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Review Cite This: Chem. Rev. 2018, 118, 8936−8982

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Exploring Chemical, Mechanical, and Electrical Functionalities of Binders for Advanced Energy-Storage Devices Hao Chen,†,§ Min Ling,†,‡,§ Luke Hencz,† Han Yeu Ling,† Gaoran Li,‡ Zhan Lin,*,⊥ Gao Liu,*,∥ and Shanqing Zhang*,†

Chem. Rev. 2018.118:8936-8982. Downloaded from pubs.acs.org by UNIV OF LOUISIANA AT LAFAYETTE on 01/23/19. For personal use only.

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Centre for Clean Environment and Energy, School of Environment and Science, Griffith University, Gold Coast Campus, Gold Coast, Queensland 4222, Australia ‡ Zhejiang Provincial Key Laboratory of Advanced Chemical Engineering Manufacture Technology, College of Chemical and Biological Engineering, Zhejiang University, Hangzhou 310027, China ∥ Electrochemistry Division, Lawrence Berkeley National Lab, San Francisco, California 94720, United States ⊥ Electrochemical NanoEnergy Group, School of Chemical Engineering and Light Industry at Guangdong University of Technology, Guangzhou, China ABSTRACT: Tremendous efforts have been devoted to the development of electrode materials, electrolytes, and separators of energy-storage devices to address the fundamental needs of emerging technologies such as electric vehicles, artificial intelligence, and virtual reality. However, binders, as an important component of energy-storage devices, are yet to receive similar attention. Polyvinylidene fluoride (PVDF) has been the dominant binder in the battery industry for decades despite several well-recognized drawbacks, i.e., limited binding strength due to the lack of chemical bonds with electroactive materials, insufficient mechanical properties, and low electronic and lithium-ion conductivities. The limited binding function cannot meet inherent demands of emerging electrode materials with high capacities such as silicon anodes and sulfur cathodes. To address these concerns, in this review we divide the binding between active materials and binders into two major mechanisms: mechanical interlocking and interfacial binding forces. We review existing and emerging binders, binding technology used in energy-storage devices (including lithium-ion batteries, lithium−sulfur batteries, sodium-ion batteries, and supercapacitors), and state-of-the-art mechanical characterization and computational methods for binder research. Finally, we propose prospective next-generation binders for energy-storage devices from the molecular level to the macro level. Functional binders will play crucial roles in future high-performance energy-storage devices.

CONTENTS 1. Introduction 2. Types of Binders 2.1. Natural Binders 2.2. Synthetic Binders 2.3. Nonreactive Binders 2.4. Reactive Binders 3. Binding Mechanism 4. Binder Properties on Electrode Fabrication 4.1. Thermal Properties 4.2. Mechanical Properties 4.3. Electrical and Ionic Conductivity 4.4. Dispersion Properties 4.5. Chemical Stability 4.6. Electrochemical Stability 5. Strategies in Developing Binders and Binding Processes 5.1. Strategies To Enhance Mechanical Interlocking 5.1.1. Manipulation of Bulk Mechanical Strength © 2018 American Chemical Society

5.1.2. Maximizing Interfacial Contact 5.2. Strategies for Enhanced Interfacial Bonding 5.2.1. Creation of Strong Intermolecular Forces 5.2.2. Electrostatic Bonding 5.2.3. Covalent Bonding 5.3. Binders with Special Properties 5.3.1. Multifunctional Polymer 5.3.2. Conductive Binders 5.3.3. Self-Healing Binders 5.3.4. Redox-Active Binder 6. Binders in Different Energy Materials 6.1. Binders in Cathodes of Li-Ion Batteries 6.1.1. Layered LiMO2 (M = Co, Ni, Mn, Al) 6.1.2. Olivine LiFePO4 6.1.3. Spinel LiMn2O4 6.1.4. Other Cathodes 6.2. Binders in Anodes of Li-Ion Batteries 6.2.1. Graphite

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8943 Received: April 14, 2018 Published: August 22, 2018

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DOI: 10.1021/acs.chemrev.8b00241 Chem. Rev. 2018, 118, 8936−8982

Chemical Reviews 6.2.2. Silicon 6.2.3. Other Anodes 6.3. Binders for Li−S Batteries 6.3.1. Polysulfide Dissolution and Shuttling 6.3.2. Volume Expansion and Electrode Destruction 6.3.3. Electronic and Ionic Insulation of Active Materials 6.4. Binders for Na-Ion Batteries 6.4.1. Cathode Materials for NIBs 6.4.2. Anode Materials for NIBs 6.5. Binders for Supercapacitors 7. Advanced Techniques for Binder Research 7.1. Characterization Techniques of Mechanical Properties 7.1.1. Binder Properties 7.1.2. Interface Properties 7.2. In Situ Characterization Techniques 7.3. Computational Calculation 8. Conclusion and Perspectives Author Information Corresponding Authors ORCID Author Contributions Notes Biographies Acknowledgments References

Review

boosting battery performance via exploring and developing novel binders. Binders, though small, could play crucial roles in energy-storage devices. In state-of-the-art LIBs, conventional polyvinylidene fluoride (PVDF) is the dominant binder due to its mechanical, chemical, and electrochemical stability benefits. Currently, the existing PVDF-based binding system has failed to meet the requirements of emerging high-capacity active materials. To address these issues, we need to understand binding mechanisms between binders and active materials, and more importantly, we need to establish a wide spectrum of multifunctional binder systems to suit the needs of different high-capacity active materials. A binder holds critical components of energy-storage devices, such as electrode material, conductive additive, and current collector, firmly together during charge/discharge processes, providing the following functions: (i) it acts as both a dispersing agent and a thickener to allow a homogeneous distribution of critical components; (ii) it bridges particles together with a current collector via certain mechanical, intermolecular, or chemical forces to maintain mechanical integrity; (iii) it maintains electronic contact upon cycling for electrons to tunnel near or through polymer chains; and (iv) it modifies the wettability and facilitates mass transport of Li ions at the electrode particle surface/ electrolyte interface. By understanding these roles and subsequently manipulating the properties of novel binders, higher energy/power density of batteries can certainly be achieved.7 In this review, we will introduce the binding mechanisms and properties effects of binders on electrodes and present recent progress in binder design. We then discuss the application of binders in different energy-storage systems and briefly introduce characterization techniques and computational methods. Finally, we provide a perspective on future development of binders for next-generation energy-storage devices.

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1. INTRODUCTION Society’s demand for energy continues to soar due to increasing global population, technological advances, and rapid development worldwide. Unfortunately, most energy is derived through the combustion of fossil fuels which leads to well-recognized adverse effects on the environment. However, recent infiltration of renewable energy technology, such as solar, wind, hydro, and geothermal power plants, into global energy infrastructure provides a promising outlook. These renewable technologies, at the very least, have already hampered rising greenhouse gas (GHG) emissions, and, with increased representation in global energy infrastructure, may halt or, ideally, begin to reverse the trend of rising annual GHG emissions. Regrettably, this idealistic scenario is unlikely to eventuate without first addressing the intermittent nature of the energy produced through renewable means. Lithium-ion batteries (LIBs) have already made significant headway into addressing this limitation through their application as load-leveling devices, both in large-scale applications adjacent to wind/solar power plants and in smaller-scale residential storage applications. The shift from internal combustion engine (ICE) vehicles to electric vehicles (EVs) may be facilitated by improved LIBs, further reducing GHG emissions.1−3 Furthermore, energy-storage devices with high power and large capacity are critical for future wireless devices and technologies.4 There are, however, numerous challenges inherent to current generation of energy-storage devices, as well as issues related to their manufacturing, including insufficient energy/ power density and longevity,5,6 the use of nonrenewable chemicals and resources, and toxic manufacturing processes. To date, such challenges have been commonly addressed by intensive research into the development of active electrode materials and electrolytes. In contrast, the role of binders has been underestimated, and less efforts have been devoted to

2. TYPES OF BINDERS Many types of the binder have been reported in the literature (Table 1). Among commonly used binders in Table 1, polyvinylidene fluoride (PVDF) stands out as a dominant binder in the LIB industry, while other polymer binders have been used to address different challenges in the electrode fabrication process.12,14,15,19,23,29,43,47,50,61,62,68,70,73,76,83,88,90,93,94,95,99,100,102,113, 126,127,130,139,142

Polymeric binder can be typically divided into different types by various methods. For example, by orgin, binders can been organized into natural and synthetic binders. 2.1. Natural Binders

Natural binders are made from naturally available organic sources such as plants and animals. In Table 1, guar gum (GG), alginate (Alg), carboxyl methyl cellulose (CMC), gum Arabic (GA), xanthan gum (XG), carrageenan, gelatin, chitosan, starch, and β -cyclodextrin are natural binders derived from Cyamopsis tetragonolobus, brown algae, cellulose, Acacia senegal, collagen, and corn (see Figure 1).79,91,96,111,112,120,143−147 Most natural binders are directly extracted from natural sources and may require some purification processes. For example, GA can be used directly, while CMC is synthesized via an alkali-catalyzed reaction of cellulose with chloroacetic acid.148,149 Natural binders commonly contain multiple components, such as polysaccharide, glycoprotein, and/or other functional components.107,112 The multiple components might form a synergetic effect to reinforce binding strength; therefore, excellent mechanical and electrochemical performance can be obtained.101,107,150 Because these bioderived 8937


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Table 1. Molecular Structure and Applications of Existing Binders for Energy-Storage Devices in the Literature

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Table 1. continued

In addition, the structures of synthetic binders can be specially designed, which allows us to design electrodes for special needs.153 Synthetic binders can be used in a wide range of applications including cathodes, anodes, Li−S, and sodium-ion batteries and are dominant in practical applications. On the basis of the reactiveness of the binding process (nonreactive or reactive), binders can be divided into nonreactive binders and reactive binders. 2.3. Nonreactive Binders

Nonreactive binders, such as PVDF and PAA, can be used directly without additional chemical reaction. In practice, the binders are first dissolved in solvent. The binding forces among the electrode active materials then form after drying, i.e., the hardening process. Nonreactive binders currently dominate energy-storage devices.

Figure 1. Chemical structures and sources of some representative natural binders.

2.4. Reactive Binders

In reactive binding processes, binder precursors need to carry out a polymerization reaction to form a polymer and simultaneously create strong binding forces between active materials and current collectors. Before the binding process, the precursors are mixed with electroactive materials to ensure sufficient mixing and wetting. The polymerization is then triggered by various means such as chemical radicals (epoxy resin),121,154 UV adiation (polysiloxane acrylate oligomer-based binder and PAA-BP binder),56,155 and heating (in situ polymerization).156 Through the reactive process, a robust interpenetrated network such as in situ cross-linked PVA−PEI binder can form rather than a simple mixture of different components.156

polymers are sustainable, abundant, low cost, and environmentally friendly, they can simultaneously address energy and environmental issues.107 As they are usually soluble in water, they can also help disperse active materials and conductive additives in water for electrode fabrication, removing the need for conventional toxic solvents, such as N-methyl-2-pyrrolidone (NMP), converting a harmful electrode fabrication process into a green technology. Among the binders in Table 1, PVDF, CMC, PAA, alginate, and PU are commonly used in both cathodes and anodes as well as other energy-storage systems, while the other binders are mainly applied individually in cathodes, carbonaceous materials, silicon anodes, and Li−S batteries.

3. BINDING MECHANISM An effective binding process can be separated into two steps: the desolution/diffusion/penetration step and the hardening step. In the first step, the binder (either dissolved nonreactive binder or reactive binder precursors) wets the substrate surface and penetrates the pores of electrode material particles, as shown in Figure 2a. In the second step, the binders are hardened via different reaction mechanisms (e.g., drying for nonreactive binders or polymerization for reactive binders) which leads to the mechanical interlocking effect (Figure 2b).157,158 Besides the mechanical interlocking effect (see Figure 2b) and the interfacial binding forces (see Figure 2c), the mechanical strength of the binded composite also depends on the mechanical strength of the binder and electrode materials. This can be illustrated by a model adapted from Marra’s report as

2.2. Synthetic Binders

Synthetic binders are synthesized via modern chemical industry. PVDF is the most widely used synthetic binder for LIBs.151 Other polymers, such as poly(acrylic acid) (PAA), styrene butadiene rubber (SBR), polyamide imide (PAI), poly(vinyl alcohol) (PVA), polyethylenimine (PEI), polyimide binder (PI), and poly(tert-butyl acrylate-co-triethoxyvinylsilane) (TBATEVS) (see Table 1), are also adopted for different fabrication processes. These synthetic binders exhibit various properties when used in different energy-storage systems. For example, PVDF and PAN are only soluble in organic solvents, while PAA and PVA are water soluble.8,9,32,55−57,152 One of obvious advantages of synthetic binders over natural binders is that the composition of the binders can be strictly controlled and fixed, which is beneficial for uniform and large-scale production. 8939


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Figure 2. Schematic illustration of the binding mechanism: (a) diffusion/penetrating process during electrode preparation; (b) formation of mechanical interlocking during the drying process; (c) interfacial bonding forces include intermolecular forces and chemical bonds; (d) polymer states in a bonding system: bonded polymer, fixed polymer, and excessive polymer.

shown in Figure 2d.159 There are generally three different states of binder polymers when the binder is in contact with particles: as shown in Figure 2d, the bonded polymer layer, the fixed layer polymer on the surface of particles, and the excessive polymer. The excessive polymers are the free polymers surrounding the fixed layer. In the bonding system, the strength of the fixed layer is commonly stronger than the excessive polymer layer due to the interaction with the bonded layer. The properties of the fixed layer and excessive layer are mostly dependent on the intrinsic property of the polymer. The interface configuration varies with the polymer and substrate.160−162 The adhesion strength can also be affected by the morphology of electrode materials. A study investigating the relationship between surface roughness and peel strength (N/m) showed that an increase in surface roughness produces an increase in the surface area (contact area), which produces higher peel strengths.163 Wake proposed an equation defining the effects of mechanical interlocking and thermodynamic interfacial interactions for estimating adhesive joint strength (G)

G = C × Mk × I i

mechanism, thermodynamic mechanism, and chemical bonding mechanism are the most commonly used.178−187 Briefly, the mechanical coupling mechanism is based on the interlocking into the surface of the substrate. This is similar to glue on wood in that the glue locks into rough irregularities on the surface of the wood, while the thermodynamic mechanism does not require a molecular interaction for good binding, only an equilibrium process at the interface.188,189 The thermodynamic mechanism works for the adhesive polymers without chemical binding sites. The chemical bonding mechanism is the most convincing mechanism to explain the binding between two surfaces in close contact. Chemical bonds include covalent, ionic, and metallic bonds and arise from atoms sharing, donating/accepting, or delocalizing electrons, respectively. This interaction results in the chemical force which binds atoms together to form molecules. These theories are in line with our proposed mechanism in Figure 2.

