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Article Cite This: Acc. Chem. Res. 2017, 50, 2642-2652
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Material and Structural Design of Novel Binder Systems for HighEnergy, High-Power Lithium-Ion Batteries Published as part of the Accounts of Chemical Research special issue “Energy Storage: Complexities Among Materials and Interfaces at Multiple Length Scales”. Ye Shi,§ Xingyi Zhou,§ and Guihua Yu* Materials Science and Engineering Program and Department of Mechanical Engineering, The University of Texas at Austin, Austin, Texas 78712, United States CONSPECTUS: Developing high-performance battery systems requires the optimization of every battery component, from electrodes and electrolyte to binder systems. However, the conventional strategy to fabricate battery electrodes by casting a mixture of active materials, a nonconductive polymer binder, and a conductive additive onto a metal foil current collector usually leads to electronic or ionic bottlenecks and poor contacts due to the randomly distributed conductive phases. When high-capacity electrode materials are employed, the high stress generated during electrochemical reactions disrupts the mechanical integrity of traditional binder systems, resulting in decreased cycle life of batteries. Thus, it is critical to design novel binder systems that can provide robust, low-resistance, and continuous internal pathways to connect all regions of the electrode. In this Account, we review recent progress on material and structural design of novel binder systems. Nonconductive polymers with rich carboxylic groups have been adopted as binders to stabilize ultrahigh-capacity inorganic electrodes that experience large volume or structural change during charge/discharge, due to their strong binding capability to active particles. To enhance the energy density of batteries, different strategies have been adopted to design multifunctional binder systems based on conductive polymers because they can play dual functions of both polymeric binders and conductive additives. We first present that multifunctional binder systems have been designed by tailoring the molecular structures of conductive polymers. Different functional groups are introduced to the polymeric backbone to enable multiple functionalities, allowing separated optimization of the mechanical and swelling properties of the binders without detrimental effect on electronic property. We then describe the design of multifunctional binder systems via rationally controlling their nano- and molecular structures, developing the conductive polymer gel binders with 3D framework nanostructures. These gel binders provide multiple functions owing to their structure derived properties. The gel framework facilitates both electronic and ionic transport owing to the continuous pathways for electrons and hierarchical pores for ion diffusion. The polymer coating formed on every particle acts as surface modification and prevents particle aggregation. The mechanically strong and ductile gel framework also sustains long-term stability of electrodes. In addition, the structures and properties of gel binders can be facilely tuned. We further introduce the development of multifunctional binders by hybridizing conductive polymers with other functional materials. Meanwhile mechanistic understanding on the roles that novel binders play in the electrochemical processes of batteries is also reviewed to reveal general design rules for future binder systems. We conclude with perspectives on their future development with novel multifunctionalities involved. Highly efficient binder systems with well-tailored molecular and nanostructures are critical to reach the entire volume of the battery and maximize energy use for high-energy and high-power lithium batteries. We hope this Account promotes further efforts toward synthetic control, fundamental investigation, and application exploration of multifunctional binder materials. space.4−6 Although a variety of novel electrode materials with decent capacity and rate capability have been developed,7−10 the performance of LIBs is still limited by the traditional binder systems11 in which a conductive additive ensures the conductivity of the entire electrode while nonconductive
1. INTRODUCTION Lithium-ion batteries (LIBs) have been dominating the market of consumer electronics as power sources for decades owing to their high-energy density, high efficiency, relatively light weight, and portability.1−3 Great efforts from both academia and industry are still dedicated to further improving the performance of LIBs to support large-scale energy storage needs in applications such as all-electric vehicles, military, and aero© 2017 American Chemical Society
Received: August 16, 2017 Published: October 5, 2017 2642
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Figure 1. Schematic illustration of conventional (a) and future battery electrodes (b) for high-energy lithium-ion batteries.
