the electroactivity and the charge state of conjugated polymers would be destroyed when charging them beyond a certain voltage limit, known as the overoxidation process.44 Conditions such as


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solvent and electrolyte species could also affect this process and result in different levels of doping. As a result, the life time of rechargeable batteries will be shortened. Examples including PAc, PPy, PEDOT, etc. have been found to suffer from this degradation process, though the exact chemical changes of these materials were not extensively studied.45-47 To get a deeper understanding, D. Aurbach and M.D. Levi wrote a short review and discussed several strategies (e.g. by the introduction of different electron-withdrawing groups) to improve the doping availability.48 One recent investigation demonstrated the changes of morphologies and crystallinity of PEDOT film upon overoxidation.46 From the perspective of ionization or doping, the more a conjugated polymer chain is doped, the more “charge island” overlaps and delocalizes over the material, thus improving the electronic conductivity of conjugated polymers.39,

49

Of

course, to get a high electronic conductivity and electrochemical performance, factors such as charge carriers, counterion species and reaction kinetics, etc. should also be considered. In addition to doping, the charge transfer and polaron formation of conjugated polymers has also been discussed.39, 50

Nanostructured conventional conductive polymers. Despite the disadvantages from over-doping, one of the greatest advantages of conductive polymers is the excellent electronic conductivity (upon pre-doping) as well as the good flexibility, particularly at their nanosized scales. Pioneering investigations have been performed to study nanostructured conductive polymerbased active electrodes.51 One early strategy to improve the electrochemical performance of conductive polymers is to utilize the increased electroactive surface area and the enhanced electronic conductivity of their nanostructures. This was confirmed by a comparison study between ClO4- pre-doped PANI bulk powders and nanotubes.52 With an almost six-time-higher


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electronic conductivity, ClO4- pre-doped PANI nanotubes (~300 nm, 7.1 S cm-1) delivered a significantly-improved specific capacity compared to the unmodified ClO4- pre-doped PANI powders (1.2 S cm-1) (Figure 2a&b). Other than PANI, a study on Cl- pre-doped PI nanoparticles (50-100 nm) also presented a considerable electronic conductivity and cycling stability.32 Nevertheless, the practical capacity of PI was moderate and could be further increased through the optimization of synthetic conditions. Moreover, an aqueous electrolyte (Li2SO4) has been successfully involved into a SO42- pre-doped PPy nanoparticles (200-400 nm),53 which were assembled with LiCoO2 to demonstrate a full battery. However, such a demonstration only delivered a low working voltage of around 0.9 V, which is unsatisfactory for the requirement of a full battery (preferably >1.5 V). To search for bipolar conductive polymers as active electrodes, PPP stands out due to its p- and n-dopable property.54 This excellent property endows PPP with a large operating voltage (>2.4 V) and a potential high energy density (Figure 2c). Therefore, a full battery employing bipolar PPP nanoparticles as both anode and cathode was successfully fabricated.55 Though the assembled cell displayed a small capacity gap between the charge (ptype) and discharge (n-type) capacity values (Figure 2d), this was one of the few earliest attempts to develop an all-organic battery.


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Figure 2. Charge-discharge profiles of (a) ClO4- doped bulk PANI & (b) ClO4- doped PANI nanotubes (inset is the TEM image of ClO4- doped PANI nanotubes). (c) Cyclic voltammetry (CV) profiles of PPP at the n-dopable (0-3 V) and p-dopable (3-4.6 V) regions (inset is the SEM image of PPP aggregates). (d) Charge-discharge profiles of an all-organic battery employing PPP as both the anode and cathode at 40 mAh g-1. Reproduced with permission from refs 52 and 55. Copyright Wiley-VCH Verlag GmbH & Co. KGaA. Copyright Royal Society of Chemistry. Note that the examples of nanostructured conjugated polymers as pure cathode materials are still rare. Although conductive polymers with pre-doped nanostructures show obvious advantages over their bulk forms, the maximum doping level of conductive polymers has not been effectively improved. Therefore, their practical capacities are quite low and far behind the theoretical capacities. It is extremely difficult to further enhance the doping availability of conductive polymers through modification and nanoengineering of their morphologies because they easily become unstable when over-doped, attributing to the serious charge repulsion among their structural units.8

Nanostructured conductive polymer composites. After realizing the pros and cons, efforts on nanostructured conjugated polymers draw great attention to the utilization of their hybrid and composite forms. State-of-the-art investigations show successful experience on the fabrication of conductive polymer based three-dimensional (3D) conductive hydrogels.56-59 The unique crosslinked networks possess large surface areas and good structural flexibilities along with high conductivity and rapid charge transportation behaviors. In addition, such cross-linked flexible frameworks can provide a great number of free space to compensate the significant volume expansions of inorganic electrode materials.