4. BINDER PROPERTIES ON ELECTRODE FABRICATION Binder makes up only a small part of the electrode composition (2−5% of the mass in a commercial electrode), but it plays an important role in the electrochemical performance of battery systems.190 Only perfect cooperation between the different electrode components can contribute to enhanced electrode electrochemical properties.191 In this section, we will focus on the physical and chemical properties of the binders. Physical properties include thermal, mechanical, conductivity, and dispersion properties. Excellent physical performances are still required when improving electrochemical performance.21,84 Chemical properties include chemical and electrochemical stability, which emphasize the stability of binders in harsh environments and wide voltage windows. These properties are highly associated with molecular weight, molar volume, density, polymerization degree, crystallinity, and functional groups.64

(1)

where C is a constant, Mk is a mechanical keying component, and Ii is an interfacial interactions component.164 To incur significant mechanical interlocking we will need to (i) increase the surface roughness and porosity of the electrode material, (ii) choose a suitable binding system, and (iii) prepare a binder solution of proper viscosity to ensure effective mixing, dispersion, and penetration of the binder into the electrode material.165 The strength of force at the interface increases with respect to the surface roughness and porosity, as more binder sites for these interactions are present.166−168 In the literature, the binding mechanisms can be described with seven models,169 including mechanical interlocking theory,170 electronic or electrostatic theory,171,172 adsorption (thermodynamic) or wetting theory,173 diffusion theory,174 chemical bonding theory,175 acid−base theory,176 and theory of weak boundary layers.177 It should be noted that these theories are not exclusive and may be occurring simultaneously in different circumstances. In general, the mechanical coupling

4.1. Thermal Properties

Thermal properties involve the changes in physical and chemical performance of polymers as heat is added or removed. 8940


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being more elastic, as shown by cyclic tensile testing.201 Apart from their beneficial effects as binders in standard electrodes, flexible and elastic materials are particularly important for application in wearable devices.122 The hardness of a material measures its resistance to indentation.202 A harder material implies better binding capability and higher stability during cycling.92 Nanoindentation, used to evaluate hardness, can also be used to investigate the elasticity of materials, the benefits of which are mentioned above. For example, guar gum binder is more elastic and homogeneous than PVDF, resulting in a binder which was better able to withstand large volume expansion in Li−S electrodes.103 Adhesive strength is a measure of the strength of bonding between an electrode film and the current collector.192 The molecular weight and functional groups of a polymer strongly influence adhesive strength.84 Various polymers have better adhesion than PVDF, including CMC, PAA, and alginate, which can provide strong hydrogen and chemical bonding. In battery applications, repeated volume expansion of active materials (silicon anodes) and bending (wearable energy devices) are the main source of loss of contact between electrode components (Figure 3). Binders with a high adhesive

Almost all of the properties of polymers are closely associated with thermodynamics.192 In practical application of binders, thermal stability, diffusivity, and expansion rate are major considerations as they can significantly affect the electrochemical performance and stability of electrodes when energy-storage devices are operated under different temperatures.6,193,194 Factors affecting thermal properties include the strength of binding forces between binders, the containing compositions and functional groups, and molecular weight.13,193 Binders performance varies when they are exposed to heat over a range of temperatures. For example, GA starts to carbonize at 210 °C, while CMC remain stable until 235 °C when they are tested under the same conditions.101 Among PVDF, PAA, and CMC binders, PVDF has largest thermal expansion rate, while PAA has the largest thermal diffusivity between 20 and 80 °C.13 Thermal property and stability of binders are crucial for electrode fabrication and operation of energy-storage devices, especially at elevated temperatures. In electrode fabrication, the electrode including the active materials and binders are subject to a high-temperature treatment for the sake of binder curing and solidifying, the removal of organic solvent (e.g., 120 °C to remove NMP solvent), and sometimes the electrochemical cycling (e.g., 100 °C for solid-state batteries) at high temperatures, which requires appropriate thermal stability of the binder.195,196 In practical application, the electrodes need to work in a wide temperature range (from −20 to 55 °C).197 Most of the binders shown in Table 1 are stable at or above 150 °C. Hence, the thermal properties of binders are crucial for electrode fabrication and operation of energy-storage devices at elevated temperatures. 4.2. Mechanical Properties

The main mechanical properties of binders involved in electrode fabrication and operation are referred to the strength, elasticity, flexibility, hardness, and adhesion of the materials.165,198 The strength of a material refers to its strength under tension or compression with the same material able to display different tensile and compressive strengths. Compressive strength relates to intrinsic properties of a material, and tensile strength more accurately conveys the strength of a material while considering its internal morphology (i.e., grain boundaries, cracks, etc.) as well as its behavior in a composite.165 As such, tensile strength will be the main focus of the discussion of properties pertaining to the strength. The tensile strength of a material is quantified as tensile stress which can withstand before mechanical failure (i.e., breaking).192 Tensile strength is mainly determined by a polymer’s molar mass and functional groups.165 In general, polymers such as polyethylene,199 PU,122 and CMC200 display a relatively high tensile strength. As a result, CMC and CMCSBR binders show higher tensile strength than PVDF, and they can better withstand forces from repeated cycling.82 Not only is the tensile strength of a material important, so too is its behavior in response to these conditions. Under tension, a material’s elasticity (elastic vs inelastic behavior) and flexibility (ductile vs brittle behavior) can be investigated. Elasticity relates to a material’s ability to recover to its original form after a stress is applied and removed. Flexibility refers to the ability of a material to handle bending without breaking.192 As with tensile strength, a polymer’s molar mass and functional groups largely determine its elasticity and flexibility.165 Alginate and CMC binders are more flexible than PVDF, as evidenced by their tendency to yield before failure, while also

Figure 3. Electrode cross-section before and after cycling: (a) loss of contact to insufficient adhesion, (b and c) no loss of contact. Reproduced with permission from ref 203. Copyright 2016 Royal Society of Chemistry.

strength can withstand this volume expansion and are able to maintain contact between the active material, the conductive additive, and the current collector, which minimizes capacity fading and enhances cycle life.203 Today, much effort is devoted to developing flexible binders.203 LIBs are currently required to possess unconventional mechanical properties to keep pace with future flexible and wearable electronic applications.122−124,204 4.3. Electrical and Ionic Conductivity

Both electrical and ionic conductivity must be considered in any electrochemical batteries. Polymers were long known to be insulating materials until the first conductive polymers were 8941


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produced in the 1970s.205,206 Polymers can be conductive with special conducting structures, including conjugated framework and free charge carries (donor/acceptor radicals). Electrical conductivity of polymers depends on the presence of highly π-conjugated polymeric chains.207 In the past few decades, over 25 conductive polymer systems have been developed, with the most common being polypyrrole (PPy), polyaniline (PANI), poly(3,4-ethylenedioxythiophene) (PEDT, PEDOT), polythiophene (PTh), polythiophene-vinylene (PTh-V), poly p-phenylene-sulfide (PPS), polyacetylene (PAc), poly(p-phenylenevinylene) (PPV), poly(2,5-thienylenevinylene) (PTV), poly(3-alkylthiophene) (PAT), poly(p-phenylene) (PPP), poly(p-phenylene-terephthalamide) (PPTA), poly(isothianaphthene) (PITN), poly(α-naphthylamine) (PNA), polyisoprene (PIP), and poly(p-phenylene-terephthalamide) (PPTA).208−212 The neutral conjugated polymers show a low conductivity (usually in the range from 10−10 to 10−5 S cm−1). However, conductivity can be increased several fold by doping with oxidative/reductive substituents or by donor/acceptor radicals, and many techniques, such as gaseous doping, solution doping, electrochemical doping, self-doping, radiation-induced doping, and ion-exchange doping, are adopted to improve polymer conductivity.208 The adoption of conductive polymers could eliminate the use of conductive additives and therefore increase the specific capacity of batteries.92 Ionic conductivity of polymers is based on the motion of solvated ions through the polymer chains.213 The crystallinity degree, porosity, and viscosity of the polymers significantly determine ionic conductivity.214 The measurement of ionic conductivity in binders demands sophisticated instrumentation and data processing, leading to rare ionic conductivity data in the literature. Nevertheless, the available ionic conductivities in the binder could be significantly different (see Table 2).215

4.5. Chemical Stability

Binders must have a certain chemical stability to resist corrosion from electrolyte and electrochemical reactions during battery operation. The chemical stability of binders depends on their chemical composition and structure as well as chemical environment. For example, PVDF, one of the most chemically stable binders in LIBs, reacts with lithiated graphite and metal lithium at elevated temperatures229,230 and swells in organic solvents (EC, DEC, DMC) as displayed in Figure 4.231−233

Figure 4. Swelling ratio vs time in solvent PC for PVDF and SBR binders at two different temperatures: 25 and 80 °C. Reproduced with permission from ref 233. Copyright 2005 the Electrochemical Society.

The reactions of PVDF with lithiated carbon (LixC6) attribute to the exothermic heat generation of lithiated carbon materials and also form LiF and hydrogen on the surface of electrodes.229,234 In lithium−air batteries, commonly used binders polytetrafluoroethylene (PTFE), carboxymethyl cellulose (CMC), poly(ethylene oxide) (PEO), polyacrylonitrile (PAN), polyvinylpyrrolidone (PVP), polystyrene (PS), poly(methyl methacrylate) (PMMA), and polyvinyl chloride (PVC) are also chemically unstable, while the polyisobutylene (PIB) binder is chemically stable even in highly reactive oxygen species atomspheres.81,82,235,236 The decomposition of PVDF binder, which was triggered by either LiOH (formed from residual H2O in the cell) or Li2O2, has been clearly evidenced in lithium−air batteries.237,238 The following side reactions will form LiF and H2O2 on the surface of electrodes. The significant deposit of LiF on the surface of electrode contributes to the capacity fading.239 The side reaction of PVDF can also turn PVDF-NMP solution into dark gel.240 The significant deposit of LiF on the surface of an electrode contributes to the capacity fading. The instability of binders may cause the collapse of electrodes and trigger safety issues. Therefore, the chemical stability of binders in a specific battery system (including types of electrode materials, electrolyte, and cycling potential range) is an essential condition of electrode design to ensure electrochemical performance and longevity of batteries.141,241 However, in some cases, binders are expected to react to form strong bonds with active materials, such as in silicon anodes and Li−S batteries. Such chemical reactions play a positive role in battery performance.

Table 2. Ionic Conductivity of SPEEK-PSA-Li, SPEEK-PSILi, and PSU-PSI-Li Membranes215 binder

ionic conductivity (s/cm)

SPEEK-PSA-Li SPEEK-PSI-Li PSU-PSI-Li

6.3 × 10−7 4.6 × 10−6 1.4 × 10−6

Many polymers, such as natural chitosan,216 starch,217 PEO,218,219 GG,105 and Nafion,220 have been explored and used to enhance the ionic conductivity of electrodes, especially in the application of solid polymeric electrolytes.221 The ionic conductivity of polymers plays an important role in the electrochemical performance of batteries, especially power density. 4.4. Dispersion Properties

Dispersion properties of polymers refer to the ability to disperse the electrode materials into the binder solution and obtain a uniform composite. A polymer that helps this process and prevents aggregation is usually called a dispersant.222 In electrode fabrication processes, polymers can act as a binder as well as an effective dispersion agent, such as CMC/ SBR,28,30,34,223−225 gelatin,226 PAA,227 PVP,33 and poly(acrylicco-maleic) acid (PAMA).223 The dispersibility of these binders can be affected by the charge density, chain flexibility, and electrostatic repulsion and depletion.224,225 The overall dispersion performance is also dependent on electrode materials. The uniform dispersion of particles, especially for silicon anodes, remains a big challenge.228

4.6. Electrochemical Stability

When the electrodes are cycled in a wide voltage window, binders are expected to withstand voltage change. One of the fundamental requirements of binders is that they cannot be reduced under a low negative potential and not oxidized under 8942


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a high positive potential. The electrochemical reaction is ruled by Nernst and Arrhenius equations,242,243 and activation energy is one of the determining factors for electrochemical stability of binders. Existing binders, such as PVDF, CMC, PAA, PVA, and PMA, show quite high electrochemical stability.137,232 However, the increase of operating voltage for high-voltage materials, such as spinel-type LiNi0.5Mn1.5O4,244 Li-rich layered oxides,245 Ni-rich layered oxides,246 and olivine-type LiNiPO4247 as well as solid-state electrolytes, may bring in bigger challenges for the electrochemical stability of binders. The electrochemical activity of binders can also be utilized to increase the specific capacity of electrodes. Such so-called “redox-active binders” are normally used in Li−S batteries.65

and (vii) adopt emerging technology, such as self-healing ability of polymers to accommodate large volume changes.153,252,253 In this section, we introduce strategies to improve overall binding forces according to the mechanisms proposed previously, as illustrated in Figure 2. We also attempt to discuss design strategies from the molecular level, at the nanoscale, and the rational combination of various binders to realize the goals above. 5.1. Strategies To Enhance Mechanical Interlocking

5.1.1. Manipulation of Bulk Mechanical Strength. The mechanical properties of binders play a large role in determining the integrity of electrodes via the mechanical interlocking mechanism and its bulk properties as indicated in Figure 2d. Although PVDF binder does not contain reactive functional groups to form direct chemical bonds with active materials, the interlocking mechanism would lock active materials firmly within its inert PVDF matrix and therefore provide satisfactory electrochemical performance. However, it is widely established that PVDF binder cannot tolerate a dramatic volume change of some active materials (such as Si and Sn) during charge and discharge processes. Stronger binders, for example, GA binder, have been explored to address this issue.101 GA contains two different components, polysaccharides and glycoproteins. As shown in Figure 5, the long spiral

5. STRATEGIES IN DEVELOPING BINDERS AND BINDING PROCESSES It is well recognized that the most widely used binder is PVDF because of its good electrochemical stability and adhesion to the electrode materials and current collector.230,248 However, there are a series of drawbacks that must be overcome for large specific capacity, high power density, and long life in energystorage devices. (i) Insufficient binding forces: the adhesive force of PVDF mainly stems from the mechanical interlocking mechanism and interfacial forces (see Figure 2). The latter is a weak van der Waals force that is not able to maintain electrode integrity, attract soluble electrode materials such as polysulfides onto the current collector, and consequently could not suppress shuttling effect. (ii) Insufficient stability: the chemical stability of PVDF could be reduced over time during electrochemical cycling, especially at the contact points between active materials and PVDF at elevated temperatures.249 This could be due to the decomposition of C−F bonds during the charge/discharge process. There are also other side reactions that might collapse the electrode. Furthermore, PVDF could be swollen, gelled, or even dissolved by organic solvents (EC, DEC, DMC) in electrolytes at elevated temperatures.250 (iii) Expensive and environmentally harmful electrode fabrication process: PVDF is expensive and not easy to recycle, while electrode fabrication with PVDF involves the use of toxic and volatile organic compounds such as N-methyl-2-pyrrolidone (NMP).74,251 (iv) Limited electrical and ionic conductivity: conductive additives such as carbon black (CB) are always needed due to the electrical insulation nature of PVDF. The addition of electrochemically inert CB decreases the mass energy density of batteries. Meanwhile, traditional binders limit Li-ion diffusion in electrodes due to the coating of active materials. To address these drawbacks of the traditional PVDF binder, we need to design or choose binders with enhanced relevant properties as mentioned previously. The specific goals can be summarized as follows: (i) introduce stronger interfacial forces by grafting reactive functional groups to form polar intermolecular forces and chemical bonds, (ii) reduce the cost of the binder and binding process, (iii) consider eco-friendliness in binder design and binding processes, (iv) accelerate excellent electron and ion transport, (v) improve mechanical flexibility and ductility, (vi) boost electrolyte uptake of binder and dispersion ability of electrode materials and conductive additive,

Figure 5. Schematics of the concept to address the volume change issue in battery materials. GA with dual functionality could maintain both the strong chemical bonding and the ductile property required to tolerate the volume expansion during lithiation/delithiation processes. Reproduced with permission from ref 101. Copyright 2015 Elsevier.

glycoprotein chain can enhance the mechanical strength of the binder by acting similar to fibers in concrete. The strong mechanical strength of GA can withstand the volume expansions of Si anodes during cycles (remaining 1000 mAh g−1 after 1000 cycles at 4200 mA g−1). The mechanical properties of binders can be enhanced via reinforcement of bonding forces between binders. For example, Wang et al. developed a layer-by-layer assembly method for a S/C cathode with Nafion and PVP binder (Figure 6).254 Crosslinks between the positively charged PVP and negatively charged Nafion were formed through strong electrostatic interactions, resulting in an electrode that maintained structural integrity even when the binder content of the composite was as low as 0.5 wt %. This low binder content enabled decreased 8943


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Figure 6. Illustration of (a) the air spray process of the cathode, (b) the sprayed cathode, (c) the layer-by-layer C/S composite, and (d) the cross-link between Nafion and PVP. Reproduced with permission from ref 254. Copyright 2015 American Chemical Society.

charge transfer impedance and ohmic resistance of electrodes, which contributed to a much higher initial capacity (1450 mAh g−1) and slower capacity decay for a 0.5 wt % N/P supported electrode to a 10 wt % PVDF binder. Cross-linking, such as dynamic and ionic cross-linking, is another way to modify polymer mechanical properties. For example, mechanical testing of Ca2+-doped-alginate binder (Alg-Ca) was carried out by Yoon et al., who found that the electrostatic cross-linking could significantly improve the stiffness, toughness, and resilience of the electrolyte-solvated alginate binder compared to Na-alginate and other commercial binders.201 Zhang et al. further investigated the effect of crosslinking density of Alg-Ca on the electrochemical performance of silicon anodes. They found that the adhesion force was enhanced with increases in CaCl2 concentration.255 Further evidence of the efficacy of electrostatic interactions in binder materials was provided by Lim et.al., where they reported a novel polymeric binder poly(acrylic acid)-poly(benzimidazole) (PAA−PBI) which was connected through electrostatic crosslinking (Figure 7).256 Because of the reversibly constructed ionic bonds between polymers, the binder exhibited excellent static strength and reversible forces. This polymer blend binder endowed the fabricated silicon anodes with a high mechanical adhesion strength and remarkable electrochemical properties. The flexibility of binders can be obtained by combining the soft and hard polymers through cross-linking. For example, polyurethane copolymers possess special chemical and polymeric structures which can be divided into hard and soft segments.122 As shown in Figure 8, the hard segments containing stiff methylene diphenyl isocyano (MDI) units could interact with each other through hydrogen bonding, providing mechanical strength to whole polymers.124 In the soft segments, connected polyethylene glycol (PEG) and polytetramethylene ether glycol (PTMEG) units provide flexible and stretchable capabilities. The high elasticity of binders can be achieved by topological design. Topological network polymers are a new class of polymers that are defined by their characteristic sliding crosslink points.257 Originally realized by threading PEG through α-cyclodextrin (α-CD) macrocycles,258−260 this concept was later applied to energy-storage materials by Choi et al.261 The group fabricated a highly elastic binder for Si anodes conceptually based on a molecular pulley design. As before, the α-CD was threaded by amine-functionalized PEG, followed by the formation of a covalent linkage between the α-CD rings and PAA. Such a composite can withstand the volume change of the

Figure 7. Schematic representation of the technical approaches used to overcome the volume changes of Si particles during cycling. (a) Conventional chemically cross-linked binder whose bonding is irreversible and hence causes fractures in the network upon lithiation (charging) and delithiation (discharging). (b) Physically cross-linked binder that can maintain the interaction between the polymers based on reversible bonding. (c) Molecular interactions of the physically cross-linked polymeric binder through reversible interactions between PAA and PBI. Reproduced with permission from ref 256. Copyright 2015 American Chemical Society.