other novel functionalities. Multifunctional binders have been designed by molecular modification in which different functional groups are introduced onto conductive polymer backbones to enable functionalities such as tunable electronic structure, mechanical adhesion, and electrolyte uptake. Conductive polymers can be also derived into multifunctional binders by rationally controlling their meso- and nanostructures.18,19 3D nanostructured conductive polymer gels that are synthesized via a dopant molecule cross-linking method have been recently explored as powerful functional binders for battery electrodes.20 The conductive polymer gels interconnect active particles and provide hierarchical pores for electrolyte diffusion and ion transport.21 They can also form a uniform coating on each particle to prevent aggregations and accommodate the volume change of active particles. With above-mentioned features, novel binders with tailored nano- and molecular structures have greatly improved the rate capability and cycle life of both anode and cathode materials in LIBs. In this Account, we present recent progress on rational design of novel binder materials in terms of both material selection and structure modification, introduce their applications for different electrode materials, discuss the fundamental studies on the working mechanisms of these novel binders, and provide a perspective on their future development.
polymers adhere active materials and other additives together (Figure 1a). However, the mixture of conductive phases lacks mechanical binding force and is randomly distributed, leading to bottlenecks and poor contacts that impede effective access to parts of the battery.11,12 For an ideal electrode, every active particle should be rationally shaped, sized, dispersed, and wired to the current collector and to the solid or liquid electrolyte with low-resistance and continuous internal pathways (Figure 1b).11,13 Thus, the development of novel binder systems that can facilitate both electron and ion transport, provide mechanical adhesion and flexibility, enhance the surface compatibility, and improve the dispersity of active particles is of particular importance for next-generation high-energy, highpower LIBs.14 Conventional binders show more serious problems in future ultrahigh-capacity electrodes that experience large volume change during electrochemical processes. These electrode materials tend to generate much higher stress than graphite, leading to electrode fracture and delamination.15 As a modification of classical dual-component binder design, polymers with high-concentration carboxylic groups such as carboxymethyl cellulose (CMC),23 poly(acrylic acid) (PAA),24 and alginate16 have been investigated as novel polymer binders in recent years. These polymers modify surfaces of active particles by building chemical bonds to promote the formation of a stable solid electrolyte interface (SEI) layer and provide a high elastic modulus to accommodate the volume change, thus greatly improving the stability of high-capacity electrodes. The carbon additives are still essential in dual-component binder systems, resulting in lower energy density. Therefore, single-component multifunctional binder systems based on conductive polymers are proposed and studied recently because they can synergize the advantages of organic conductors and conventional polymers, thus acting as both conductive additives and adhesive components.17 Different strategies have been developed to modify their properties and endow them with
2. RATIONAL DESIGN OF NOVEL BINDER SYSTEMS 2.1. Novel Binders with Rich Carboxylic Groups
Conventional binder systems consisting of nonconductive polymers and carbon additives offer functions of mechanical adhesion and conductive connection separately. PVDF is one of the most widely used polymer binders in these systems due to its binding capability, good electrochemical stability, and the ability to transport ions to the active material surface.22 However, the weak van der Waals force between PVDF and active particles usually fails to bind particles strongly to 2643
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Figure 2. (a) The origin and chemical structure of alginate. (b) Reversible Li-extraction capacity of alginate-based Si anode electrodes. Reprinted with permission from ref 16. Copyright 2011 AAAS. (c) Catechol conjugated polymer binders. (d) The cycling performance of different electrodes. Reprinted with permission from ref 25. Copyright 2012 WILEY-VCH Verlag GmbH & Co. (e) Schematic of PR−PAA binder to dissipate the stress during volume changes of SiMPs and chemical structures of polyrotaxane and PAA. Reprinted with permission from ref 26. Copyright 2017 AAAS.