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By adopting this concept, nanostructured conductive polymer hydrogels have been greatly investigated. One example showed the demonstration of a 3D porous PANi-Silicon (PANI-Si) hydrogen nanocomposite (60-100 nm) (Figure 3a) with a foam-like network as anode materials.56 The as-fabricated batteries exhibited an ultra-stable cycling performance and there was no significant capacity fading after 5000 cycles (Figure 3b). Following this direction, a PPySi-carbon nanotube (PPy-Si-CNT) ternary network was developed (Figure 3c)57 and delivered an outstanding electrochemical performance. More recently, a high performance nanocomposite hydrogel PPy-Fe3O4 anode (Figure 3d, right) was developed and the structure of the hydrogel framework could be well maintained after 50 cycles of reversible charging and discharging (Figure 3e).59 With these strategies, more nanocomposite-based electrodes with high performance can be obtained to overcome the challenges (e.g. volume expansion, short cycle life and low capacity retention) of traditional metal- or metal oxide-based inorganic electrode materials.

Figure 3. (a) TEM image of PANI-Si. The Si nanoparticles (blue arrow) are coated with uniform PANI layers (red arrow). (b) Ultra-long cycling of PANI-Si nanocomposite electrode between 0.01 and 1.0 V at 1 A g-1. (c) SEM of PPy-CNT-Si nanocomposite ternary electrode. (d)


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Illustration of a traditional electrode system (left) and a novel nanostructured PPy-Fe3O4 conductive hydrogen electrode system (right). (e) SEM image of PPy-Fe3O4 electrode after the cycling test. Reproduced with permission from refs 56, 57 and 59. Copyright Nature Publishing Group. Copyright American Chemical Society. Copyright Wiley-VCH Verlag GmbH & Co. KGaA. Although conductive polymer-based nanocomposites present promising electrochemical performance as electrode materials, the capacity mainly contributes from the active inorganic materials. In above studies, conductive polymers in these composite structures only dominate as the coatings or networks to promote the electron transportation as well as the ion diffusion. Highly-performed pure nanostructured conjugated polymers are still rare and yet to be discovered.

Nanostructured novel heterocyclic conjugated polymers (1D). Toward the target of achieving sustainable and high performance organic-based batteries, more investigations should be performed on the employment of pure nanostructured conjugated polymers as the active electrode materials. As mentioned previously, conventional conductive polymers are usually used as the active cathode materials because they prefer to exist in their p-doped state, either through pre-doping or electrochemical oxidation to a high voltage. Much less investigations have been conducted on exploring the electrochemical performance of conjugated polymers in their ndoped states, though few conductive polymers showed n-dopable behaviors, where the cations can be n-doped into the organic structures.40 Rather than only employing conventional conductive polymers, it is therefore highly desirable to design and develop novel conjugated polymers that are n-dopable with good performance. Unlike the conventional conductive


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polymers that the doping process occurs through delocalization of the ions (mostly cations) along the polymer backbone,38 novel conjugated organic materials show high energy storage through the insertion of metal cations (e.g. Li+, Na+, etc.) onto the atoms with lone pair electrons (e.g. oxygen and nitrogen) and the unsaturated carbon atoms. This novel mechanism was first discovered by Sun et al on a small conjugated molecule NTCDA and further confirmed by several groups.60-63 However, even though a large amount of cations can be inserted onto the organic structure of these small molecules, the high Li+ storage can only be sustained at the initial cycles. The cycling stability of these molecules were poor due to serious dissolution problems. From the perspective of molecular design, our investigations revealed that conjugated ladder polymers could be good substitutes for high performance and fast storage electrode materials.21, 64