Figure 8. Schematic illustration of the PU molecular structure consisting of hard and soft domains. Reproduced with permission from ref 124. Copyright 2013 American Chemical Society.

active material during cycling due to the ring-sliding behavior of the α-CD along the PEG backbone as well as the covalent interactions between α-CD and PAA,253 as displayed in Figure 9. 5.1.2. Maximizing Interfacial Contact. The maximum mechanical interlocking effect is ensured by the wetting of the surface and penetration of binder into the internal structure of active materials, i.e., maximizing the interfacial contact between 8944


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binding process. Currently, SBR/CMC composite is widely applied in anodes for large-scale LIBs manufacturing. In the experimental stages, many water-soluble binders have been already trialled in electrode manufacturing, such as CMC-based binders,77,82,272 PAA-based binders,24,273−275 chitosan-based binders,110,276 alginate-based binders,91,201,255,277 β-cyclodextrin polymer-based binders,278,279 and nature gum-derived polymer binders such as gum Arabic,101,103 gellan gum,280 xanthan gum,281 guar gum,104−106,282,283 and Karaya Gum.284 Watersoluble binders are more environmentally friendly and less expensive, pose less risk to health, and are safer than organic solvent-based binding systems. Consequently, water-based binding systems have become a key topic of practical significance.271 It should be noted that solvent-free manufacturing of electrodes has been developed to eliminate the large cost of solvent recovery. These unconventional manufacturing methods can be achieved through pulsed laser and sputtering deposition.285−287 Such solvent-free manufacturing methods also remind us to explore liquid solvent-free binders which can be hardened by reactive binding processes. 5.2. Strategies for Enhanced Interfacial Bonding

Because the van der Waals force of PVDF is weak there is a need to create stronger binding forces, such as hydrogen bonds, electrostatic bonds, and convalent bonds, at the interfacial junction to enhance interfacial bonding forces between binders and active particles as shown in Figure 2. 5.2.1. Creation of Strong Intermolecular Forces. According to the proposed binding mechanism in Figure 2, binders with strong intrinsic mechanical strength, better mechanical interlocking, and interfacial forces could address the issue of weak binding. The H−F bond on the PVDF is not reactive, and therefore, no chemical bonds can be built with active materials. The first approach of molecular tailoring is to bring in reactive functional groups such as hydroxyl (−OH) and carboxylic groups (−COOH). Polymers such as carbonxymethyl cellulose (CMC),194 poly(acrylic acid) (PAA),288 alginate,91,289 and guar gum105 possess abundant hydroxyl and/or carboxylate groups (−COOR) that are able to build strong hydrogen bonds, ion− dipole interactions, and even chemical bonds with active material particles. It is well estabilizhed that these forces are much stronger than van der Waals forces. The availability of these binding forces on the interface can bring in excellent mechanical properties and stability for electrodes and also hinder the occurrence of side reactions and improve structural stability in cathode materials.13,109 As a result, cathode materials with these binders deliver better charge transfer capability, initial Coulombic efficiency, reversible capacity, and cyclability in comparison with PVDF.10,52,11,74,290 With regard to anode materials, these binders can provide a high elastic modulus to accommodate volume change, such as in graphite and Si anodes, and facilitate the formation of a stable solid electrolyte interface (SEI) during cycling. For example, in graphite anodes, the hydrophobic parts of CMC adsorb onto the hydrophobic graphite surface and carboxylate groups stabilized the graphite surface.291 Other binders with hydroxyl and/or carboxylate groups have also been investigated, such as xanthan gum150 and TEMPO-oxidized cellulose nanofibrils.292 Other functional groups have also been added to enhance the intermolecular interaction between binders and other components in electrodes via molecular design. As shown in Figure 10, Choi et al. introduced catechol groups onto the PAA and alginate backbone (forming PAA-C and Alg-C separately).293

Figure 9. Proposed stress dissipation mechanism of PR-PAA binder for SiMP anodes. (a) Pulley principle to lower the force in lifting an object. (b) Graphical representation of the operation of PR-PAA binder to dissipate the stress during repeated volume changes of SiMPs, together with chemical structures of polyrotaxane and PAA. (c) Schematic illustration of the pulverization of the PAA-SiMP electrode during cycling and its consequent SEI layer growth. Reproduced with permission from ref 261. Copyright 2017 American Association for the Advancement of Science.

active materials and binders (see Figure 2). This is achieved by controlling the electrode fabrication process, more precisely, the preparation of the active material paste according to the binder system. As discussed previously, the achievement of mechanical interlocking inevitably involves a dissolution of nonreactive binders into proper solvent, e.g., PVDF binder into NMP solvent and CMC binder into water. For this discussion, we define nonaqueous binders and aqueous binders as binders that can dissolve in organic solvents and aqueous solution, respectively. The solubility of binders partially determines the corresponding types of solvents. Electrode fabrication using nonreactive binders involves a dissolution process in a suitble solvent, mixing with electrode materials, and a drying process. The selection of binder and solvent significantly affects the performance and stability of the resultant battery.262−264 It is well established that for the same binder the choice between different solvents could significantly affect binding performance and therefore electrochemical performance.13,265 Nonaqueous binders, such as PVDF, PAN, and PA, are readily soluble in organic solvents but hardly soluble in water; therefore, organic solvents rather than water would be a logical component of the binding process. In the current LIBs manufacturing industry, the use of PVDF binder involves the use of NMP as a solvent. Other organic solvents to dissolve binders have also been investigated in battery research, such as N,N-dimethylacetamide (DMA), N,N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO), acetone, dimethyl ether, ethanol, and acetonitrile.266−270 Aqueous binders are soluble in water, due to their abundant hydrophilic functional groups, such as −COOH, and −OH.271 This property allows the use of water as the solvent in the 8945


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Figure 10. Catechol-conjugated polymer binders and Si anode structure. (a) Mussel; (inset) chemical structure of dopamine inspired from mussel foot proteins. (b) Structural formula of Alg-C and PAA-C alongside a simplified structure of a conjugated polymer binder; black solid line represents the polymer backbone with carboxylic acid functional groups attached, and red circles represent catechol moieties conjugated to the backbone. (c) Graphical illustration of the Si NP anode structure. Reproduced with permission from ref 293. Copyright 2013 Wiley-VCH.

Figure 11. (a and b) Structural formulas and graphical representations of (a) the Alginate (Alg) and (b) β-CD polymer (β-CDp) binders. Side chains of β-CDp could be dihydroxypropyl moieties in monomer, dimer, or trimer form. Bridges could be glyceryl moieties in each form. (c and d) Schematic representations of Si-binder configurations for (c) SiAlg and (d) Siβ‑CDp during lithiation/delithiation of Si (sphere). Hyperbranched structure of β-CDp offers multidimensional hydrogen-bonding interactions, which enable the binder to maintain the interactions with Si during continuous volume change of Si via a self-healing process. Reproduced with permission from ref 278. Copyright 2014 American Chemical Society.

They suggested that function-enhanced binders employed dual adhesion mechanisms of hydrogen-bonding and catecholic interactions with silicon particles. The contact between binders and silicon particles was robust due to wetness-resistant adhesion provided by catechol modification. Yue et al. reported that carboxymethyl chitosan containing three different functional groups (−OH, −COOH, and −NH2) formed strong hydrogen bonding with the silicon surface, thus showing better cycling and rate performance for Si anodes compared to PVDF, CMC, and alginate.110 The branched polymers tend to resist external forces via intrachain interactions and form multidimensional contacts.253,294 The multidimensional contacts of branched polymers can provide stronger binding forces with active materials than linear-chain polymers. Although branched polymers, such as amylopectin,112 exist naturally, specifically designed branched polymers can be synthesized to form star polymers, graft polymers, polymer networks, dendrimers, and hyperbranched polymers. The hyperbranced structures of binders can provide multidimensional hydrogen-bonding interactions with active materials and produce robust contacts between both

components. As shown in Figure 11, compared to a linear polymeric binder alginate (Alg), hyperbranched β-cyclodextrin polymer (β-CDp) forms intimate adhesions with the silicon surface through congested bulky side groups, which are able to accommodate a huge volume expansion of silicon particles and maintain interactions between binders and silicon during cycling (Figure 11b and 11c).278 Subsequently, they choose β-CDp and adamantine (AD) as host−guest pairs to design a host−guest complex via “dynamic cross-linking”, allowing for intimate Si−binder interactions, structural stability of the electrode film, and controlled SEI formation, thus leading to superior cycling performance with 90% capacity retention after 150 cycles.279 Murase et al. even investigated the effect of branching density of polymermeric binders on silicon/graphite anodes. Their results suggested that appropriate amounts of branching density are critical to improve the electrochemical performance of electrodes.295 Cross-linking and grafting are efficient ways to design polymer architectures and combine different function binders. The reaction can be realized via dynamic cross-linking (in situ cross-linking via heating or irradiation) or ionic cross-linking. 8946


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Figure 12. Effect of the alginate hydrogel binder on the mechanical integrity of the electrode during the cycling processes. Reproduced with permission from ref 277. Copyright 2014 Royal Society of Chemistry.

The cross-linking agents, such as hexamethylene diisocyanate (HDI)296 and glutaraldehyde,297 are also widely used to achieve cross-linking. Many grafted or cross-linked polymer binders have been explored, including PVDF-G-PtBA,298 Nafion,299 c-PAA-CMC,55 PAA-CMC/SBR,300 PAA−PVA,57 PAAx(PCD)y,53 GS-GA,276 PEI-PVA,156 and PAA-BP.56 Wei et al. used CMC and acrylic acid as precursors to prepare poly(acrylic acid sodium)-grafted-carboxymethyl cellulose (NaPAA-g-CMC) copolymer as a binder for Si anodes.301 Unlike the linear structured PAA and CMC, branched binders present more contact points between binders and silicon particles, providing stronger hydrogen-bonding and van der Waals interactions. As such, this branched structure maintains contact with silicon particles upon repeated volume change from lithiation−delithiation cycles. An example of ionic crosslinking is shown in Figure 12, in which Sun et al. used Ca2+ ions to cross-link SA binder.277 The α-L-guluronic acid (G) blocks in alginate chains were cross-linked by Ca2+ ions, forming a network in water. This structure was able to significantly improve the cycling performance of the Si/C anode. After 100 cycles, the electrode with the alginate hydrogel binder retained a capacity of 1308 mAh g−1. 5.2.2. Electrostatic Bonding. Ionic bonds are a strong bonding force found in nature, which are a result of electrostatic interactions. The electrostatic interactions on the interface between binders and active materials have shown a promising ability to enhance the mechanical robustness and electrochemical performance of electrodes for energy-storage applications. The electrostatic bonding on the interface has been widely used in Li−S batteries to resolve the dissolution of sulfur and shuttling effects of polysulfides. Polar polysulfides can be efficiently adsorbed by binders in order to alleviate polysulfide dissolution and shuttling. For example, the amino groups in polyethylenimine (PEI) polymer can form strong electrostatic interactions with polysulfide intermediate.302 Helms et al. developed an active cationic polyelectrolyte, i.e., poly[(N,N-diallyl-N,N-dimethylammonium) bis(trifluoromethanesulfonyl)imide] (PEB-1) as binder for a lithium−sulfur battery (Figure 13).303 The strong electrostatic bonding between soluble lithium polysulfides and the cationic polymer backbone could prevent polysulfides diffusion from the cathode on cycling. Ling et al. designed a cationic poly[bis(2-chloroethyl) ether-alt-1,3-bis[3-(dimethylamino) propyl]urea] quaternized (PQ) with high-density quaternary ammonium cations, which could use the electrostatic

Figure 13. Illustration of the fabrication of sulfur electrodes with PVDF or PEB-1 binder. (a) Cathode is comprised of sulfur-active materials loaded into N-doped mesoporous carbon (N-MC) hosts, “Super P” as the conductive additive, and a polymer binder (PEB-1 or PVDF). (b) Conventional sulfur cathode cast onto an aluminum current collector. (c) Highly loaded sulfur cathode cast onto a carbon nanofiber current collector. (d) Schematic illustration of the formation of complex ion clusters via anion metathesis when PEB-1 encounters soluble polysulfides during Li−S cell cycling. Reproduced with permission from ref 303. Copyright 2017 Nature Publishing Group.

attraction between positively charged quaternary ammonium (R4N+) and negatively charged polysulfide (Sx2−) for improved sulfur electrochemistry (Figure 14).304 Such electrostatic interaction was both experimentally and computationally verified to be capable of immobilizing the active sulfur within the electrode to achieve mitigated polysulfide dissolution/ shutting and capacity fading during the battery cycling. The PQ-based electrode achieved good cycling stability with 40% capacity retention after more than 100 cycles at C/3. 5.2.3. Covalent Bonding. In some cases, the inherent properties of active materials can enable the formation of covalent bonds to some binder materials. The electron-rich groups, such as carboxylate, hydroxyl, catechol, nitrogenous, carbonyl, and sulfonic groups, can facilitate interactions between binders and polysulfides in lithium−sulfur batteries.296,305−307 Lithium polysulfides are more likely to form C−S, N−Li, or O−Li bonds with functional groups within PVP,308 PAMAM,307 PAN,309 and GA103 binders. For example, 8947


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Figure 14. Polysulfide confinement through cationic polymer. Electrostatic attraction between the PQ quaternary ammonium cations and the polysulfide anions. Reproduced with permission from ref 304. Copyright 2017 American Chemical Society.

which can undergo covalent bonding with silicon active materials.311,312 The chemical bond is usually created by chemical reaction between functional groups and oxide or hydroxide layers of silicon particles. For example, the etherification reaction between silicon particles and PVA can form a C−O−Si bond.156 The free carboxylic acid of PAA reacts with hydroxyl groups on the surface of silicon nanoparticles to form covalent ester bonds between the binder and the silicon particles.55 Combining functional groups in PAA and PVA, Wang et al. developed a gel polymer binder through in situ cross-linking of water-soluble poly(acrylic acid) (PAA) and poly(vinyl alcohol) (PVA) precursors.57 The results showed that condensation reaction occurred between PAA−PVA binder (−COOH) and silicon particles (Si−OH) and formed − COO-Si- bonds. The strong covalent bonds could improve the integrity of electrodes and mitigate electrical network destruction upon lithiation/delithiation, resulting in an excellent cycling stability and high Coulombic efficiency at high current density. Similar results can also be observed for PVDF-g-PAA binder.298 The covalent bonds vary in strength in different particle surfaces. Liu et al. designed an all integrated silicon anode by multicomponent interlinking among carbon@void@silica@ silicon (CVSS) nanospheres and cross-linked carboxymethyl cellulose and citric acid (CA) polymer binder (c-CMC-CA) (Figure 16a).313 Owing to the high adhesivity and ductility of cCMC-CA binder and strong binding forces between CVSS and c-CMC-CA, the electrode exhibited high mechanical strength. Contrary to chemical bond between silicon particles and binders, the binding energy between the binder and the carbon layer is higher than that between the binder and the silicon as displayed in calculations through density functional theory (DFT) (Figure 16b), so a carbon coating layer can provide a stronger binding force for the electrode. The roles of various bonding forces should be viewed from a dialectic point. Among those mentioned above, the strongest covalent bonding is not always the optimal design. Kwon et al. compared the effects of different binding mechanisms, namely, van der Waals forces, hydrogen bonding, and covalent bonding, on silicon anodes.314 They suggested that covalent bonding could indeed improve cycling performance to an extent; however, strong intermolecular interactions affect the performance to a greater degree. Whether covalent or noncovalent bonding should be prioritized depends on the particular application. The selection of binders depends on the properties of active materials. For small volume change materials, such as LiFePO4 and Li4Ti5O12, covalent bonding is the best choice,

a coordinated N−Li−S bond was observed when a modified polybenzimidazole (mPBI) polymer was used as a binder in Li−S batteries.310 The bonding is attributed to the abundant nitrogen-containing functional groups with lone pairs in mPBI which could act as a strong polysulfide trapper to inhibit the dissolution and migration of polysulfides. Cui et al. illustrated that Li2S2 and Li−S species bind most strongly to the carbonyl functional group, like those contained in esters, ketones, and amides, through a “lithium-bonding” coordination similar to the hydrogen-bonding mechanism.33 Chen et al. developed a hydrophilic binder through the polymerization of poly(ethylene glycol) diglycidyl ether (PEGDGE) with PEI to form the crosslinked hyperbranched polymer (denoted as PPA). 268 The abundant polar groups in the molecular structure showed a sufficiently strong affinity to trap polysulfides and inhibit the shuttle effect. Moreover, the multidimensional chemical bonding between the PPA binder and the sulfur species evidenced by the presence of Li−O, Li−N, and S−O bonds could also be observed and was theoretically confirmed by DFT calculations (Figure 15).