between the polymer binder and the particles, as well as stronger adhesion between the electrode layer and the current collector. With these advantageous features, alginate-based Si anodes show high stability for more than 1200 cycles’ charge/ discharge (Figure 2b). The properties of carboxylic-rich binders can be improved by introducing functional groups onto polymeric backbones. Choi et al. developed mussel-inspired binders for high-performance Si electrodes by modifying PAA and alginate with catechol groups (Figure 2c).25 Compared to PAA or alginate, the modified binders employ the dual adhesion mechanisms of hydrogen bonding and catecholic interaction with the Si nanoparticle (SiNP) surfaces, thus showing exceptional wetness-resistant adhesion and robust contacts between SiNPs, promoting efficient electrical conducting pathways to decrease the portion of dead Si. As a result, the Si electrodes showed
maintain the electrode integrity when high-capacity active electrode materials with substantial volume change such as silicon are used, leading to a drastic capacity fade.23 To mitigate this problem, polymers containing carboxylic groups such as CMC, PAA, and alginate are adopted as binders because they can promote the formation of a stable SEI layer and accommodate the volume change of active particles.24 Yushin et al. reported a stable Si anode by applying alginate, which is a high-modulus natural polysaccharide, as the binder (Figure 2a).16 Alginate experiences virtually no swelling in commonly used electrolytes but still provides good access of Li ions via hopping of Li ions between the adjacent carboxylic sites, which are uniformly distributed along the chain. This unique chemical structure also enables uniform coverage and efficient assistance to form a stable SEI layer. The large polarity of alginate macromolecules ensures better interfacial interaction 2644
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Figure 3. (a) The concept of a conductive polymer binder with dual functionalities. (b) The molecular structure of the PF-type conductive polymers. (c) Cycling performance of Si/PFFOMB electrode. Reprinted with permission from ref 27. Copyright 2011 WILEY-VCH Verlag GmbH & Co. (d) Molecular structure of conductive polymers with electrolyte uptake capability. (e) Cycling performance of Si/PEFM electrode. Reprinted with permission from ref 15. Copyright 2013 American Chemical Society.
independent optimization of them without detrimental effects on electronic properties (Figure 3a). Liu et al. synthesized a conductive polymer based on polyfluorene (PF)-type polymers and introduced two functional groups, carbonyl (CO) and methylbenzoic ester (PhCOOCH3, MB), to tailor the LUMO electronic states and improve the polymer adhesion (Figure 3b).27 In order to achieve a properly tailored electronic structure, they applied synchrotron-based soft X-ray absorption spectroscopy (XAS) and density functional theory (DFT) calculations on a series of polymers to monitor the unoccupied conduction states. The results demonstrate that the introduction of carbonyl groups significantly lowers the LUMO level while enhancing the lithium binding energy, thus cathodically doping the polymer with electrons to achieve adequate electronic conductivity. The MB units were introduced to improve the flexibility of polymer chains and strengthen the mechanical binding force, thus leading to high electrochemical performance (Figure 3c). In another subsequent work, they further modified the polyfluorene based conductive polymers with triethylene oxide monomethyl ether (E) side chains to enhance its electrolyte uptake capability (Figure 3d).15 This tailored multifunctional binder has been applied for Si anodes and enabled full-capacity cycling of Si in the initial cycle (Figure 3e). The design concept, methodology, and practical application of the functional group modified conductive polymers can be applicable for other highcapacity electrodes.