In addition, the nanostructured conjugated polymers can show high electronic conductivity

even without pre-doping.22 We demonstrated the design and development of several heterocyclic nanostructured conjugated polymers as anode materials for lithium-organic batteries. Lithium cations (Li+) were found that can be effectively inserted into the organic structures between 3.0-0 V. Unlike small organic molecules that encounter serious dissolutions, conjugated polymers with large aromatic backbones and repeatable units could not be easily dissolved into the non-aqueous organic electrolytes. These ladder-conjugated polymers can be synthesized through a one-pot synthesis.21-22 Different from the traditional methods to obtain polymer nanostructures (e.g. oxidation

polymerization,

electrochemical

polymerization,

self-assembly,

etc.),65

the

nanostructures of novel heterocyclic conjugated polymers can be obtained through a simple precipitation method (Figure 4a). The rigid ladder-like structures with extended π-conjugations are believed to be beneficial to the charge transfer during the electrochemical reactions.66 In


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addition, the presence of oxygen and nitrogen heteroatoms in the conjugated backbones can enhance the electrochemical reactivity of the materials,60,

67

and hence improve the overall

battery performance in terms of specific capacity, cycling stability and rate performance. The nanoparticles of conjugated BBL and its derivative SBBL were obtained with diameters around 30 and 70 nm, respectively (Figure 4b&c).21 Compared to the as-prepared microsized BBL and SBBL aggregates, nanostructured BBL and SBBL show much improved electrochemical performance. Attributing to the large π-conjugated backbone and heteroatomrich multi-aromatic system, BBL and SBBL nanoparticles can accept a large amount of around 22.3 and 15.2 Li+, respectively (per repeating unit). These remarkable high Li+ storage led to a large initial reversible capacity of 1787 and 1603 mAh g-1 of BBL and SBBL, corresponding to a high value of 93.1% and 84.9% of their theoretical capacity values. However, the capacity performance of BBL and SBLL showed rapid declination and only 66% and 58% of the retentions were achieved after 50 cycles. The fast deterioration of the two materials might be due to the low polymerization of the compounds with large backbones. Therefore, our further studies focused on conjugated polymers that could be obtained by small backbone reactants.


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Figure 4. (a) Schematic illustration of a simple precipitation method. Step I: The conjugated polymer/acid solution is added dropwise into a rapid-stirring solvent. Step II: The nanostructured conjugated polymers will be rapidly precipitated out from the solvent. (b) SEM image of BBL nanoparticles. (c) SEM image of SBBL nanoparticles. (d) Cycling performance of PQL nanoparticles at 100 mAh g-1 (Inset is the SEM image). (e) Charge-discharge profiles of PPCQ nanoparticles for the first two cycles at 100 mAh g-1 (Inset is the SEM image). (f) Long-term cycling of PDA electrode at 500 mAh g-1 (Inset is the SEM image of as-prepared PDA). Reproduced with permission from refs 21, 22, 64 and 36. Copyright Wiley-VCH Verlag GmbH & Co. KGaA. Copyright American Chemical Society. To search the better stability of organic materials, one promising type of conjugated polymers is heterocyclic conjugated polyazaacene, which possesses a rich number of nitrogen atoms. To find out the doping availability of such a type of conjugated polymer, a novel polyazaacene PQL was synthesized and investigated.22 Confirmed by the four-point measurement, PQL nanoparticles showed significantly improved electronic conductivity (2.1x10-3 Scm-1) comparing to its pristine state (1x10-5 Scm-1) due to slight doping by acid. Investigated by both the electrochemical cell fabrication and density functional theory (DFT), PQL nanoparticles (~50 nm) could be maximum inserted with 13.6 Li+, approximating to a high theoretical capacity of 1822 mAh g-1. Although the practical capacity (Figure 4d) could not reach the maximum theoretical capacity due to the kinetic reaction limitation, it belongs to one of the highest values for organic anodes (1444 mAh g-1). To be highlighted that, other than the superior battery performance at room temperature, BBL, SBBL and PQL nanoparticles were found to exhibit better performance at a high temperature of 50 °C, demonstrating the possible development of high-temperature organic batteries in future.