Figure 15. Schematics of the binder. (a) Traditional approaches to maintain electrode construction using PVDF as mechanical binder indicating that intermediate sulfur species dissolve rapidly with time. (b) Polar polymer with abundant amino and amide groups. (c) Reducible molecular structure of multiactive sites binder of PPA for tailoring the chemical and physical ability to improve the Li−S battery performance. Reproduced with permission from ref 268. Copyright 2018 Wiley-VCH.

Pure silicon is spontaneously oxidized in the atmosphere to form a SiO2 layer, and as a result Si−O and Si−OH bonds form with which covalent interactions can occur. This leads to a phenomenon which can be exploited in the search for binders 8948


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Molecular Design. A good example of the rational design of a binder with multiple functions is shown in Figure 17a.315 A bis-imino-acenaphthenequinone (BIAN)-fluorene copolymer (π-conjugated polymer bearing BIAN and fluorene units) binder was designed for graphite anodes. In this polymer, the planar naphthene group could form π−π stacking with the graphitic framework, and the cisoid conformation of the diamine group could facilitate adsorption on the surface and enhance the interaction with the copper current collector. Meanwhile, alkane chains are beneficial to polymer solubility in solvents, and the fluorene group improved the electronic conductivity of electrodes. Thus, each individual component had a function in the polymer binder. Similar rational design can also be seen in Figure 17b.314 Such a complex structure could integrate multiple functions, including providing covalent bonding to the Si surface (K group) and enhancing stiffness (S group) and flexibility (M group) of the binder and selfhealing effects (C group). The results showed that electrodes with such binder exhibited good mechanical, interfacial, and electrochemical properties. Conductive polymers are another base material used to attain multiple functions. For example, Liu et al. further modified the aforementioned PPFOMB by introducing triethylene oxide monomethyl ether (E) side chains to enhance its electrolyte uptake capability (Figure 18).2 First, the fluorenone (F) group could tailor the electronic structure of the polymer to improve its electric conductivity. Second, methyl benzoate ester (M) groups were introduced to enhance the chain flexibility of the polymer and strengthen the mechanical adhesion force on the interface. Third, triethylene oxide monomethyl ether (E) side chains were incorporated to increase the electrolyte uptake capability. Finally, they systematically studied the effects of the polarity of polymer binders on silicon anodes by manipulating the molar ratio of polar triethylene oxide side chains, suggesting that optimizing the polarity of polymer binders would lead to superior performance of silicon anodes.316 Such design concept is also applied in conductive polyfluorene (PF) polymers as mentioned before. The pyrene units, conductive agent (graphene), carboxyl groups, and butyl segments were combined together through molecular design, showing excellent mechanical flexibility and conductivity of electrodes in silicon/graphene composite material.317 Similar designs based on PF conductive polymer were also synthesized by introducing carbonyl groups and carboxyl groups.318,319 Such conceptual design of binders can incorporate functional groups into polymeric backbones and realize multifunctionality.

Figure 16. Proposed working mechanism of the all-integrated electrode and binding-energy calculation based on silicon/binder interaction. (a) As-prepared double-shelled-yolk-structured silicon (CVSS) electrode with c-CMC-CA as binder, in which the chemical bonding between CVSS and the binder and the cross-linking between CMC and CA are graphically presented. (b) Calculation of binding energy based on the relaxed models b1 (CO−OCH3), b2 (SiO− OCH3), and b3 (SiOH−H3COOCH3), representing the ester linkage between the carbon shell of CVSS and the c-CMC-CA binder, the covalent attachment between Si and the binder, and the hydrogen bonding between Si and the binder, respectively. It is indicated that the binding strength between the binder and the carbon shell of CVSS (b1) is higher than that between the binder and silicon (b2 and b3). Higher binding energy for our designed electrodes accounts for the electrode stability and high electrochemical performance. Reproduced with permission from ref 313. Copyright 2017 Wiley-VCH.

because covalent bonding is strong and therefore could maintain the integrity of electrodes, while for large volume change materials such as silicon and graphite, noncovalent binding is preferred as the binding is flexible and the binding strength to electrode materials might recover during the charge/discharge processes, namely, a self-healing effect. 5.3. Binders with Special Properties

This section mainly discusses the strategies for adding special functions to polymer binders, including conductivity, selfhealing ability, and redox activity. 5.3.1. Multifunctional Polymer. The multifunctionality of binders can be attained by molecular tailoring, nanostructure design, and composite binders.

Figure 17. (a) Structure and functional components of BIAN−fluorene. Reproduced with permission from.315 Copyright 2017 Royal Society of Chemistry. (b) Chemical structures of polymers and copolymers incorporating various monomeric units with distinct functionalities. Reproduced with permission from ref 314. Copyright 2014 Wiley-VCH. 8949


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and active RFID tags and card, which require excellent flexibility for the electrodes as well as the binders.322−326 As shown in Figure 19, microfibrillated-cellulose nanoparticles (MFC) were used as a novel binder to manufacture extremely flexible paper-like graphite/MFC electrodes.78 The microfibrillated material was used for a thin, self-standing electrode, showing high specific discharge capacity close to the theoretical specific capacity of graphite and good cycling performances. The graphite/MFC anode exhibited a high capacity retention of 77% at a C/5 current rate. Nanostructured conductive polymers, as mentioned above, are another main type of binders that have been widely investigated by researchers. For some energy-storage materials, such as silicon, metal oxides, and sulfur, conductive polymers have a high potential for practical application because of their excellent intrinsic properties.2 Cui et al. developed a tunable 3D nanostructured conductive gel framework for silicon anodes via in situ polymerization of PANI hydrogel (Figure 20).327 The unique nanoarchitecture exhibited exceptional electrochemical stability, which can be ascribed to several factors: (i) the porous matrix and 3D network can accommodate large volume change greatly during cycling while simultaneously providing a continuous 3D pathway for electronic conductivity, (ii) it can reduce the weight fraction of the binder and conducting filler of a battery, and (iii) it can form a deformable and stable SEI on the silicon nanoparticle surface. The nanostructured conductive binders with multiple functions were deeply studied by Yu et al. A 3D nanostructured hybrid inorganic-gel framework electrode, in which the conductive gel served as the binder system for battery electrodes, was developed by in situ polymerization of conductive polymer gel onto commercial lithium iron phosphate particles.328 In the gel materials, copper(II) phthalocyanine tetrasulfonate salts (CuPcTs) cross-linked with polypyrrole (C-PPy) as a multifunctional framework were used as the binder and showed a higher electrical conductivity than pure PPy. Due to their conjugated polymer chains, high doping level, and hierarchically porous structure, the hybrid 3D framework could promote electron transport and ion diffusion. Meanwhile, the in situ polymerization could form a uniform coating to embed the active material particles, thereby preventing active materials from aggregating. As a result, the robust framework showed excellent long-term electrochemical stability. In their subsequent work, they applied the same gel framework to Fe3O4 nanoparticles. As seen from Figure 21, the highly conductive and continuous 3D conductive polymer framework provided a good electrical connection between the particles and the current collector and maximized

Figure 18. Synthetic scheme and design purpose of the functional groups incorporated in the polymer binder. Functional groups contribute specific functionalities when the polymer is used as a binder in Li-ion batteries. Reproduced with permission from ref 2. Copyright 2013 American Chemical Society.

However, the synthesis complexity and product control are still big challenges before their practical applications. Nanostructural Design. Owing to the rapid development of nanoscience and nanotechnology, high potential for new binder development is offered by controlling the structure of materials at the nanoscale. The nanoarchitecture of energy-storage materials has many advantages as controlled nanostructures can facilitate electronic and ionic transport and improve contact between the active material, the conductive additive, and the current collector.207,320 For binders in battery systems, the nanostructure can also be designed to not only hold active materials together but also offer multiple advantages, including a continuous electrically conductive framework, porous space for volume changes upon repeated Li + insertion and deinsertion, and excellent flexibility during repeatedly folding. Guo et al.321 prepared sodium carboxymethyl cellulose with a porous scaffold structure via a slurry spary technique as a binder for silicon anodes. This nanostructure was realized by spraying the slurry (a mixture of Si, carbon black, and CMC) onto a preheated copper current collector. Results showed that silicon anodes exhibited high energy and power density and superb cycle stability, as the binder with porous scaffold CMC could accommodate immense volume change of Si and enhance lithium-ion transport in electrodes. Recently, paper-like electrodes have been widely used in rollup display, wearable devices,

Figure 19. Microfibrillated cellulose (MFC)-graphite anode at (a) 10 000× and (b) 20 000× magnification. Reproduced with permission from ref 78. Copyright 2010 Royal Society of Chemistry. 8950


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Figure 20. Schematic illustration of 3D porous SiNP/conductive polymer hydrogel composite electrodes. Each SiNP is encapsulated within a conductive polymer surface coating and is further connected to the highly porous hydrogel framework. SiNPs have been conformally coated with a polymer layer either through interactions between surface −OH groups and the phosphonic acids in the cross-linked phytic acid molecules (right column) or through the electrostatic interaction between negatively charged −OH groups and positively charge PANi due to phytic acid doping. Reproduced with permission from ref 327. Copyright 2013 Nature Publishing Group.

matrix. Thus, the tubular PPy actually served as the conductive additive and binder at the same time. Porous carbonaceous materials have been used to address the issues of Si-based LIBs and Li−S batteries and obtained promising outcomes. This success of this strategy is mainly due to fact that binder can build excellent conductive and adhesive networks together with the porous carbonaceous structures. In these networks, the electrode active materials are confined and adsorbed in the carbonaceous structures while the interlocking and physicochemical attraction force maintain the intact of the entire electrode. In conclusion, nanostructure design is an efficient method to improve the performance of electrode binders. Normally, nanostructured polymers, especially conductive polymers, possess various advantages. First, the kinetic process of electron and ions transport can be significantly enhanced due to the 3D pathways. Second, the conductive polymers simultaneously serve as conductive additive and binders, which will improve the gravimetric energy density by reducing inert materials in electrodes. Third, through in situ polymerization, active materials can be uniformly dispersed in electrodes and better electrical contact with conductive polymers can be achieved. Fourth, electrodes with nanocontrolled structures can show excellent flexibility, with high potential for flexible and wearable electrical devices. However, some limitations hinder the practical use of nanostructured binder systems, like low volumetric capacity and relatively low conductivity compared to carbon black.207 Composite Binder. The binder materials reviewed thus far typically possess outstanding properties in only a few areas. As such, it is unlikely that a single material can meet all of the demands required of a multifunctional binder for highperformance electrodes. For example, CMC is known to contain carboxylic functionality as well as improved mechanical strength when compared with PVDF; however, it is hindered by its brittle nature.332 Composite binders are a practical method to combine the advantages of different materials. The most commonly used composite binder is CMC/SBR, in which SBR is an elastomeric additive to CMC to form a CMC:SBR composite binder.28 As the CMC:SBR-based electrode only exhibits a small volume increase with no loss of contact between the electrode and the current collector, it has been widely applied to different electrode materials.16,28,71 Choi et al. fabricated a composite binder for high-performance silicon anodes in LIBs by combining polymerized β cyclodextrin

Figure 21. (a) In a traditional electrode system, bottlenecks and poor contacts may impede effective access to parts of the battery and aggregation of active particles occurs. (b) Conductive polymer gel framework constructs a 3D network for electron transport and a porous structure to facilitate the transport of ions in hybrid gel electrodes. Reproduced with permission from ref 329. Copyright 2017 Wiley-VCH.

the interface between the active material and the electrolyte. Due to multiple functionalities in the binder system, the mass ratio of active material in the hybrid gel-based electrodes could reach as high as 85%. Meanwhile, the hybrid gel electrodes showed high rate capability (1260, 1002, and 845 mAh g−1 at a rate of 0.1C, 1C, and 2C, respectively, 1C = 926 mA g−1) and excellent cycling performance (1100 mAh g−1 within 50 cycles at 100 mA g−1).329 A conventional binder system consists of a single component providing separate functionalities.330 In contrast, nanocontrolled structures can combine binder and conductive additions into a conductive binder, simultaneously providing strong bonding force and conductivity. In some cases, through nanoarchitecture design, active materials are trapped in the nanostructure of conductive polymers, resulting in so-called “binder-free” electrodes. For instance, Zhang et al.331 developed PPy nanotubes to embed sulfur molecules for Li−S batteries. The composites of tubular polypyrrole and sulfur were prepared by coheating the sublimed sulfur with the tubular PPy at a suitable temperature. Results showed that the composite binder exhibited significantly improved conductivity for Li−S batteries and retained a constant capacity of ca. 650 mAh g−1 after 80 cycles. It was also suggested that polysulfides could be greatly retarded by the tubular PPy 8951


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Figure 22. Proposed mechanism of the synergistic effect in the β-CDp/Alg hybrid binder. (a) Agglomerative property of β-CDp due to the intramolecular hydrogen bonding. (b) Stretched feature of Alg due to the electrostatic repulsions between carboxylates. (c) Configuration of the hybrid binder. Positively cooperative effects promote a more homogeneous polymeric network. Moreover, the Na+ originally from Alg also contributes to the good distribution and stability of the hybrid binder by creating a “glue effect”. Reproduced with permission from ref 278. Copyright 2014 American Chemical Society. Figure 23. Schematic diagram of the sulfur cathodes with different binders: (a) PAA, (b) PAA/PEDOT:PSS, and (c) PEDOT:PSS.336 (d) Conductive graphene oxide-poly(acrylic acid) (GOPAA) binder for Li−S battery. Reproduced with permission from ref 335. Copyright 2017 Elsevier.