increased capacities and superior cycling performance (Figure 2d). The binders can be also applied to other electrode materials because catechol groups can provide effective adhesion with various surfaces. In addition to modification by simple chemical groups, functional components with rationally designed architectures can be introduced to further improve the binder performance. Recently, Choi et al. developed a new binder for Si microparticle (SiMP) anodes by covalently integrating a small amount of ring-slide polyrotaxane (PR) with PAA (Figure 2e).26 The polyrotaxane’s ring components can move freely to substantially lower the tension exerted on the polymer network, resulting in highly stretchable and elastic binder network which makes even pulverized Si particles remain coalesced without disintegration during repeated delithiation−lithiation. 2.2. Multifunctional Binders with Tailored Molecular Structures
Although polymeric binders such as CMC, PAA, and alginate have demonstrated the capability to achieve high stability for high-capacity inorganic electrodes, conductive carbon additives are still needed to ensure electronic connection, thus leading to lower specific energy density considering the significant weight and volume fraction occupied by carbon additives. Ideally, a single-component binder system that can provide both electronic conductivity and mechanical adhesion can alleviate the aforementioned limitation. Conductive polymers are promising candidates as multifunctional binder systems because they combine the advantages of organic conductors and conventional polymers, thus eliminating the use of carbon additives and improving the overall energy density of batteries.14 However, conductive polymers may lose their conductivity in a strong reducing environment, and their rigid and brittle chains may cause failure of mechanical integrity of the electrode due to the stress generated during electrochemical reactions. To modify their properties and endow them with more functionality, their molecular structures have been tailored by introducing different functional groups to bring multiple functions such as high electronic conductivity, mechanical adhesion, ductility and electrolyte uptake. This method allows
2.3. Multifunctional Binders with Controlled Nanostructures
The properties of conductive polymers have been modified by tailoring their molecular structures. However, conventional electrode processing methods, which mechanically mix conductive polymers and active materials may lead to low ionic conductivity, poor dispersity, and inferior compatibility.28 One strategy to solve these problems is to develop conductive polymer based binders with rationally designed nanostructures because of their unusual mechanical, electrical, and optical properties endowed by the confined dimensions. Binders with controlled nanostructures can provide shortened pathways for electronic and ionic transport, create higher electrode/electro2645
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Figure 4. Schematic illustration (a) and TEM image (b) of 3D porous SiNP/PANI hydrogel composite. Scale bar, 50 nm. (c) Electrochemical cycling of the composite Si-gel electrodes. Reprinted with permission from ref 21. Copyright 2013 Nature Publishing Group. (d) SEM image of Si/ PPy/CNT ternary electrode. (e) Electrochemical cycling of different electrodes. Reprinted with permission from ref 32. Copyright 2013 American Chemical Society.
polymer gels and SiNPs (Figure 4a,b). The PANI gel framework serves as continuous 3D pathways for electronic conduction and its hierarchical pores facilitate ion diffusion, leading to superior rate performance of the composite electrodes. The electrode also shows excellent stability because the 3D porous framework and conformal coating layer on SiNPs accommodate the large volume expansion of the SiNPs, which enables a deformable and stable SEI (Figure 4c). These dopant cross-linked 3D conductive polymer gels are promising multifunctional binders for other battery electrode materials suffering from large volume expansion and unstable SEI formation. They also show great versatility for various types of electrodes from anodes to cathodes.30,31 The multifunctional gel binder systems could be further extended to a ternary system,32 in which PPy gels and singlewalled carbon nanotubes (SWCNTs) were used to wrap and interconnect SiNPs (Figure 4d). The SWCNTs confine SiNPs in the hybrid framework to ensure good electronic connection, greatly enhancing transport kinetics and the electrode integrality in long-term operations (Figure 4e). Another important function of conductive polymer gel binders is the interface modification to improve the homogeneous dispersity of active particles within the bulk electrodes.30,31 The aggregation of active particles hinders the electron and ion transport from the surface of aggregates to the center particles, thus preventing full utilization of active materials especially at high current density (Figure 5a).33−36 