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In

the

following

works,

a

nanostructured

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oligomer

poly(1,4-dihydro-11H-

pyrazino[2',3':3,4]cyclopenta[1,2-b]quinoxalin-11-one) (PPCQ) (>100 nm) with the insertion of a five-membered ring was synthesized and showed a high initial lithiation that about 13 Li+ (~1678 mAh g-1) can be accepted by the conjugated structure (Figure 4e).64 Inspired by the redox availability of catechol/o-benzoquinone, Zhang et al. investigated the biodegradable polydopamine (PDA) as both the binder and active anode materials.36 The PDA microspheres (~500 nm) could accept about 10 Li+ at a high current density of 500 mAh g-1. Such a high Li+ storage could be well maintained with a high retention of 93% after 580 cycles (Figure 4f). Most importantly, all above-mentioned nanostructured conjugated polymers show excellent cycling stability and possess high capacity retentions even after ultra-long cycles of charging and discharging. The investigations on novel heterocyclic nanostructured conjugated polymers point out a promising direction to achieve high-performance rechargeable batteries. Considering the poor solubility and amorphous state of polymer materials, comprehensive understandings of the Li+ doping as well as the solid electrolyte interphase (SEI) mechanisms through in-situ or ex-situ studies become essentially important. Of course, prior to study the mechanisms, the exact information of this novel types of 1D conjugated polymers, for examples, the molecular weight, number of repeating units, structural defects, etc., should be obtained through further advanced characterizations. Although it is very challenging, a better understanding of the electrochemistry of such a promising type of material is of great importance.

Nanostructured novel heterocyclic conjugated polymers (2D). Regarding the two-dimensional (2D) conjugated systems, investigations demonstrated that nanostructured conjugated polymers


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including CMPs, COFs and similar aromatic frameworks could be potential electrode materials with superior performance (e.g. high specific capacity, long cycles and high power density). These polymers show several unique features such as poor solubility in most solvents, large surface area, high structural stability and more importantly synthetic diversity with different building blocks. It was reported that microporous hexaazatrinaphthalene-based CMP (HATNCMP) could be a good redox-active cathode material (Figure 5a &b) with the power density ranges from 70 to 706 W kg-1.37 The mechanism was further confirmed by a recent study on a single-crystal based small-molecule HATN (or so-called triquinoxalinylene (3Q)),68 which shows an excellent reversibility of Li+ doping. 3Q delivered a high specific capacity (395 mAh g1

) (Figure 5c) and a super-high stability of 10000 cycles at 8 A g-1, and the power density was as

high as ~8 kW kg-1 . Jiang et al. synthesized several nanostructured triphenylamine (TPA)-based CMPs (OPTPA, SPTPA and YPTPA) with high surface areas24 (Figure 5g). These cathode materials exhibit good rate stability and fast charge-discharge capability during the electrochemical doping of PF6- anions, especially that YPTPA can provide a stable capacity of ~93 mAh g-1 at 2 A g-1 after 1170 cycles (Figure 5h). The power density of YPTPA could reach ~6.4 kW kg-1. As for anode applications, Cui et al. obtained a CMP named PDCzBT with a high reversible capacity of 1042 mAh g-1 (vs. Li+) and a good rate performance.25 These outstanding results revealed successful developments of CMPs to be used as effective cathode materials.


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Figure 5. (a) Molecular structure of HATN based conjugated polymer (HATN-CMP). (b) SEM image

of

HATN-CMP.

(c)

Initial

charge-discharge

profiles

of

a

small-molecule

hexaazatrinaphthalene (or triquinoxalinylene 3Q) between 1.2 and 3.9 V at 400 mAh g-1. (d) Structure of an aromatic porous-honeycomb conjugated polymer (BPOE). (e) TEM image of BPOE showing a sheet-like morphology. (f) Ultra-long cycling performance of BPOE as the electrode for a sodium-ion battery between 1.3 and 4.1 V at 1.0 A g-1. (g) Structures of the three CMPs. Reproduced with permission from refs 37, 68, 24 and 23. Copyright Royal Society of Chemistry. Copyright Nature Publishing Group. Copyright Elsevier Ltd. The development of crystalline or semi-crystalline organic structures with strong covalent bonds is also of great potential and a lot of COFs have been developed by Jiang and coworkers.69 Other than as electrode materials, a demonstration revealed that COFs could also be suitable materials for solid-state electrolyte applications, attributing to the fast lithium-ion conductivity.70 20 ACS Paragon Plus Environment