(β-CDp) and sodium alginate (Alg, Figure 22). 278 Due to its branched 3D structure, β-CDp can provide abundant H-bonding sites between the silicon active material and the copper current collector. However, β-CDp also tends to aggregate due to intramolecular H bonding. Conversely, alginate tends to stretch due to Coulombic repulsion originating from its carboxylate (COO−) moieties. Therefore, the group proposed that when combined the alginate can diminish the agglomeration in the β-CDp while the β-CDp can neutralize the Coulombic repulsion between the COO− units in the alginate. This effect was realized with a β-CDp:Alg ratio of 83:13, as evidenced by cycling performance. However, alginate amounts above and below this ratio had a detrimental effect on performance. To improve binder conductivity, conductive polymers or conductive additives (e.g., reduced graphene oxide) are usually added to form conductive binder blends.125,333−335 Pan et al. developed a PAA/PEDOT:PSS composite binder for an aqueous base electrode fabrication and improved battery performance (Figure 23a−c).336 PEDOT:PSS in the composite binder delivered the advantages of both good electronic conductivity and strong chemical affinity to polysulfides, while the PAA improved the electrode adhesion as well as the electrolyte swelling/uptake to promote Li+ transfer within the sulfur electrode. The hybridization between conductive agent and conventional polymer binder is also an attractive strategy of developing conductive binders for Li−S batteries. Xu et al. developed a composite binder with higher electronic conductivity, superior mechanical property, and strong adsorption of polysulfides through a facile and straightforward strategy (Figure 23d).335 The reduced graphene oxide-poly(acrylic acid) (GOPAA) composite binder was constructed via the hydrogen bonding between rGO and the PAA molecule, where rGO takes up 1/10 of the composite weight. Due to the favorable combination of the flexibility and electronic conductivity of the rGO and the strong adhesion and good polysulfide chemisorption of PAA, the obtained sulfur electrode with GOPAA binder achieved significantly higher capacity, rate capability, and cycling stability than that based on pure PAA binder. With respect to the application of ionically conductive binders, Lacey et al. reported a mixed binder of PEO and PVP

for Li−S batteries.308 The composite binder combines the local improvement to the solvent system offered by PEO and the polysulfide-confining effect of PVP. A PEO to PVP ratio of 4:1 was optimized for the best cyclability with a capacity retention of 800 mAh g−1 after 200 cycles at 1C. Li et al. developed a composite of Li-Nafion, PVP, and nanosilica as a multifunctional binder for high-performance Li−S batteries.337 In this recipe, Li-Nafion offered easier access to Li+ supply for sulfur redox reactions while PVP provided strong chemical adsorption of polysulfides to alleviate the polysulfide shuttling as well as improved mechanical bonding and material dispersion within the sulfur electrode. The impregnated nanosilica could further chemically confine sulfur species due to strong affinity between its polar surface and polysulfides and simultaneously introduced abundant interfaces within the electrode for improved electrolyte wetting and enhanced adsorption sites. Due to these favorable functionalities, the sulfur electrode based on the composite binder achieved a high sulfur utilization with initial discharge capacity of 1373 mAh g−1 at 0.2C, excellent sulfur redox kinetics with highly reversible capacity of 470 mAh g−1 at a high current rate up to 5C, and superb cycling stability over 300 cycles at 1C. Composite binders can go beyond dual-component systems. Lee et al. investigated ternary composites of SBR, CMC, and PAA for graphite anodes in LIBs.227 CMC was used as a thickening agent to prevent graphite particles from agglomerating during processing, while SBR was used as an elastomeric binder. Varying amounts of PAA were introduced to examine whether it could stabilize the precursor suspension via surface active dispersion as well as improve adhesion between the active material and the current collector. A peel test revealed that the adhesion strength of the PAA-containing electrode was three times greater than the SBR:CMC electrode without PAA. Zheng et al. fabricated another ternary composite binder for graphite anodes using a combination of PVDF, PMMA, and PMALi (Figure 24).338 When the ratio between polymers was 6:4:2 (PVDF:PMMA:PMALi), an outstanding rate performance 8952


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Figure 24. SEM images (left) and molecular structure (middle) of PVDF, PMMA, and PMALi. Discharge rate capability of the graphite electrodes with ternary composite binders of different weight ratios (right). Reproduced with permission from ref 338. Copyright 2016 Elsevier.

Figure 25a, the functional group carbonyl (CO) was introduced to lower the lowest unoccupied molecular orbital (LUMO) energy level of the polyfluorene (PF) backbone, leading to improved electronic properties. When PFFOMB was used as a binder in nanosilicon particles, micrometer-size silicon monoxide (SiO) anodes, porous SiO anodes, and Li−S batteries, all of these electrodes showed enhanced electrochemical performance.97,98,330,347 The side chains of conductive polymers can also be modified to realize binder application. After being functionalized with ionic alkyl carboxylate groups of various lengths (Figure 25b), electrically conductive poly(thiophene) could work as a binder for silicon and graphite anodes.348 Compared to commercial PEDOT:PSS and PVDF for graphite anodes and PEDOT:PSS and NaCMC for nanoscale silicon-based anodes, the n-doping side chain was able to increase the electronic conductivity of the polymer, leading to enhanced initial capacities and cyclability in both anodes. Similarly, Gao et al. employed sulfuric acid to coordinate with the PANI chain in m-cresol solvent to form an extended chain structure (Figure 25c). The conductive polyaniline promoted the electron transport between sulfur and the conductive matrix and gave rise to high sulfur utilization, resulting in a higher discharge capacity (showing initial capacity of 872 mAh g−1 and remaining 439 mAh g−1 after 50 cycles at current density of 122 mA g−1).349 In addition to conductive groups in polymeric backbones, conductive units can also be introduced into side chains. Liu et al. introduced conductive side chain pyrene moieties to a methacrylate backbone to form a new class of electric conductive binders and applied it in silicon anodes. The asassembled batteries exhibited a high capacity and excellent rate capability.115 As shown in Figure 25d, pyrene units are crosslinked into the side chain of binders. After synthesis, the ordered phase characteristic of pyrene is still maintained in the binder. The pyrene units are expected to provide the conductivity of the binder, thereby resulting in remarkable electrochemical performance. In their other works, similar conductive binders were synthesized via this strategy.317,350 Ionic Conductivity. Ionic conductivity plays a significant role in rate capability during intercalation/deintercalation within an electrode. The efficient movement of ions becomes especially important at high current rates.351,352 Ionic conductivity for organic-based binders is partially determined by the amorphocity of polymer materials. To verify this, organic-based

with 96.2% reversible capacity retention at 50C was possible, compared with only 16.2% retention for the pure PVDF electrode. The authors ascribed the superior mechanical properties of the anode to the coexistence of PVDF and PMMA and the improved rate performance to the appropriate addition of PMMA and PMALi. Creating composite binders for high-performance electrodes has been a remarkably successful method to combine benefits offered by unifunctional binder components. However, some results have displayed that this fact is not always the case; therefore, careful consideration of the interaction between all binder components and their respective active materials is required to construct the desired synergistic effect.227 5.3.2. Conductive Binders. Binders can provide either electric or ionic conductivity, leading to new electrode systems which shift the status quo from conventional electrodes comprised of active material, polymer binder, and conductive additive. In other words, conductive binders can act as binders and conducting additives simultaneously. Therefore, the transition from conventional electron/ion-inert binders to ones with inherent conductive properties provides an alternative route to enhance the overall capacities of electrodes without relying on mechanical and chemical advances. The conductive properties for binders can be classified into two categories, i.e., electronically conductive and ionically conductive. The conductive polymers can only be effective binders if they can present both strong interactions and high conductivities.97,115,253,327 This section will focus more on the conductivity of binders at the molecular level. Electric Conductivity. For battery applications, conductive polymers were first introduced as coating layers to protect active materials from electrolyte corrosion or as conductive additives in different battery systems, such as cathode materials,339,340 anode materials,341−343 and Li−S batteries.344,345 With the exception of several studies on commonly used conductive polymers, such as PEDOT79 and PEDOT:PSS,346 conductive polymers are rarely used directly as binders. Their electric conductivity can be improved by doping in backbones or side chains. Conductive binders usually adopt a conductive functional group as the backbone and then introduce others groups to achieve binding function and doping effects. An n-doped conductive binder designed by Liu et al. is a good example to explain this strategy. In a PFFOMB conductive binder shown in 8953


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Figure 25. (a) Molecular structure of the PF-type conductive polymers with two key function groups in PFFOMB, carbonyl and methylbenzoic ester, for tailoring the conduction band and improving the mechanical binding force, respectively. Reproduced with permission from ref 97. Copyright 2011 Wiley-VCH. (b) Chemical structure of the three poly(thiophene) binders, each anchored with an ionic carboxylate group and different lengths of the organic side chain. Reproduced with permission from ref 348. Copyright 2016 Wiley-VCH. (c) Extended conducting polyaniline chains in m-cresol. Reproduced with permission from ref 349. Copyright 2017 Elsevier. (d) Generic synthesis of poly(1-pyrenemethyl methacrylate) (PPy, top) and poly(1-pyrenemethyl methacrylate-co-triethylene oxide methyl ether methacrylate) (PPyE, bottom). Reproduced with permission from ref 115. Copyright 2015 American Chemical Society.

PVDF binders were modified to achieve higher amorphocity through conjugation with hexafluoropropylene (HFP). HFP is supposed to possess a higher Li-ion diffusion capability and lower glass transition temperature (Tg) than PVDF. PVDFHFP binder showed the highest rate capability and cycling property for LiFePO4 material, which is ascribed to a higher Li-ion diffusion coefficient derived from higher amorphocity and lower Tg.69 Another way to improve ionic conductivity is ionization. Salem et al. improved the electrochemical performance of silicon anodes, particularly the rate capability, through the enhancement of the ionic conductivity of the binder by introducing ionic alkyl carboxylate groups into poly(thiophene).348 Peled et al. investigated and compared the binder effect on Li2S-based cathodes containing five different binders, including PVDF-HFP, PVP, a mix of PVP with PEI, PANI, and LiPAA.353 Due to the improved localized Li+ support, the LiPAA binder endowed the corresponding Li2S electrode with the highest initial specific capacity of 1200 mAh g−1 and energy efficiency of 90.2% of all of the binders, indicating its positive role in sulfur utilization. Many polymers inherently have a relatively high ionic conductivity,354,355 such as GG,105 PEO,218,219 and Nafion.220 The introduction of ionically conductive polymers into polymer blends can greatly improve the overall ionic conductivity of the material. Lim et al. developed a binder system based on poly(acrylic acid) (PAA) and poly(ethylene glycol-co-benzimidazole) (PEGPBI) containing an ion-conducting PEG group. A silicon electrode with this PAA−PEGPBI-based binder delivered a high capacity of more than 1600 mAh g−1 at a high rate of 2C. The high rate originated from the addition of ionically conductive PEG groups.356 Other binder systems have been designed based on these ionically conductive polymers, such as triblock copolymer polydopamine-polyacrylic-polyoxyethylene (PDA-PAA-PEO), 357 c-PEO-PEDOT:PSS, and c-PEO-PEDOT:PSS/PEI3 as shown in Figure 26. Nafion is

Figure 26. (a) Chemical structure of the polymers PEDOT:PSS, PEO, and PEI used to prepare the binders. (b) Schematic diagram of the interactions between the polymer components (PEI and PEO) and lithium ions that improve lithium-ion transport. (c) Schematic diagram of the interactions inside the binders with high ion and electron conductivities. (1) Cross-linking between PEO and PSS. (2) Electrostatic interaction between PEI and PSS. (3) PEI chemically reducing PEDOT. Reproduced with permission from ref 3. Copyright 2018 Wiley-VCH.

ion selective due to the fixed highly electronegative sulfate termini on the PTFE skeleton. By wrapping the Li-Nafion layer on the sulfur composite surface, diffusion and migration of polysulfide anions can be well regulated due to the electrostatic repulsion effect.358−360 The biggest challenge for the application of conductive polymers as binders in energy storage lies in their potential 8954


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Figure 27. (a) Schematic illustration of self-healing behavior and conventional binder. (b) Chemical structure of the SHP. (c) Capacity retention of Si marcoparticle electrode with various binders. (d) Images show the small cracks (as white arrows pointed) healed after 5 h. Reproduced with permission from ref 362. Copyright 2013 Nature Publishing Group.

efficiency of 81% and a retained capacity of 1454 mAh g−1 after 400 cycles with only 0.04% capacity decay per cycle. It is superior to PAA binder, as long as the content of the selfhealing polymer is sufficient. Figure 28 shows the healing

conductivity loss in harsh conditions of battery systems and their rigid and brittle chains having negative effects on electrode integrity.153,361 5.3.3. Self-Healing Binders. Self-healing of a polymeric material is an attractive characteristic that captures the attention of many researchers. This self-healing ability comes from strong intermolecular interactions (e.g., hydrogen bonding) that can enable reconnection between either the polymer itself or the polymer, the active material, and/or the additive and the current collector (i.e., the intermolecular forces on the interface). Previously, we discussed various approaches that enhance selfhealing adhesion between the binder and the remaining electrode materials, including the way to improve hydrogen-bonding strength. Here, however, we will briefly review self-healing materials strictly from the perspective of interbinder interactions. Bao et al.362 coated silicon microparticles (SiMP) with a thin layer of hydrogen-bond-directed self-healing polymer (SHP) to withstand the great Si volume change during charge and discharge. As illustrated in Figure 27a and 27b, the self-healing capability is ascribed to abundant hydrogen-bonding sites in SHP. This specifically designed polymer can maintain electrode integrity even after the pulverization of Si particles through its abundant hydrogen-bonding sites. The discharge capacity can reach 1617 mAh g−1 for the first cycle at a current density of 0.4 A g−1 and shows a good cycling stability when compared with PVDF, CMC, and alginate (Figure 27c). The self-healing effects are clearly evidenced by SEM images in Figure 27d, in which small cracks can heal themselves over time. Bao et al. further synthesized a tougher self-healing polymer that has covalent cross-links and hydrogen bonds.363 This material can partially heal itself due to its high density of hydrogen-bonding sites, and the recovered sample can reach strains over 100% after 30 min of healing at room temperature. Fang et al.364 investigated the self-healing ability of materials by introducing a quadruple hydrogen-bonding ureidopyrimidinone (UPy) moiety into polyethylene glycol (PEG) oligomers to enable it to heal the mechanical cracks caused by volume change. This conjugated polymer achieved an initial Coulombic

Figure 28. Optical microscopy images of the self-healing of cuts: (a, b, and c) as prepared and after 1.5 and 3 h of UPy-PEG-Si-15 electrode, respectively; (d, e, f) as prepared and after 13 and 25 h on UPy-PEGSi-8 electrode, respectively; and (g, h, i) as prepared and after 13 and 25 h on PAA-Si-15 electrode, respectively. Reproduced with permission from ref 364. Copyright 2018 Elsevier.

situation for this UPy-PEG self-healing polymer and the reference PAA binder on Si. Many conductive polymers also exhibit self-healing capability.365 For instance, Yu et al.366 synthesized a hybrid 3D-spaced PPy/G-Zn-tpy conductive gel material that could provide self-healing properties during cycling. When this kind of conductive self-healing polymer was applied as the binder for LIBs, the volumetric capacity and cycle life can be improved. The self-healing capability highly depends on the reorganization of dynamic covalent bonds or through noncovalent 8955


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interactions, mostly involving hydrogen bonds.367−369 Recently, a mechanically robust, readily repairable polymer with dense hydrogen bonds was developed by Yanagisawa et al.370 This work provides essential structural elements for the design of mechanically robust yet healable polymeric materials, which also give us some new ways to develop suitable self-healing polymers as binders for battery systems. 5.3.4. Redox-Active Binder. Normally, binders will not react with other components during cycling, but Ouatani et al. revealed that the chemical reactivity of CMC toward the electrolyte contributes to higher first cycle irreversible capacity loss in graphite electrodes compared with PVDF/graphite electrodes. This result shows that the chemical reactivity between CMC and the electrolyte takes part in the formation of the surface film and contributes to the good properties of CMC as a binder.72 In this regard, redox-active binders are developed to enhance electrode performance, especially in Li−S batteries. The redox activity not only provides more specific capacity but also hinders the shuttle effect. A simple and straightforward strategy for reactive binders is realized by incorporating active sulfur into the binder structure, which can contribute to the overall capacity while maintaining good electrode integrity. Trofimovet et al. prepared an organopolysulfide bis[3-(vinyloxyethoxy)-2-hydroxypropyl]polysulfides (BVPS) binder by reacting ethylene glycol vinyl glycidyl ether (EGVGE) with Na2S4 in the presence of NaHCO3 and a phase transfer catalyst triethylbenzylammonium chloride. The obtained BVPS contained 24.5% sulfur (n = 2, 3, where n represents the length of the polysulfide chain in the BVPS molecule) bridging symmetric organic moieties, which was further copolymerized with elemental sulfur at 130 °C for 1 h to yield a structured polymer containing up to 32.6% sulfur (n = 4). The polymerization leads to the formation of cross-linked polymers, which were used as the active binder for Li−S batteries. The obtained binder contributed additional capacity due to the redox reactivity of the sulfur incorporated in the binder molecule.65 Thiokol is a type of synthetic rubber polycondensationed by alkyl dihalide and alkali metal or alkaline earth metal polysulfide and proposed to act as a polysulfide scissor whose thiols could promote the breaking of the disulfide bonds in long-chain soluble polysulfides to form short-chain ones, thus reducing the polysulfide dissolution and shuttling behavior.371 A similar mechanism was also reported when using dithiothreitol as electrolyte additive for the suppressed shuttle effect.372 These merits endowed the thiokol-based electrode with much improved capacity and cycling stability compared to that based on the conventional PVDF binder. Imide-rich polymers are redox active due to the incorporation/release of lithium within a certain potential range.373 Mecerreyes et al. reported using the polyimide-polyether as a reactive binder for Li−S batteries.374 The properties of the imide groups redox mediate the charge−discharge behavior of active sulfur, thus enabling a suppressed shuttle effect while promoting mass/charge transfer for enhanced sulfur utilization and expedited redox kinetics. Three copolymers with different imide structures were investigated as redox-active binders for sulfur electrodes including pyromellitic, naphthalene, and perylene polyimide-polyether. Among these, the naphthalenepolyether held the appropriate redox potential (2.2−2.5 V vs Li/Li+) that coincided most with the sulfur redox, thus offering the strongest localized Li+ support from the prestorage of Li+ by its own electrochemical redox. Benefiting from such a mechanism, the corresponding sulfur electrode obtained

significantly enhanced sulfur utilization, improved redox kinetics, and stabilized cycling, even with a carbon additive as low as 5 wt %. A similar mechanism used π-stacked perylene bisimide (PBI) as an aqueous redox-mediating supramolecular binder for sulfur electrodes.375 Beyond the above-mentioned modes of binder actions, researchers are also actively searching and developing new bonding mechanisms between binders and active materials to benefit the sulfur electrochemistry and boost the electrochemical performance of Li−S batteries toward practical viable standards. Representatively, Ling et al. proposed a novel nucleophilic substitution mechanism between polysulfides and binders to make the Li−S batteries more durable and efficient.120 The reactivity of a nucleophilic reaction is usually governed by several factors including the ability of both the nucleophile and the leaving group to polarize, the stability of the leaving group, and the interaction between the nucleophile and the leaving group, etc. In a Li−S system, a good leaving group should hold a low pKa for its anion’s conjugate acid to ensure the stable large negative charge in both its transition and its product states. In addition, the leaving group should also not introduce side-reactive species to the system. On the basis of these considerations, sulfate was adopted as the leaving group in the binder design. The synthetic poly(vinyl sulfate) potassium salt (PVS) was employed as their first example for nucleophilic substitution due to its well-controlled sulfate-rich structure. Carrageenan, a bioderived polymer from polysaccharides in seaweed, was ultimately chosen due to its combination of the nucleophilic substitution and the much better mechanical binding strength than PVS. The substitution reaction proceeded by replacing the sulfate terminuses in PVS or carrageenan with lithium polysulfides, which was verified by the XPS spectra with the detection of C−S bonding as the indicator of the successful substitution (Figure 29). Due to this nucleophilic substitution mechanism, the intermediate polysulfides can be covalently bonded onto the binder skeleton, serving as a strong sulfur immobilizer that efficiently alleviates the shuttling behavior and the capacity loss during the battery operation. On this basis, the carrageenan binder delivers additionally a much higher adhesion strength than either PVS or conventional PVDF binder, enabling a micrometer-sulfur mass loading of 24.6 mg cm−2 with an intriguingly high initial areal capacity of 33.7 mAh cm−2. This is the highest reported for Li−S batteries based on polymer binders, indicating great potential of the nucleophilic substitution bonding mechanism in the development of high-energy Li−S batteries.