Ideally, the active particles should be uniformly dispersed and electron and ion transport pathways need to be optimized for each particle (Figure 5b). In our recent studies, TEM microtome imaging of bulk electrodes revealed that the hybrid gel−inorganic framework greatly suppresses the aggregation of active particles when compared to traditional binder systems (Figure 5c−h). The reason is that the conformal coating and the strong molecular framework can separate and support active particles. In addition to unique structural features of nanostructured conductive polymer gels, the scalable solution-based electrode fabrication for gel-based binders is also compatible with industry manufacturing. In the future, binders with more ordered microscopic structures could be developed by techniques such as lithography and 3D printing to further regulate the electron/ion transport pathways and enhance the mechanical properties. More detailed investigation and fundamental understanding of transport properties in these
lyte contact area, and better accommodate the generated strain during cycling. Our recent works on 3D nanostructured conductive polymers are good examples to show how structural design promotes multifunctionality of binders and improves their electrochemical properties. 2.3.1. 3D Nanostructured Conductive Polymer Gels As Multifunctional Binders. Inspired by the unique 3D structures of biogels,29 conductive polymer gels with interconnected network nanostructures were developed and recently explored as unique binder materials because they possess many advantageous features such as good compatibility, high surface area, high electronic conductivity, and hierarchical porosity. Unlike conventional template-guided methods,17 we developed an efficient dopant cross-linking method to synthesize the conductive gel framework with multiple functionalities.20 In a typical synthesis, dopant molecules with multiple functional groups such as phytic acid and copper phthalocyanine3,4′,4″,4‴-tetrasulfonic acid tetrasodium salt (CuPcTs) are adopted as the cross-linkers to interconnect conductive polymer chains. The morphology and nanostructure of the resulting gels could be tuned by controlling synthetic conditions such as types of initiators, monomers, and temperature. The conductive polymer gels can serve multiple functions when applied as binder materials for LIBs. First, the conductive polymer gels provide highly conductive pathways connecting all active particles for continuous electron transport. Second, the hierarchical pores facilitate ion and electrolyte diffusion. Third, gel binders enable a uniform coating on active particles for surface modification and prevent particle aggregation. Meanwhile, the ductile surface coating and porous structure can accommodate volume change of electrode particles during cycling. Last, the chemical stability and robust gel framework further provide mechanical strength and improve the long-term stability. With above-mentioned functionalities, the gel binder materials show great potential to promote the development of future high-performance LIBs. The gel binders were first demonstrated useful for SiNP based electrodes.21 A polyaniline (PANI) framework, which encapsulates SiNPs, was in situ polymerized using phytic acid as both gelator and dopant, forming a uniform conductive coating on the surface of SiNPs. The hydrogen bonding and electrostatic interaction between PANI and the SiNP surface also contributes to improved compatibility between conductive 2646
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Figure 6. (a−c) SEM images of different nanostructured PPy gels synthesized with various cross-linking dopants. Scale bar: 1 μm. (d) Film conductivities of different PPy gels. Reprinted with permission from ref 39. Copyright 2015 American Chemical Society.
explored as bifunctional binders for Fe3O4 particles based battery anodes.30 The microstructure of C-PPy framework is different from that of P-PPy since CuPcTs with planar structure can facilitate the supramolecular assembly of PPy chains (Figure 7a−d). When investigated as binders, C-PPy framework based electrodes show better electrochemical characteristics compared to those of P-PPy based electrodes (both are significantly better than conventional PVDF binder based electrodes), because of improved conductivity and continuous 3D network microstructure (Figure 7e,f). The tunable microstructure and electrochemical properties of the gel framework can control transport properties within electrodes and provide the ability to further optimize the resulting performance of LIBs.
Figure 5. (a,b) Schematic of active particle dispersion in conventional and gel binder systems. (c−e) TEM microtome images of the control sample. (f−h) TEM microtome images of the PPy/Fe3O4 hybrid gel electrode. Panels e and h are color enhanced images in which blue represents the Fe3O4 nanoparticles and red is organic binder. Reprinted with permission from ref 30. Copyright 2017 WILEYVCH Verlag GmbH & Co.