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A recent review from our group provides sufficient examples of COFs as potential candidates for energy-related applications including rechargeable batteries,26 therefore it is not necessary for us to provide a comprehensive overview on COFs in this perspective. To be noted that, so far only a few nanostructured conjugated polymers (e.g. PAc and PPP) show bipolar property that can be either n-dope or p-doped in an assembled electrochemical cell. Investigations show that bipolar COFs including their nanostructures might be good substitutes for bipolar electrode materials with high energy storage.23, 71 To be highlighted that, an aromatic porous-honeycomb conjugated polymer (BPOE) (Figure 5d) with a size of around 300 nm (Figure 5e) was investigated to show both p-dopable and n-dopable (vs. ClO4- and Na+, respectively) capability, sustaining a long cycle life (80% of the initial capacity value72) of 7000 cycles (Figure 5f).23 Remarkably, BPOE has a power density of 10 kW kg-1. However, there is a drawback that such a material displays a capacitive behavior, probably resulting from the pesudo-Faradaic reactions that occur at the surface of this material.73 Therefore, regarding such a pesudocapacitive process, equation 4 is not suitable. Similarly, many conjugated polymers (including 1D and 2D structures) actually show such an undesirable sloping behavior (Figure 6a) due to the generation of a broad distribution of redox states during the electrochemical reactions. To overcome this issue, a novel strategy was proposed by designing π-conjugated redox polymers that combine the advantages of conjugated units and redox-active sites (Figure 6b).74 As a result of this strategy, a high performance cathode material P(NDI2OD-T2) with a flat plateau, a reduced polarization and a significant high level of doping (n-type, vs. Li+) can be obtained. The reversible doping was maintained for 3000 cycles with a high capacity retention of 96%. This strategy has also been applied on conjugated poly(2,5-dimethoxyaniline) PDMA and cross-conjugated quinones to obtain high performance


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organic batteries.75-77 Even 2D conjugated or non-conjugated polyimide-based COF and CMP structures with redox-active carbonyl groups were developed, providing stable performances.2728, 78

Very recently, Stoddart JF et al. performed a promising study on redox-active

macrocycles.79 They suggested that the electron repulsion can be minimized by controlling the conformational dispositions of redox-active units in macrocycles. Different voltage profiles of pyromellitic diimide (PMDI) derivatives were obtained (Figure 6c), in particular, the triangular ((-)-3PMDI-∆) macrocycle exhibit through-space electron delocalization with a well-defined single plateau compared to the multi-plateau behaviors in other derivatives. It will be quite interesting to investigate the nanostructures of these materials through future studies. All these strategies provide opportunities to achieve high performance organic-based rechargeable batteries.

Figure 6. (a) Different behaviors of voltage curves during the discharge. (b) Design strategy of conjugated redox polymers. (c) Voltage profiles and structures of the three PMDI derivatives. Reproduced with permission from refs 74 and 79. Copyright American Chemical Society.


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Summary and Future Outlook. Numerous studies have been conducted to achieve promising nanostructured conjugated polymer electrodes and most of them have already met the competitive requirements of specific capacity, cycling stability and Coulombic efficiency. With novel design strategies and in-depth investigations, we believe that nanostructured conjugated polymers might not only be good candidates for rechargeable metal-ion batteries, but also be potential organic materials for hydronium80 and proton81 batteries. As mentioned in the previous section, there is one major challenge for currently developed conjugated polymers. The highly sloping voltage behavior becomes a severe disadvantage for them to be effectively used in real applications. This characteristic becomes more distinct at higher current densities. Curves with flat plateaus indicate a stable electrochemical activity without disturbing side reactions and kinetic limitations.82 For the cases of nanostructured conjugated polymers, factors contributing to this behavior could involve the intrinsic insulation property, amorphous state of polymers, morphology & size and charge repulsion between the repeating units in the polymer chains during the electrochemical reactions. To achieve practical conjugated polymer electrodes, it is therefore highly recommended to carry out further investigations on the polarization losses as well as the hysteresis effect of nanostructured conjugated polymers. With novel design strategies, we believe that nanostructured conjugated redox-active polymers would open a new window for future development of practical polymer-based rechargeable batteries. The doping availability of conventional p-type conductive polymers is usually low, leading to a low specific capacity of the batteries. Much less studies have been performed on the electrochemistry of the n-doped states of conductive polymers. Through n-doping of small-sized cations (usually metal ions), the energy storage of conductive polymers could be increased compared with that of bulky-sized anions. Very recently, a n-doped (Li+ doped) PANi with a