6. BINDERS IN DIFFERENT ENERGY MATERIALS In this section, we introduce the specific strategies in designing binding systems for individual electrode materials. The applications of various binders to different electrode materials are summarized. The different electrochemical environments for the anode and cathode of a battery impart different requirements on binders applied in these components. Put simply, an anode binder’s lowest unoccupied molecular orbital (LUMO) must be higher than the chemical potential of the anode material; otherwise, the binder itself will be reduced before the anode materials upon charging. Conversely, the cathode binder’s highest occupied molecular orbital (HOMO) must be lower than the chemical potential of the cathode material; otherwise, it will be oxidized upon charging. For PVDF, both of these requirements are met, which explains its successful application across anode and cathode materials 8956


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Figure 29. (a) Molecular structures of PVS and carrageenan and their replacement reactions with polysulfide to form immobilized polysulfides on the polymer backbones. (b) Visual effects of the polysulfide solution exposed to different binders over 24 h. (c) Schematic of the in situ XAS measurement setup. (d) Relative polysulfide concentration changes with discharge show the superiority of carrageenan binder in immobilizing polysulfide. Reproduced with permission from ref 120. Copyright 2017 Elsevier.

energy density and excellent cycling stability for LIBs. The common binders used in LIBs are listed in Table 3. 6.1.1. Layered LiMO2 (M = Co, Ni, Mn, Al). Layered LiCoO2, proposed by Goodenough et al. in 1980, was the first commercialized cathode material in LIBs and remains the most popular because of its superior cycle stability, excellent reversibility, high energy density, and ease of preparation.397,398 In LiCoO2 batteries, commercialized PVDF is the dominant binder. LiCoO2 suffers from the formation of Co3O4 on its surface as well as Co2+ dissolution into the electrolyte, both of which result in capacity fading. When PVDF is used as a binder, this process is accelerated at the contact points between these materials which exacerbates the problem.9 Lee et al. used an aqueous-based poly(butyl acrylate) (PBA)-PAN as a binder for LiCoO2 cathodes, which resulted in less capacity fading per cycle, which might be due to the presence of polar functional groups and improved adhesive properties of PAN-based binders in LiCoO2 cathodes.67 Layered Li-rich oxides (LLRO) have the chemical formula Li1+xNiyCozMn1−x−y−zO2 and are an attractive replacement for LiCoO2 cathodes because of their superior energy/power density.399,400 Unfortunately, structural changes occur within the LLRO’s during long-term cycling which fragments the active material and causes significant capacity fading.401 Often these drawbacks are avoided through complicated synthesis routes and structure modification; however, Zhang et al. simplified this solution by using guar gum as the binder.109 The water-soluble and natural polymer binder hindered detrimental side reactions and provided a clear performance enhancement over PVDF-based electrodes. 6.1.2. Olivine LiFePO4. Olivine LiFePO4 (LFP) is a promising cathode material for rechargeable LIBs due to its high capacity, long cycle life, environmental friendliness, low cost, and safety. However, intrinsic drawbacks of the olivine structure induce a poor rate performance due to low Li-ion diffusion rate and poor electronic conductivity,402,403 especially at high temperature.404 LFP-based electric vehicles have started to infiltrate transportation infrastructure in various applications globally, but their uptake is hindered by their travel range.405

alike. This requirement also explains SBR’s success as an anode binder but not as a cathode binder. SBR’s LUMO is high enough for application as an anode binder; however, the double bonds present within its structure are prone to oxidation upon charging when applied as a cathode binder.376 According to the composition of the electrode materials, the electrode materials can be classified into inorganic electrode materials and organic electrode materials. Different from most inorganic electrode materials (such as LiCoO2, LiFePO4 cathodes, and graphite anode), organic electrode materials are made up of pure organic compounds where functional groups (such as quinones and dianhydrides) are responsible for the storage of Li ions.377 In order to facilitate the ionic conductivity of the resultant electrode, the organic electrode materials are commonly made into nanostructured materials. As a result, the protocol of the organic electrode fabrication is fundamentally similar to the classical PVDF/CB/NMP method, i.e., involves the use of organic solvent, binder, and conductive additives.378 Because the binders are also made of organic compounds, incorporation of the binding functions into the organic electrode materials via combining the functional groups of ionic transport, ionic adsorption, and binding force is a promising strategy and development direction. As such, binder effects on common anode and cathode materials will be discussed separately in this section before moving onto other energy-storage systems, including Li−S batteries, NIBs, and supercapacitors. 6.1. Binders in Cathodes of Li-Ion Batteries

Transition-metal oxides with layered structure, manganese oxides with spinel structure, and polyanion-based olivine structure have been highlighted as cathode materials for future applications. Each material has its own merits and disadvantages compared with other candidates. However, as the modification of the cathode itself is limited in the improvement of energy density,379,380 researchers tend to study the other electrode components which include binders. In this section, we focus on the effects of binders on cathode materials for high 8957


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Table 3. Applications of Some Representative Binders in LIBs active materials

binder

initial discharge capacity (mAh/g)

reversible discharge capacity (mAh/g)

C rate

ref

LFP LFP LFP LFP LFP LFP LFP LFP LFP LFP LFP LiCoO2 LiCoO2 LiCoO2 LiMn2O4 LiMn2O4 LiMn2O4 LiMn2O4 LiMn2O4 LiMn2O4 Li1.2V3O8 Li1.2V3O8 Li1.2V3O8 graphite graphite graphite graphite graphite graphite Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si SiO2 SiO2 SiO2 SiO2 Li4Ti5O12 Li4Ti5O12 Li4Ti5O12 Li4Ti5O12 Li4Ti5O12 CoFe2O4 SnO2 SnO2 TiO2 TiO2 TiO2

PVDF-HFP PAA carboxylmethyl chitosan alginate xanthan gum PEDOT:PSS/CMC PVA PTFE PANI PMMA SA-PProDOT PVDF PEGMA/MMA/IBVE PA/PBA PVDF CMC PAA alginate PAMMA PVAc/PVDF PVDF PVDF-HFP/PEO PTFE PVDF PEDOT:PSS xanthan gum LiCMC CMC AMAC PVDF PAA PVA PVDF CMC CMC/SBR PAA/CMC PAA/BP PAA/PVA alginate gum Arabic carboxylmethyl chitosan PEI PPy TBA-TEVS PAI PEO/PEDOT:PSS/PEI PVDF PAA CMC PVA CMC PEGMA/MMA/IBVE Acryl S020 guar gum tara gum alginate CMC alginate PVDF PVDF-HFP CMC/SBR

∼125 ∼135 ∼150 ∼140 ∼160 ∼140 ∼80 150.3 ∼120 170 ∼140 ∼140 ∼138 ∼140 ∼100 ∼100 ∼100 ∼121 110 110 270 ∼280 ∼190 375 375 375 290 375 ∼200 ∼2700 ∼500 ∼1300 ∼1400 ∼1400 ∼3000 ∼2800 ∼1600 ∼2660 ∼2000 ∼4000 ∼4000 ∼1600 ∼2000 ∼2500 ∼2000 2440 ∼100 ∼800 ∼400 ∼400 ∼180 ∼170 ∼150 179.7 176.3 ∼1600 ∼1400 ∼2000 175.5 172.4 163.3

∼120 (120 cycles) ∼120 (200 cycles) 147 (200 cycles) ∼130 (100 cycles) 151.1 (100 cycles) ∼140 (100 cycles) ∼80 (80 cycles) 146.5 (100 cycles) ∼145 (100 cycles) 160 (30 cycles) ∼120 (400 cycles) ∼110 (80 cycles) ∼130 (20 cycles) ∼130 (30 cycles) ∼80 (200 cycles) 60 (200 cycles) ∼90 (200 cycles) 117 (200 cycles) 100 (200 cycles) 88 (100 cycles) 220 (100 cycles) ∼280 (50 cycles) 68 (100 cycles) 337 (100 cycles) 330 (100 cycles) 350 (100 cycles) 300 (100 cycles) 310 (100 cycles) ∼200 (60 cycles) ∼800 (50 cycles) ∼600 (50 cycles) ∼625 (50 cycles) ∼25 (500 cycles) ∼500 (35 cycles) 2221 (30 cycles) ∼2400 (100 cycles) ∼1400 (100 cycles) ∼2000 (50 cycles) ∼2000 (100 cycles) ∼2200 (300 cycles) 950 (50 cycles) ∼800 (50 cycles) ∼1500 (1000 cycles) ∼2000 (100 cycles) ∼1800 (20 cycles) 2027 (500 cycles) ∼0 (50 cycles) ∼700 (50 cycles) ∼0 (50 cycles) ∼0 (50 cycles) 153 (100 cycles) ∼155 (20 cycles) ∼150 (200 cycles) 160 (100 cycles) 150.1 (100 cycles) 1040 (30 cycles) ∼400 (50 cycles) ∼800 (50 cycles) 158.8 (140 cycles) 158.4 (140 cycles) 153.6 (140 cycles)

1C 1C 0.2C 0.1 mA/cm 0.2C 1C 1C 0.2C 1C 0.1 mA/cm 1C 0.125C 0.5C 0.2C 1C 1C 1C 1C 0.5 mA/cm 0.2C 0.5C charge, 0.2C discharge 0.13C 0.1C 0.083C 0.083C 0.083C 0.083C 0.083C 0.5 mA/cm ∼0.01C 0.5C 0.5C 0.5C ∼0.02C ∼0.05C ∼0.07C ∼0.05C 1C 1C 0.1C ∼0.12C 0.2C 2C 0.05C 0.1C ∼0.25C 0.02C 0.02C 0.02C 0.02C 1C 0.5C 1C 1C 1C 0.1C 0.1C 1C 0.5C 0.5C 0.5C

69 10 381 289 150 382 383 384 385 232 92 9 48 67 13 13 13 386 250 387 388 389 390 79 79 79 79 79 63 26 18 18 18 25 28 55 56 57 91 101 110 114 115 391 392 3 27 27 27 27 84 51 393 108 108 394 86 395 16 16 16

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Table 3. continued active materials TiO2 SnS2 SnS2 SnS2

binder

initial discharge capacity (mAh/g)

CMC PVDF carboxylmethyl chitosan CMC

172.4 661 899 937

reversible discharge capacity (mAh/g) 166.7 (140 cycles) 265 (50 cycles) 479 (50 cycles) 544 (50 cycles)

C rate 0.5C 0.5C 0.5C 0.5C

ref 16 396 396 396

capacity of 148 mAh g−1. Unfortunately, LiMn2O4 tends to exhibit capacity fading, especially at elevated temperatures, because of the dissolution of Mn into electrolyte.386 Functional binders could solve these problems, for example, uniform coatings of PAA and CMC binders on the LiMn2O4 particle surface effectively alleviate the dissolution of Mn2+ into electrolyte for better cycling performance.13 Similarly, watersoluble and fluorine-free alginate extracted from brown seaweed as the binder could improve the Mn2+ chelation effect with the binder and therefore prevent active material loss to the electrolyte, leading to excellent cyclability in both LiPF6-based and LiBOB-based electrolyte.386 LiMn1.5Ni0.5O4, a derivative from LiMn2O4, possesses a high operating voltage of over 4.7 V while also exhibiting a high rate capability due to the three-dimensional diffusion path of lithium ions.409 However, it also suffers from structural change because of Jahn−Teller distortion and Mn dissolution.410 Tanaka et al. synthesized a PAN-grafted PVA copolymer for use in LiNi1/2Mn3/2O4 cathodes. The grafted PAN provided the composite with suitable oxidation resistance required for this high-voltage material, and the PVA backbone afforded a high adhesive strength to withstand volume change during cycling.411 6.1.4. Other Cathodes. Li1.2V3O8 was also investigated as a promising positive electrode material due to its high theoretical capacity of 360 mAh g−1. The experimental capacity, however, could achieve only ca. 180 mAh g−1. A polymeric binder preplastified PEO was synthesized for Li1.2V3O8 cathodes. Homogeneous and efficient carbon black (CB) distribution resulted in good CB/Li1.2V3O8 interfaces due to the dispersion effect of the as-prepared plastified PEO and consequently optimized electrode performance.412 Long-term cycling performance, however, needs further improvement since PEO has limited electrochemical stability during oxidation.389,413 The above research confirms critical roles of binders in enhancing the electrochemical performance of cathode materials for LIBs. For layered LiMO2 (M = Co, Ni, Mn, Al) materials, the functionalities of the binders should be focused on conductivity, phase stability, and inhibition of side reactions and corrosion during cycling. For olivine LiFePO4, special attention should be paid to the ionic and electronic conductivity of binders to improve the cycling performance of LiFePO4 due to its insulating properties. Finally, spinel LiMn2O4 tends to exhibit capacity fading at elevated temperatures; thus, the influence of temperature on binder performance needs to be considered. Several other detrimental effects, including Jahn−Teller distortion-induced volume variation and dissolution of Mn into the electrolyte, should also be improved by special designs of binders. In summary, different cathode materials have their own sources of capacity fading, which guide the design of novel binders with special functionalities to achieve good cycling performance in the near future.