confined nanostructures and at the hybrid organic−inorganic interfaces are also critically needed. 2.3.2. Structural and Property Tuning in Multifunctional Binders. An ideal binder system should provide excellent tunability in their structures through facile synthetic modification, and their physical and chemical properties need to be effectively tuned, thus regulating their structure-derived functions to achieve the best possible performance of different battery systems. The micro- and nanostructures, as well as the resulting physical and chemical properties, of conductive polymer gel binders could be tuned by various means. This can be realized by adjusting the synthetic conditions such as using specific synthetic routes, adopting different dopants, initiators, and monomers, and varying dopant or precursor concentrations. For instance, hierarchically nanostructured conductive polymer gels with structure-derived elasticity have been synthesized by an interfacial polymerization method37,38 and explored as effective binders for several high-energy LIB electrodes. Conductive polymer gels with tunable nanostructures and electronic properties have been also demonstrated by changing dopant molecules with different molecular and geometric structures (Figure 6a−c).39 Importantly, different dopants have great influence on electronic properties of the resulting polymeric gels.39 For example, the PPy hydrogel cross-linked by CuPcTs shows a high conductivity of 7.8 S cm−1, which is two orders higher than that of pristine PPy (Figure 6d). In our recent studies, PPy gels based on two kinds of dopant molecules, which also serve as cross-linking agents, phytic acid (named as P-PPy) and CuPcTs (named as C-PPy), were
2.4. Multifunctional Binders Based on Conductive Polymer Composites
Another strategy to develop multifunctional binders is hybridizing conductive polymers with other functional materials. In such composites, conductive polymers construct pathways for electron transport, and the other component, such as functional polymers, natural gels, and carbon materials, provides functionalities including ionic conductivity, mechanical robustness, surface compatibility, and binding capability. Zhang et al.40 synthesized a multifunctional polymer binder by combining a natural binder, sodium alginate (SA), with 3,4propylenedioxythiophene-2,5-dicarboxylic acid (ProDOT). With the synergetic effects of the functional groups, the resulting binder maintains the binding capabilities, enhances the mechanical integrity, provides high lithium ion diffusion coefficient, and improves electronic conductivity in LiFePO4 electrode. In another work, Reichmanis et al.41 introduced conjugated polymer of poly[3-(potassium-4-butanoate) thiophene] (PPBT) and poly(ethylene glycol) (PEG) as binder components (Figure 8a). PPBT undergoes electrochemical doping to form effective electronic bridges between active particles (Figure 8b), while PEG reduces aggregated size, enables effective dispersion of active materials, and facilitates ionic conduction (Figure 8c). The resulting electrode shows greatly improved capacity and cyclic stability (Figure 8d). In addition, carbon nanomaterials were also hybridized with conductive polymers for multifunctional binders. Bao et al.42 developed an interpenetrating network of carbon nanotube (CNT)−conductive polymer (PEDOT:PSS) hydrogel provid2647
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Figure 7. (a, b) SEM and TEM images and electron diffraction pattern of P-PPy/Fe3O4 hybrid gel electrodes. (c,d) SEM and TEM images and electron diffraction pattern of C-PPy/Fe3O4 hybrid electrodes. (e,f) Rate and stability tests of different electrodes. Reprinted with permission from ref 30. Copyright 2017 WILEY-VCH Verlag GmbH & Co.
Figure 8. (a) Schematic depicting electron and ion transport in the electrode modified by PPBT and PEG. (b) PPBT’s electronic and chemical structure changes during reduction (n-doping). (c) Schematic of PEG coating on the Fe3O4 surface. (d) Cycle and rate performance of PPBT and PEG modified electrodes. Reprinted with permission from ref 41. Copyright 2016 American Chemical Society.
3. MECHANISTIC UNDERSTANDING OF BINDER ROLES AND DESIGN RULES FOR NOVEL BINDERS Experimental efforts on developing multifunctional binders greatly improve the electrochemical properties of electrode materials in LIBs. However, it is far from clear how these binder materials function in many aspects such as electronic/ionic
ing good mechanical properties, high electronic conductivity, and better ion transport to achieve facile electrode kinetics and accommodate volumetric changes of TiO2- and Si-based electrodes. 2648
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Figure 9. Time-resolved TEM images of “self-delithiation” process of a lithiated SiNP (a−c) and multiple SiNPs (d−f) after removal of external bias. (g) Modeling study showing inter-SiNP Li diffusion and “self-delithiation” process of single SiNP. Reprinted with permission from ref 43. Copyright 2015 American Chemical Society.