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high electronic conductivity of 3 S cm-1 was obtained, delivering a high specific capacity of 230 mAh g-1 with a stable capacity for 400 cycles.83 The results are much superior to the conventional p-doped conductive polymers. Considering the high operating voltage of conductive polymers, it is of critical importance to conduct more investigations to increase the capacity of conductive polymers through the investigation of their n-doping capability. To achieve this, the exploration of novel synthetic routes, different pre-doping or doping species and a variety of morphology controls should be taken into account. Concerning the specific capacity of conjugated polymers, the values calculated by different scientists might not always follow the same basis. For example, a ClO4- predoped (doping level, Cl/N ≈ 0.5) possess a theoretical specific capacity of 95.2 mAh g-1 in contrast to that of 147 mAh g-1 referring to PANI alone.31 To be consistent, there should be a standard with regard to the calculation, otherwise the capacity difference will be obvious, especially for the cases of large molecular weight doping anions. Since conjugated polymers have low intrinsic electronic conductivities, a large amount of carbon additives (e.g. Super P, graphene, CNT) are usually added to improve the electron transportation within the cells. Sometimes, the weight percentage of carbon additives could reach a high value of >80%.84 This will decrease the utilization of active conjugated polymers and reduce the energy density of the cells accordingly. Therefore, studies on high-loading conjugated polymers are also welcomed to achieve practical rechargeable batteries. Last but not least, the consideration of a suitable electrolyte system is also important to achieve high-performance organic electrodes. It is highly encouraged to select a compatible electrolyte system with a favorable ionic mobility and a good ion-solvent interaction with the electrode materials during the electrochemical reactions.85-86 This requires a lot of prior studies regarding


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the testing and modification of different electrolyte systems (e.g. ester or ether based solvents). In addition, electrolyte solutions with different concentrations and additives could affect the behavior of electrode materials dramatically.87-88 More recently, advanced electrolyte (quasisolid or solid state, aqueous, etc.) systems are believed to show better performance.89-91 Therefore, through further exploration and optimization of the electrolyte systems, improved electrochemical performance of conjugated polymers can be obtained in near future. With deeper investigations on advanced electrode and electrolyte materials, it is promising to design all organic-based full batteries on the basis of nanostructured conjugated polymers. By adopting this concept, future high density flexible or wearable batteries can be obtained with extensive application prospects.

AUTHOR INFORMATION Corresponding Author *Email: [email protected] Notes The authors declare no competing financial interest. Biographies Jian Xie received his B. Eng degree in Materials Science and Engineering (MSE) from Nanyang Technological University (NTU). He is a PhD candidate in Prof. Qichun Zhang’s group, NTU, Singapore. He currently focuses on the development of organic electrode materials for rechargeable lithium and sodium batteries.


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Dr Pei yang Gu received his BSc, MSc, and PhD degree in applied chemistry at College of Chemistry, Chemical Engineering and Materials Science of Soochow University. He is currently a research fellow at MSE of NTU under the supervision of Prof Qichun Zhang. His research expertise is in synthesis and application of organic semiconductor materials. Dr. Qichun Zhang is an associate Professor at School of Materials Science and Engineering and an adjunct associate professor at Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences at Nanyang Technological University (NTU, Singapore). He received TCT fellowship in 2013 and lectureship from National Taiwan University in 2014. Currently, he is an associate editor of J. Solid State Chemistry, the Advisory board member of Materials Chemistry Frontiers, and the Advisory board member of Inorganic Chemistry Frontiers. Currently, he is a fellow of the Royal Society of Chemistry. He has published > 251 papers and 4 patents (H-index: 51).

Quotes: 1.

However, due to their low doping availability during the electrochemical reactions,

rechargeable batteries based on pure conventional conductive polymer electrodes have already reached the bottleneck of their performance. 2.

Despite the disadvantages from over-doping, one of the greatest advantages of conductive

polymers is the excellent electronic conductivity (upon pre-doping) as well as the good flexibility, particularly at their nanoscopic scales.


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3.

After realizing the pros and cons, efforts on nanostructured conjugated polymers draw

great attention to the utilization of their hybrid and composite forms. 4.

Toward the target of achieving sustainable and high performance organic-based batteries,

more investigations should be performed on the employment of pure nanostructured conjugated polymers as the active electrode materials. 5.

To achieve practical conjugated polymer electrodes, it is therefore highly recommended

to carry out further investigations on the polarization losses as well as the hysteresis effect of nanostructured conjugated polymers.

ACKNOWLEDGMENT Q. Z. acknowledges financial support from Academic Research Fund Tier 1 (RG13/15 and RG 8/16) from the Ministry of Education, Singapore.

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Nanostructured Conjugated Polymers: Toward High Performance

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