Improving LFP capacity, and therefore LFP-based EV travel ranges, through active material modification is unlikely, as the electrodes based on traditional PVDF binders are already approaching the maximum theoretical capacity (170 mAh g−1) based on the mass of the active material.406 Despite this an increased capacity can still be achieved with respect to the mass of the entire electrode by reducing the content of conductive additives and binder. LFP’s low ionic and electrode conductivity can also be circumvented through the use of multifunctional binders, as demonstrated by our group.92 A PProDOT (conjugated 3,4-propylenedioxythiophene-2, 5-dicarboxylic acid)-functionalized sodium alginate binder (Figure 30) was able to not only remove the need for

Figure 30. Schematic design of the synthesis of SA-PProDOT polymer. SA and ProDOT molecules self-assemble along with hydrophilicity of functional groups, forming an interface where the esterification reaction takes place. Produced water is removed from the interface to the hydrophilic phase, which advances the reaction equilibrium. Reproduced with permission from ref 92. Copyright 2017 American Chemical Society.

conductive additives in the electrode, thereby increasing LFP loading, but also increase ionic conductivity resulting in improved rate performance. PAA has also been applied as an aqueous binder for LFP cathodes.10,52 Zhang et al. demonstrated that when PAA is used as a binder it can help to negate the drawbacks associated with high-temperature operation from which LFP suffers.407 A lithiated PAA (PAALi) binder enabled a high LFP loading of 82% and capacity which almost reached the theoretical limit for this material that has also been investigated.11 As a binder for LFP, CMC also shows favorable properties. The CMC-based LFP electrode fabricated by Lux et al. displayed good mechanical properties and showed 75% capacity retention over 1000 cycles.74 Furthermore, a lithiated CMC (CMCLi) was able to improve ionic conductivity and rate capability in LFP electrodes.408 All in all, the aforementioned binders are able to minimize LFP’s shortcomings by increasing electronic and ionic conductivity as well as minimizing the need for conductive additives and improving high-temperature performance. 6.1.3. Spinel LiMn2O4. Spinel LiMn2O4 is attractive as it is low cost, and environmentally benign and has a high specific

6.2. Binders in Anodes of Li-Ion Batteries

To date, graphite is the dominant material for LIB anodes. Although graphite already has a much higher specific capacity 8959


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6.2.2. Silicon. Silicon is widely considered by many to be the “Holey Grail” of LIB anode materials, due to its outstanding theoretical capacity of 3580 mAh g−1 for partial lithiation to Li15Si4 and 4200 mAh g−1 for complete lithiation to Li22Si5. If this capacity is fully realized, a 10-fold improvement over theoretical capacity of graphite could be obtained. Additionally, silicon anodes have the advantages of low lithiation/delithiation potential, natural abundance, and environmental benignity. Unfortunately, Si-based anodes are hindered in their application as they undergo up to 400% volume change upon lithiation and suffer from serious interfacial side reactions, which have spurred concerted efforts to overcome these challenges.420 Significant progress has been made through the synthesis of various nanostructured Si materials, such as nanocrystals,421 nanowires,422 core−shell nanofibers,423 nanotubes,424 nanospheres, nanoporous materials, and Si/carbon nanocomposites;321,425,426 however, the performance has still not been optimized. The role that binders could play in the application of silicon anodes was largely overlooked until the pioneering research by Kovalenko et al. revealed that an algae-based binder could deliver a reversible capacity 8 times higher than state-of-the-art graphite anodes.91 Since then the development and application of multifunctional binders mainly aim to solve two big challenges in silicon anodes: huge volume change during charge−discharge process and poor electron and ion transport. The mitigation of the effects resulting from volume change, pulverization, and low conductivity through the use of multifunctional binders is thoroughly discussed in section 5 with many of the examples arising from silicon anode systems. As such, in the interest of brevity, steps toward addressing these challenges through binder application are only briefly re-examined below. The most widely used binders for silicon anodes are listed in Table 3. Strategies To Address Drastic Volume Changes and Pulverization. As mentioned previously, binders with excellent mechanical properties, interfacial interactions, and self-healing abilities are required to withstand large volume expansion associated with the lithiation/delithiation of silicon. In addition to the Na-alginate binder used by Kovalenko et al., binders such as PAA/CMC composite,55 gum Arabic,101 and guar gum105 display the required strength, flexibility, and adhesive properties to withstand large volume changes and deliver high-performance Si anodes. Strong interfacial interactions, including H bonding and covalent bonding, are also crucial to maintain electrode intergitiy and contact during volume change. To enhance these interactions, CMC,194 PAA,288 and alginate91 are the first three groups of binders that have been widely explored in silicon anodes because of their abundant carboxylic and hydroxyl groups. These functional groups interact with surface Si−O and Si−OH moieties present on the silicon active materials and can also help to withstand volume change. Subsequently, other groups, such as catechol groups and methylbenzoic ester groups, were also found to be able to strengthen the interfacial forces.97,293 On the basis of these binders, cross-linking polymers and composite materials are designed to combine these functional groups and provide more contact points beween binders and active materials.53,227,261,276 For example, as shown in Figure 31, the aforementioned host− guest complex binder comprised of hyperbranched β-cyclodextrin polymer (β-CDp) and a dendritic gallic acid was synthesized via a dynamic cross-linking process.279 This cross-linking structure increased noncovalent interactions between binders and silicon particles, thus contributing to electrode integrity and enhanced

than most cathodes in LIBs, there is strong demand to develop alternative anode materials with low cost, good safety, high energy/power density, and long cycle life. In this section, we investigate the effects that multifunctional binders can provide when combined with the most commonly researched LIB anode materials, including carbonaceous materials, silicon (Si), and other representative anodes, e.g., spinel lithium titanate (Li4Ti5O12), spinel cobalt ferrite (CoFe2O4), tin oxide (SnO2), titanium oxide (TiO2), and tin disulfide (SnS2).414−416 6.2.1. Graphite. Graphite is commercially successful as an anode material for LIBs due to its long and stable cycle life, abundance in nature, and relatively low cost. Typically the binders used in graphite anodes are either PVDF or, more recently, SBR composites.376 However, by applying novel functional binders to graphite anodes, progress can be made toward a longer cycle life, improved performance, and lower cost for electrodes. As mentioned, binder functional groups can mediate the formation of H bonding or chemical bonds with active material to enhance adhesive properties.194 For example, CMC, with abundant carboxylic acid groups was used as a binder in CMC/ SBR composite-based graphite electrodes. The electrode slurry displayed better dispersion properties, and the electrodes fabricated from this slurry displayed stronger adhesion to the copper current collector than reference PVDF-based electrodes. As a result, more than 90% of the initial discharge capacity was retained over 200 cycles.417 Furthermore, by carefully manipulating the content of CMC, SBR, and graphite in an electrode, these properties can be optimized.291 Now that graphite anodes have experienced commercial success, attention has shifted toward a more cost-effective fabrication method. Water-soluble binders can help reduce manufacturing costs by removing the need for NMP solvent through the switch to aqueous processes. Therefore, watersoluble PAA- and PI-based binders could help reduce manufacturing costs while still providing good adhesion and functional groups to promote the reversibility of graphite anodes.17,64,418 Gelatin-based binders are also particularly attractive in this regard. Not only are they water soluble but also their distinct binding mechanism can reduce the required quantity of binder. Gelatin binds neighboring particles via a bridging mechanism which requires a lower binder content than the binders using a network mechanism and may further lower the manufacturing cost.77 In the application of aqueous binders, controlling the drying process of slurries can facilitate a homogeneous distribution and enhance the mechanical interlocking between binders and active materials, thereby optimizing the mechanical strength of the graphite electrode. As an example, the water-based SBR/ CMC composite exhibited a better uniformity than PVDF after drying.419 In an effort to improve electrode performance, Chong et al. used a composite binder containing SBR and neutralized PAAX (X = Li, Na, K) for spherical natural graphite.11 The results illustrated that the PAALi/SBR composite forms a more homogeneous SEI layer, which resulted in an improved discharge capacity and initial CE compared with a PVDF-based graphite electrode. Graphite-based anodes have already proven to be remarkably effective LIB components; however, as described above, the enhancement of these electrodes can be achieved through the use of multifunctional binders. 8960


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Figure 31. (a) Graphical representation and chemical structures of hyperbranched α-, β-, and γ-CDp. (b) Proposed working mechanism of dynamic cross-linking of β-CDp and 6AD in an electrode matrix along with graphical representations and chemical structures of guest molecules incorporating adamantane moiety. Reproduced with permission from ref 279. Copyright 2015 American Chemical Society.

titanium oxide (TiO2), and tin disulfide (SnS2), to demonstrate the effectiveness and potential of binder strategies on their electrochemical performance. Li4Ti5O12. Spinel lithium titanate (Li4Ti5O12) (theoretical capacity of 175 mAh g−1) has attracted great interest as an anode material for rechargeable LIBs because it offers a great safety improvement due to high and flat Li insertion voltage at ca. 1.55 V vs Li/Li+. Binder effects on this material have been investigated using CMC, PEG, PAN, and Acryl S020.32,51,393,431 Cyclic voltammetry (CV) was conducted on Li 4 Ti 5O 12 electrodes using PVDF and CMC binders, with both returning profiles suggesting good reversibility of lithium intercalation at a scan rate of 0.1 mV s−1. When the scan rate was increased to 5 mV s−1, the CMC-based Li4Ti5O12 electrode still displayed good reversibility and rate capability; however, the PVDF-based electrode was unable to perform adequately at this faster rate. Moreover, the CMC-based electrode had lower charge transfer resistance, smaller activation energy, and higher Li-ion diffusion coefficient.431 On the basis of the promising performance of CMC-based Li4Ti5O12, a synergistic effect was also demonstrated when it was applied with electrolytic solutions based on ionic liquids.75 Poly(PEGMA-co-MMA-co-IBVE) copolymer binder was also demonstrated on >200 μm thick Li4Ti5O12 electrodes. The PEG-based copolymer is a unique and multifunctional material that plays many crucial roles including effective dispersion of electrode slurries, ensuring the cohesion of electrode materials, and maintaining ionic pathways throughout the electrode.51 According to the study by Gong et al., highly polar PAN was also a promising binder for Li4Ti5O12 anodes due to a high polarity for strong adhesion and low irreversible capacity. The PAN-containing electrodes also showed good electrolyte wettability and low charge transfer resistance.32 CoFe2O4. Spinel cobalt ferrite (CoFe2O4) is a promising anode material for LIBs due to its low cost and high chemical

cycling performance (up to 90% capacity retention after 150 cycles at 0.5C). Another strategy to tackle volume change in silicon anodes is to develop self-healing binders. To date, designed self-healing binders, such as SHP,362 UPy-PEG,364 and PPy/G-Zn-tpy,366 have been quite effective in accommodating volume change as they can regenerate the interfacial forces between the binder and silicon which may have been broken during lithiation/ delithiation. Furthermore, self-healing polymers can form new interfacial binding interactions with pulverized silicon which may otherwise have lost contact. Strategies To Address Issues Related to Electrical and Ionic Conductivity. A common approach to enhance electrical conductivity is through the use of conductive polymers, such as pyrene-based,115 polyfluorenen-based,97 polyaniline-based,327 and PEDOT:PSS-based polymers,3,427 as binder. Another way is adding conductive polymers into binder systems to form binder blends such as CMC/PEDOT:PSS binder.428 Similarly, these strategies can also be adopted to facilitate ionic transport. The ion-conductive polymers, such as PEG (PEO) and PEI,429,430 are generally adopted to synthesize ionic conductive binders, e.g., PAA-PEGPBI,356 c-PEO-PEDOT:PSS and c-PEOPEDOT:PSS/PEI,3 and (PDA−PAA-PEO).357 Multifunctional binders such as the aforementioned PFFOMB binder,2 3D PANI binder,327 and CMC/PEDOT:PSS binder428 are prime examples which demonstrate the ability of binder to tackle many challenges faced in the manufacture of high-performance silicon anodes. 6.2.3. Other Anodes. Other materials, such as metal oxides, sulfides, and phosphides, are also considered advanced anodes for Li-ion batteries due to their relatively high capacity, low cost, and enhanced safety. In this section, we choose some representative anode materials, such as spinel lithium titanate (Li4Ti5O12), spinel cobalt ferrite (CoFe2O4), tin oxide (SnO2), 8961


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SnS2 particles in the electrode. These favorable characteristics significantly improved both the cycling and the rate performance of the SnS2 electrode.436 In summary, anode materials such as silicon, metal oxides, and sulfides are promising alternatives to graphite due to their improved capacity and energy density. However, poor conductivity and large volume change during cycling hinder their commercial development. In this case, aqueous binders stand out as a highly effective strategy to address the issues mentioned above. Such binders can mechanically and chemically integrate active materials and conductive additives to achieve good electrode integrity and decent conductivity with improved electrochemical performance. These strategies can be considered in future R&D for LIB anode development.

stability. CoFe2O4 possesses a high theoretical capacity of 914 mAh g−1 but suffers from a severe drawback of high capacity fading during cycling. A porous CoFe2O4 nanoclustergraphene composite was synthesized by a solvothermal process and investigated as an anode with Na-alginate binder. It exhibited a high stable capacity of 1040 mAh g−1 at C/10 with excellent rate capability, while the alginate binder held the integrity of the electrode.394 The performance was much better than that of PVDF binder, which is attributed to the stronger binding strength of Na-alginate binder. SnO2. Tin oxide (SnO2) was proposed as a promising alternative anode for LIBs due to its high theoretical capacity (790 mAh g−1), low cost, low toxicity, and widespread availability.432 CMC has been applied as an aqueous binder for SnO2-based LIBs, which improved the initial capacity and capacity retention of SnO2 anodes compared with those based on PVDF binder.85,86 Nanoporous SnO2 hollow microspheres have also been prepared for LIB anodes. The electrochemical properties of SnO2 hollow microspheres were investigated by employing different binders, e.g., PVDF, Na-CMC, and Na-alginate. EIS measurement confirmed the lowest charge transfer resistance was attained from the Na-alginate-based electrode due to stronger adhesion between electrode film and current collector. With this improved integrity, the Na-alginate binder-based electrode exhibited a remarkably enhanced capacity retention of ∼92% after 50 cycles.395 TiO2. TiO2 with a theoretical capacity of 167.5 mAh g−1 has been attractive in LIBs because it is abundant, low cost, and environmentally friendly and shows small volume change (1200 1000 1000 3000 5000 1000 1000 10 000

87 90 94

1000 1000 1000

current density or scan rate 0.1 A/g 1 A/g

1.28 A/g 1 A/g 1 A/cm2 5 A/g 50 mA/cm2 2 A/g 1 A/g 2 mV/s 2 A/g 1 A/g 0.2 A/g 10 A/g 2.5 mA/cm2 50 mV/s 1 A/g 0.2 V/s

ref 22 128 129 138 66 472 473 474 131 132 133 60 44 45 134 475 46 140 135 136

pseudocapacitance is a faradaic process involving fast redox reactions between the electrolyte and electrode.468 Porous activated carbons (AC) are the most commonly used materials for EDLC due to their low cost, high surface area, and tunable pore structure. Common carbons include pyrolyzed organic carbons, CNTs, and graphene. Their high specific surface areas result in high capacitances due to increased available area for electrolyte−electrode interactions.469 For pseudocapacitance, transition-metal oxides or conductive polymers are typically investigated.470 Previously, RuO2 was a heavily researched transition-metal oxide due to its high theoretical capacitance, but its high cost has led to the investigation of other low-cost transition-metal oxides, among which the most promising material is MnO2. Carbon-transition-metal oxide composite electrodes are always fabricated to combine EDLC and pseudocapacitance to maximize overall capacitance.471 Similarly, due to unfavorable mechanical properties and structural deterioration upon cycling of conductive polymers, composite conductive polymer electrodes were usually fabricated.140 Like with battery electrodes, SC electrodes are usually fabricated by coating a slurry of active material, conductive additives (if present), and binder (typically PVDF or PTFE) onto a current collector.466 The binder content in SC electrodes should be minimized for several reasons: (i) to maximize active material loading, therefore increasing gravimetric/areal capacitance, (ii) to reduce the amount of inaccessible pores on active materials due to binder blockage, and (iii) to reduce electrode resistance by reducing electronically insulating binder material.22 Generally speaking, the binders for LIBs can be applied to the fabrication of electrodes for SCs. The proposed binding mechanism for electrode active materials of LIBs can be also applied to that of SCs. The binders in SCs are listed in Table 6. Taberna et al. fabricated high-performance ac electrodes with high active material loadings by using 3 wt % CMC and 2 wt % PTFE as binder.476 The use of CMC is in accordance with “green” chemistry; however, the use of fluorinated binders was unavoidable in this case. Aslan et al. avoided the use of

e.g., Na15Sn4. The employment of PAA binder led to improved reversible capacity, high initial Coulombic efficiency, and good cycling stability by providing favorable mechanical strength and surface modification to the Sn-based electrode. Metal Oxides/Sulfides. Metal oxides/sulfides are a class of widely studied anode materials for NIBs. They have high specific capacity, benefiting from sodium conversion electrochemical process between metal oxides/sulfides that can exchange multiple electrons per transition metal.464,465 Furthermore, the problem of the large ionic radius of Na+ ions hindering its intercalation will not occur in conversiontype anodes.42 Among these metal oxides/sulfides, MoS2 has attracted much attention in NIBs as a typical 2D-layered metal dichalcogenide.54 With respect to binder research on MoS2 as anodes for NIBs, Kumar et al. reported on MoS2 microflower electrodes with water-soluble Na-alginate as binder for NIBs.40 Significantly superior to electrodes with conventional PVDF and PEG binders, the Na-alginate-supported electrode showed an excellent cycling stability with a high discharge capacity of 595 mAh g−1 after 50 cycles as well as an outstanding high rate capability of 236 mAh g−1 at 10C without any carbonaceous materials. The enhanced cell performance is attributed to the synergetic effect of the microflower morphology of MoS2 and excellent binding strength of the alginate binder to tolerate electrode volume expansion and shrinkage during cycling. 6.5. Binders for Supercapacitors