conduction, surface chemistry, and electrochemimechanical effect. Advanced characterizations and modeling studies are powerful tools to gain fundamental understanding of these behaviors, thus illustrating rational design rules for better binders.43 In situ TEM was used to investigate the effect of PPy coating layer on lithiation/delithiation kinetics of SiNPs. It was discovered that this polymer coating could lead to “selfdelithiation” of SiNPs at different stages of lithiation. Time resolved TEM images showed the fluctuation lithiation/ delithiation phenomenon and self-delithiation of both single SiNP (Figure 9a−c) and multiple SiNPs (Figure 9d−f) after the removal of the external bias, indicating that the surface coating plays a critical role in electrochemical processes. Combined with a modeling study, it was revealed that this self-discharging behavior originated from the constraint effect of the coating layer due to the large volume expansion during lithiation. The constraint of the coating causes compressive stress in the lithiated regions and at the reaction front as the SiNP swells, leading to inter-Si diffusion when two particles touch (Figure 9g). Despite the detrimental effect of “selfdischarge” during lithiation, the polymer coating provides a mechanically protective role for SiNPs to decrease the tensile stress of the particles to suppress the pulverization. These observations and chemomechanical modeling reveal the functions of surface coatings in multiple roles: they act as a protective layer to mitigate side reactions and buffer the volume change of the anode while the constraint of the coating layer generates compressive stress, retarding lithiation or even causes self-delithiation. According to the knowledge obtained, general guidelines for designing an effective surface coating can be illustrated. An ideal polymer coating from binders should be highly deformable and the polymer should exhibit weak resilience to avoid the strong residual stress, which can cause self-delithiation of Si (Figure 10). The above study is a good example showing that the combination of advanced characterizations and modeling calculations can shed light on rational design of binder coatings. To further understand the roles of polymer binders in battery electrodes, other advanced characterization techniques can be used such as X-ray microfluorescence for elemental mapping of the electrode,44 X-ray absorption spectroscopy for determining the oxidation states of the metal centers resident in the solid electrolyte interphase and at the electrode surface,45
Figure 10. Schematic illustrating the effects of surface coatings on lithiation behavior of Si electrodes. Reprinted with permission from ref 43. Copyright 2015 American Chemical Society.
energy-dispersive X-ray diffraction for achieving spatial resolution of the electrochemical reduction process within the electrode,46 transmission neutron diffraction analysis for mapping lateral inhomogeneity, and synchrotron radiation Xray tomographic microscopy for dynamic mapping of changes in both electrode structure and chemical content with cycling.47 These tools can monitor the electrochemical processes within a battery electrode, which are closely related to binder properties. Modeling studies are also important to interpret the experimental observations and understand the working mechanisms of multifunctional binder systems. With knowledge obtained from these studies, one can systematically modify the chemical and microstructures of binders to achieve the best possible LIB performance.
4. PERSPECTIVE For future development, other novel functionality such as selfhealing properties, mechanical flexibility and stretchability, and environmental responsiveness can be introduced into binder systems by molecular design or hybridizing materials, thus further improving battery performance. Bao et al.48 demonstrated a self-healing binder that improves cycle life of microsized Si particle based electrodes since electrode cracks and damage formed during cycling can be healed spontaneously (Figure 11a,b). Recently, we also developed a conductive, self2649
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Figure 11. (a) Schematic of different behavior of conventional silicon electrode and self-healing electrode. (b) Capacity retention of Si microparticle electrodes with different polymer additives. Reprinted with permission from ref 48. Copyright 2013 Nature Publishing Group. (c) Illustration of selfhealing mechanism of PPy/supramolecule hybrid gel. (d) Self-healing behavior of PPy/supramolecule hybrid gel. Reprinted with permission from ref 49. Copyright 2015 American Chemical Society.
healing hybrid gel by incorporating a supramolecule into conductive PPy gel matrix (Figure 11c,d),49 which could heal cracks at room temperature without external stimuli. This conductive, room-temperature self-healing gel may be a promising binder candidate. To enhance flexibility and stretchability of binders, gels synthesized by interfacial methods can be exploited since they show greatly enhanced elasticity due to the interconnected hollow-sphere microstructure.37 Another strategy is the design of intrinsically flexible molecular structure. Bao et al.50 introduced chemical moieties into conjugated polymers to promote dynamic noncovalent cross-linking, which undergoes an energy dissipation mechanism through the breakage of bonds when strain is applied. The obtained polymer shows improved intrinsic stretchability while retaining high charge transport abilities. LIBs that can respond to environmental stimuli such as temperature and pH are attracting broad interest due to their improved safety and stability.51 In our recent study, we demonstrated that an electrochemical energy storage device with thermally self-regulating behavior could avoid thermal runaway.52 Hybrid gels based on poly(N-isopropylacrylamide) (PNIPAM) and PPy have been also developed with a unique combination of high electrical conductivity, high thermoresponsive sensitivity, and good mechanical properties.53,54 By proper modification and device design, these hybrid gels could play an increasingly important role in environmentally responsive binder systems, thus enabling smart LIBs or other smart energy storage devices. In addition, the conductivity and surface properties of the binder should be further optimized. The chemical structure of conductive polymers such as PANI, PPy, and PTh can be modified by introducing functional groups on their backbones or side chains, thus tuning the electronic structures including HOMO/LUMO level and band gap. Building ternary hybrid structures by introducing highly conductive and flexible materials can also enhance the overall conductivity of the electrode. As for the surface properties, chemical modification can be applied on polymer binders for surface passivation to avoid undesirable reactions. Additionally, functional ligands or surfactants can be introduced onto active particle surfaces to
improve their electrical and chemical contacts with polymer binders.
5. CONCLUSION For next-generation ultrahigh-capacity battery electrode materials, large structural/volumetric change with conventional binders is a serious problem. Polymers with high-concentration carboxylic groups have been applied to suppress the volume change and improve the electrode stability. However, conductive additives are still needed in these dual-component binder systems, resulting in lower energy density. Singlecomponent conductive polymers are regarded as promising bifunctional binders for high-performance LIBs. Multifunctional binders have been designed by tailoring molecular structure of conductive polymers with different functional groups to introduce multifunctionality. Properties of the binders can be separately optimized without detrimental effect on electronic property, thus achieving full-capacity cycling of active particles. 3D nanostructured conductive polymer gels represent a unique class of multifunctional binders with controlled nanostructures showing enhanced mechanical, electrical, and electrochemical properties. They offer 3D continuous pathways for fast electron and ion transport, provide mechanical strength to accommodate volume change, and prevent aggregation of active particles, thus greatly improving the rate capability and cycle life. In addition, their micro- and nanostructures and derived properties could be tuned by various methods to meet requirements of different electrode materials and achieve highperformance LIBs. In addition, hybridizing conductive polymers with other components to provide different functionalities can be useful to obtain multifunctional polymeric binders for high-performance LIBs. To further understand the roles of polymer binders in battery electrode, powerful tools such as advanced characterizations and modeling studies need to be adopted to understand the working mechanisms of multifunctional binder systems, thus further improving their electrochemical, mechanical, and surface properties. Additionally, by molecular design or hybrid material synthesis, more functionalities such as self-healing property, mechanical flexibility and stretchability, and environ2650
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Accounts of Chemical Research
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mental responsiveness can be introduced into future-generation binder systems to further improve battery performance.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Guihua Yu: 0000-0002-3253-0749 Author Contributions §
Y.S. and X.Z. contributed equally to this work.
Notes
The authors declare no competing financial interest. Biographies Ye Shi is a postdoctoral researcher in Professor Yu’s group. He received his B.S. and M.S. degrees in Polymer Science and Engineering from Zhejiang University and Ph.D. in Materials Science and Engineering under supervision of Prof. Yu from University of Texas at Austin. Xingyi Zhou is a graduate student at University of Texas at Austin under supervision of Professor Yu. She received her B.S. degree in Chemistry from University of Science and Technology of China. Guihua Yu is an Assistant Professor of Materials Science and Engineering and Mechanical Engineering at University of Texas at Austin. He is an elected Fellow of Royal Society of Chemistry (FRSC), Sloan Research Fellow and Camille Dreyfus Teacher-Scholar. He received his B.S. degree with the highest honor from University of Science and Technology of China and Ph.D. in chemistry from Harvard University, followed by postdoctoral research at Stanford University.
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ACKNOWLEDGMENTS G.Y. acknowledges the Center for Mesoscale Transport Properties, an Energy Frontier Research Center supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, under award # DE-SC0012673 for financial support.
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