Supercapacitors (SCs) are electrochemical devices investigated for high-power applications. Compared with LIBs, SCs display a higher power density (1−10 kW kg−1 for SCs vs 150 W kg−1 for LIB), extremely long cyclability (up to 1 000 000 cycles in some cases), and low cost per cycle. However, SCs have a limited energy density (ca. 5 Wh kg−1) and a high overall cost.466 SCs have been applied in a wide range of devices including hybrid electric vehicles and portable electronics.467 SCs store energy by electrode−electrolyte interactions resulting from two types of capacitive behavior: electric double-layer capacitance (EDLC) or pseudocapacitance. EDLC is a nonfaradaic process where energy is stored electrostatically at the electrode−electrolyte interface, whereas 8966


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fluorinated polymers by fabricating ac-based EDLC using PVP as a binder for nonaqueous SCs.22 They found that the electrode with 3.5 wt % PVP displayed better mechanical properties than the electrode with 10 wt % PVDF. Furthermore, the PVP-based electrode displayed better specific capacitance, which was likely due to a smaller reduction of specific surface area (SSA) in the PVP compared with the PVDF-based electrode (ca. 3% reduction for PVP, 30% for PVDF). Sun et al. also avoided the use of fluorinated polymers by fabricating ac electrodes using LA135, which is mainly composed of polyacrylonitrile and styrene−butadiene rubber, as binder for nonaqueous supercapacitors. Among the binders of PTFE, PVDF, CMC, and LA135, the electrode with LA135 exhibited the highest thermal stability as displayed in Figure 33.

including mechanical and chemical characterization, in situ characterization, and even computational calculation, are essential. This section briefly summarizes characterization techniques and in situ techniques for mechanical properties needed in binder research and some recent computational calculation successes in the development of multifunctional binders. 7.1. Characterization Techniques of Mechanical Properties

7.1.1. Binder Properties. The bulk properties of binders concerning batteries include chemical properties, mechanical properties, and thermal stabilities. The binder properties are strongly related to chemical structures and functional groups, which can be investigated by various spectra, including XRD,77,86,477 XPS,55,92,103,296 FTIR,55,103,477 NMR,55,296 and Raman.24 The mechanical properties such as tensile strength, hardness, and rheological properties can be determined by mechanical tensile-stress experiments, nanoscratch and nanoindentation experiments, and dissolution experiments.71,92,101,103,227,362 The thermal stability of binders is examined by thermogravity analysis (TGA) and a differential scanning calorimeter (DSC).56,101,362 7.1.2. Interface Properties. The adhesion between the active materials and th current collector is critical to the performance and stability of the resultant battery. A direct way to quantify the strength of this adhesion is provided by the mechanical peel test. The test can be carried out at 90° or 180° and quantifies the force required to separate the electrode film from the current collector.24,28,77,92,227,278,338 On the basis of the interlocking mechanism, the interfacial adhesion is partially determined by the surface and morphology of materials which can be observed through SEM, TEM, and AFM.16,219,293,327,338,477 To investigate interfacial property changes, spectra tests are used to confirm the functional groups and bonding types. For example, XPS can be used to confirm the presence of functional groups and examine the bonding between active materials and binders.55,92,103 FTIR and NMR spectra have been used to confirm functional groups and bonding as well as cross-linking between binder components.55,103,296,477 Raman spectrometry, when combined with optical microscopy, has been used to examine the distribution of electrode components.24 Other spectra are also explored, such as S K-edge X-ray absorption spectroscopy (XAS) and UV−vis spectroscopy.103,296

Figure 33. Thermal stability of activated carbon electrodes with different binders. Reproduced with permission from ref 472. Copyright 2013 Springer.

The group found that the SCs fabricated with this binder provided a higher specific capacitance than PTFE-, PVDF-, and CMC-SBR-based SCs.472 In SC electrodes, because the capacitance arises from electrode−electrolyte interactions, an increased active material loading may not necessarily increase the areal capacitance of the electrode as the electrolyte may not be able to penetrate the entire electrode.466 Lee at al. reduced this phenomenon through the use of a super absorbent PAA binder for MnO2-based supercapacitors.60 The introduction of PAA in the electrode facilitated the distribution of electrolyte throughout the electrode due to the superior wettability of the PAA binder. As a result, the electrode delivers a high specific capacitance at a very high areal material loading. In summary, the role of binders in SC electrodes is relatively underexplored, and targeted research may provide further improvements in these electrochemical devices.

7.2. In Situ Characterization Techniques

In situ characterization techniques have two main advantages: they (i) continuously provide physical and electrochemical information during testing and (ii) eliminate external interferences, especially for materials that are sensitive to air or moisture. Today, various in situ measurements, such as in situ scattering, in situ microscopy, and in situ spectroscopy, have been designed and widely used in energy-storage research.478 In situ characterization can provide us with direct information about the presence or absence of products during operations, which is extremely useful for the exploration of reaction mechanisms. These methods therefore offer a reliable means to select and design the opimal binders for energystorage materials. As an example, in situ UV−vis spectroscopy can detect the species of polysulfides during discharge, thereby confirming the adsorption ability of different binders, namely, PVDF and PVP, as well as binders with amino functional groups (AFG), for polysulfides in Li−S batteries.296 As shown

7. ADVANCED TECHNIQUES FOR BINDER RESEARCH As discussed previously, the exact binding mechanism, including important chemical and mechanical parameters, is crucial to the success of an electrode and therefore a battery. In the literature, chemical parameters including chemical composition, oxidation state, structure, and crystalline phase, together with electrochemical parameters such as electrochemical impedance, electrical conductivity, and redox potential, are commonly well characterized. Mechanical properties of binders and electrodes such as adhesion strength, tensile strength, hardness, and morphology before and after electrochemical cycling are often examined. To completely understand the binding mechanism, multidisciplinary collaborations, 8967


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Figure 34. In situ UV−vis investigation of S@AFG and S@PVDF electrodes during discharge. (a) Cell configuration with a sealed glass window for in situ experiments. (b and c) “Standards” of reflectance and corresponding first-order derivative curves of different polysulfides (Li2S2, Li2S4, Li2S6, and Li2S8) at a concentration of 20 × 10−3 m. (d and e) First derivative curves of S@AFG and S@PVDF electrodes at a current rate of C/6. (f) Comparison of the Li2S2 concentration changes with discharging between S@PVDF and S@AFG electrodes. Reproduced with permission from ref 296. Copyright 2017 Wiley-VCH.

uncover the role of binders in the mechanical integrity of electrodes.101,480

in Figure 34a, a cell configuration was designed to enable continuous real-time measurement of UV−vis spectra. To compare the adsorption ability of different binders to polysulfides, “standards” were prepared and measured as shown in Figure 34b and 34c. From the obtained first-order derivatives of the UV−vis spectra for S@AFG (Figure 34d) and S@PVDF (Figure 34e) electrodes, a continuous shift from long to short wavelengths was observed in both samples with the intensity increasing with further discharge. However, the S@PVDF exhibited a faster decrease in the long-wavelengths region and then increased dramatically toward short wavelengths, showing that this binder released polysulfides more easily into the electrolyte. Therefore, AFG binders possessed stronger adsorption ability than conventional PVDF binder, which was further confirmed through the comparison of Li2S2 concentration changes in both electrodes as displayed in Figure 34f. In situ characterization is also a powerful tool to monitor physical and chemical changes during battery cycles. Bridel et al. used in situ SEM and EIS to investigate the evolution of volume expansion of Si electrodes during charge/discharge. The results showed that the as-prepared porous Si/CMC electrode was able to buffer the volume change of silicon anodes up to 1.7−2 lithium ions per silicon atom, while further lithiation would lead to the collapse of electrode integrity, demonstrating the positive effect of weak Si-CMC hydrogen bonding.147 Jackel et al. used in situ electrochemical dilatometry (eD) to demonstrate the dimensional changes of battery electrodes containing PVDF binder (stiff binder) or NaCMC (soft binder).479 The combination of eD and an electrochemical quartz-crystal microbalance with dissipation monitoring (EQCM-D) provided more evidence to help choose the optimal binder, which should not be too rigid or too soft that volume changes in the electrode can be accommodated. More in situ techniques, including in situ scanning probe microscopy images of nanoscratch tests and in situ microscratch and digital image correlation (DIC) techniques, have also been adopted to

7.3. Computational Calculation

The combination of characterization techniques and computational calculation leads to a more powerful and comprehensive understanding of binder mechanisms than a single research technique. Incorporating computational analysis into binder research not only permits more thorough analysis and interpretation of the obtained results but also provides invaluable guidance in new research directions.320 In binder research, computational calculations are often used to calculate the band energetic structures of binders to investigate their properties and interfacial bonding forces. Computational calculations can provide us with the enegy states of binders that are relevant to their physical and chemical properties. For example, to investigate the electronic states of different binders, Liu et al. calculated the band energies of binders through density functional theory (DFT), as shown in Figure 35.2 The low-energy LUMO states obtained from soft X-ray absorption spectroscopy (XAS) (Figure 35a) were qualitatively consistent with the DFT calculations (Figure 35b). The differences in the low-energy LUMO state illustrated that the introduction of functional groups greatly affected the binders conductivity. Similarly, Matsumi et al. used DFT calculations to design a binder with an appropriate HOMO and LUMO which provides electrochemical activity before the reduction of the EC electrolyte.315 Computational calculation is an effective tool to compare bonding and adsorption strength between binders and active materials. With comprehensive use of characterizations and computational calculations, we can systematically probe working mechanisms behind the phenomenon observed and obtain strong evidence toward optimal binder section and rational design, thereby achieving the best electrochemical performance. For instance, Chen et al. used DFT calculations to investigate adsorption binding strength between polysulfides and AFG binder.296 As shown in Figure 36a and 36b, 8968


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Figure 35. (a) Synchrotron-based soft X-ray absorption spectra of a series of polymer binders with different chemical structures. (b) DFT calculation confirms all polymers with an F group feature a low-energy LUMO state (red) with different high-energy states. Reproduced with permission from ref 2. Copyright 2013 American Chemical Society.

Figure 36. (a) Adsorption binding energies for Li−S composites at six different lithiation stages (Li2S8, Li2S6, Li2S4, Li2S3, Li2S2, Li2S) on a reducible molecular structure of PEI. White, gray, blue, yellow, and magenta balls symbolize hydrogen, carbon, nitrogen, sulfur, and lithium atoms, respectively. (b) Trend of adsorption binding energy, and overall adsorption strength gradually decreases with lengthening of the lithium polysulfide chain beside the Li2S2 species. Reproduced with permission from ref 296. Copyright 2017 Wiley-VCH.

according to the proposed mechanisms, we identify a wide spectrum of binders and binding systems to tackle the problems associated with and meet the requirements of active materials with different physical properties and energy-storage characteristics. Given that various functional groups can be integrated onto the polymer molecular backbone structure, we suggest that corresponding functions can be created in combination with conventional polymers. The design and preparation of multifunctional binders is a promising approach to achieving future high-energy/power density materials for safer, cheaper, and greener energy-storage systems. To achieve these goals, future R&D studies on binders for energy-storage devices can be oriented to the following aspects. (i) Establishing standard methods for evaluating mechanical properties, such as adhesion strength, tensile, ductility, and electrolyte uptake of various binders, which help us understand the binding mechanism and optimize binder selection. (ii) Setup in situ probing methods to directly observe physical and chemical changes during charge and discharge in collaboration with theoretical computation to study the roles of binders on SEI formation. (iii) Exploring advanced binders and binding systems with high loadings as well as electronic and ionic conductivity for fast large capacity and electrode reaction kinetics. With such a comprehensive understanding of binding, binding processes, and binder changes in battery operation processes, we can design high-performance, low-cost, and environmentally friendly binders for a wide range of energystorage devices in the near future.

short-chain polysulfides had stronger binding strength with AFG binder than long-chain polysulfides, and Li2S2 exhibited the highest binding strength of up to 1.2889 eV. The results illustrate that the AFG binder contributes to better electrochemical performance than PVDF and PVP binders. Xu et al. designed a reduced graphene oxide-poly(acrylic acid) (GOPAA) binder and applied it in Li−S batteries.335 On the basis of DFT calculations and the projected augmented wave (PAW) method, the lithium sulfide adsorption was calculated using the Vienna ab initio simulation package (VASP). In the PAA binder, the coterminous oxygen would interact with lithium polysulfides and the PAA binder exhibited a higher binding energy with lithium polysulfides compared with PVDF. The results showed that electron density around Li2S and LiS* decreased between Li and S atoms but increased between Li and O atoms, implying a strong binding between polysulfides and PAA binders. Similar calcalutions have been conducted in other studies for silicon anodes and Li−S batteries.33,313

8. CONCLUSION AND PERSPECTIVES This work first proposes that the binding mechanisms between binders and active materials are mainly based on mechanical interlocking and interfacial binding forces consisting of intermolecular forces and chemical bonds. We review the state-of-the-art binders in the applications of different energystorage systems, including LIBs, Li−S batteries, NIBs, and supercapacitors, stematically presenting the advantages and drawbacks of traditional binder systems. More importantly, 8969


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AUTHOR INFORMATION

awards including R&D 100 Awards in 2013 and 2015 and the FMC Scientific Achievement Award in 2014.

Corresponding Authors

Shanqing Zhang obtained his Ph.D. degree in Electrochemistry in 2001 at Griffith University, Australia. Since then he has been working on the synthesis, modification, and characterization of nanostructured materials for sensing, energy conversion, and energy-storage devices. He has developed a series of patented and commercialized nanotechnologies for environmental monitoring based on functional nanomaterials. He was awarded as a Australia Research Council Future Fellow for the period of 2009−2013. Currently, he is leading his group conducting research on the synthesis of functional nanomaterials and functional polymers for lithium-ion batteries, sodium-ion batteries, supercapacitors, and all solid-state lithium ion batteries.

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: s.zhang@griffith.edu.au. ORCID

Min Ling: 0000-0001-6727-9585 Zhan Lin: 0000-0001-5009-8198 Shanqing Zhang: 0000-0001-5192-1844 Author Contributions §

H.C. and M.L. are equal contributing authors

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was financially supported by the Australian Research Council Future Fellowship and Discovery Projects, Griffith University PhD Scholarships, “Thousand Youth Talents Program” of the Natural Science Fundation of China, Zhejiang Province Science Fund for Distinguished Young Scholars (Project LR16B060001), Key Technology and Supporting Platform of Genetic Engineering of Materials under State’s Key Project of Research and Development Plan (Project 2016YFB0700600), and Science and Technology Planning Project of Guangdong Province, China (Project 2017A010104024).

Biographies Hao Chen received his Bachelor’s and Master’s degrees from Central South University (China) in 2010 and 2014, respectively. He is currently a Ph.D. candidate under the supervision of Prof. Shanqing Zhang at Griffith University, Australia. His current research interests include binders and solid-state electrolyte in lithium-ion batteries and lithium−sulfur batteries. Min Ling received his Ph.D. degree in 2015 at Griffith University, Australia. Afterward, he was a postdoctoral fellow at Lawrence Berkeley National Lab until 2017. Currently, he is working as a full faculty member at Zhejiang University. His research is focused on polymer binders for energy-storage devices.

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Luke Hencz obtained his Bachelor of Science (Chemistry) degree with First Class Honours from Griffith University in 2017. Recently, he joined Professor Shanqing Zhang’s research group as a Ph.D. candidate. His research involves investigating multifunctional binders for lithium−sulfur batteries. Han Yeu Ling received his Honours Bachelor’s and Master’s of Engineering (Materials) from the University of Wollongong in 2000 and 2001. Currently, he is a Ph.D. candidate in Professor Shanqing Zhang’s group at Griffith University. His research includes binders for anode in LIB and AIBs. Gaoran Li is currently a postdoctoral researcher in the Department of Chemical Engineering at University of Waterloo, Canada. He obtained his Ph.D. degree in Chemical Engineering from the College of Chemical and Biological Engineering, Zhejiang University, China, in 2016. His research is focused on polymer binders for advanced lithium-ion and lithium−sulfur batteries. Zhan Lin obtained his Ph.D. degree from North Carolina State University in 2010. After that he worked as a postdoctoral research associate in Oak Ridge National Laboratory and University of CaliforniaBerkeley from 2011 to 2013. He returned to China as the National Youth 1000-Plan Professor in 2014. He is a Group Leader of the Electrochemical Nano Energy Group in the School of Chemical Engineering and Light Industry at Guangdong University of Technology. His research mainly focuses on advanced materials for energy storage and conversion, which is supported by the national and local government. Gao Liu is a Group Leader of the Applied Energy Materials Group at Lawrence Berkeley National Laboratory, specialized in lithium battery research. He has led energy-storage R&D projects for the Department of Energy and industries and developed key technologies to improve battery performance. He has also collaborated with companies to commercialize new battery technologies. He has received numerous 8970


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