Harnessing the Power of Plastics: Nanostructured Polymer Systems in

Harnessing the Power of Plastics: Nanostructured Polymer Systems in LithiumIon Batteries Melody A. Morris,† Hyosung An,§ Jodie L. Lutkenhaus,*,§,∥ and Thomas H. Epps, III*,†,‡ †

Department of Chemical and Biomolecular Engineering, University of Delaware, Newark, Delaware 19716, United States Artie McFerrin Department of Chemical Engineering, Texas A&M University, College Station, Texas 77843, United States ∥ Department of Materials Science and Engineering, Texas A&M University, College Station, Texas 77843, United States ‡ Department of Materials Science and Engineering, University of Delaware, Newark, Delaware 19716, United States §

ABSTRACT: Nanostructure-forming polymers have tremendous potential to enhance the performance and safety of lithium-ion batteries (LiBs) as a result of their ability to simultaneously optimize often contradictory properties, such as ionic conductivity and mechanical stability, in a single material. These macromolecules can be harnessed in both LiB electrolyte and electrode components. With respect to electrolytes, advances in salt-doped and single-ion systems are highlighted herein with a focus on strategies that improve conductivities to rival that found in gel and liquid electrolytes, while also permitting further enhancements in electrochemical and mechanical stability. In the arena of electrodes, three major functions are considered: binders to maximize active material efficiency, polymer electrodes to enable fully organic LiBs, and sacrificial constructs that template high surface area, well-ordered metal oxide or metallic electrodes to improve electrode capacity. Additionally, the application of theory and simulation to streamline the development of key structure−property−processing relationships in ion-conducting nanomaterials is discussed. Finally, several next steps and future directions are suggested to accelerate the fabrication of next-generation LiBs.

B

years and over 6000 cyclesare essential. To meet these stringent requirements, improving all aspects of battery performance is vital. LiBs are composed of an electrolyte/separator, anode, and cathode. The electrolyte conducts lithium ions and prevents short-circuiting between the two electrodes. Traditionally, a separate conducting medium (for ionic conductivity) and separator membrane (for mechanical strength) are required to simultaneously satisfy both criteria. The electrodes store and convert chemical energy to electrical energy by shuttling lithium-ions between the cathode and anode through the electrolyte while transferring electrons through an external circuit to power a device. Current LiBs have electrolyte ionic conductivities on the order of 10−2 S/cm, the ability to perform up to 1000 charge/discharge cycles, and an electrochemical stability window of ∼0−4 V,3 well below DOE goals. Furthermore, LiBs often suffer from dendrite formation at the

atteries are essential components in devices ranging from laptops and electric vehicles (EVs) to renewable energy storage platforms. In particular, with a worldwide market share of over 60%,1 lithium-ion batteries (LiBs) have demonstrated enormous potential as they have among the highest energy densities of all battery technologies, by weight and by volume, and they possess other desirable features, such as low self-discharge rates and minimal memory effects.1 However, significant advances are needed to enable the realization of next-generation, high-performance devices. For example, the U.S. Department of Energy (DOE) has set a series of goals for energy density, power density, reliability, and cost of batteries for EVs with a target year of 2022.2 Current EV LiBs have a specific energy of approximately 100−200 Wh/kg, and the DOE seeks to at least double this figure while simultaneously increasing the lifetime of battery packs and lowering the cost by a factor of 4. Faster recharging, which is related in part to power density and ionic conductivity (among other properties), also is desired to ease the incorporation of low-cost and high-performance EVs for consumer use. For renewable energy storage, high specific energies may be less critical, but high-performance stabilitylifetimes of over 20 © 2017 American Chemical Society

Received: May 2, 2017 Accepted: July 17, 2017 Published: July 17, 2017 1919

DOI: 10.1021/acsenergylett.7b00368 ACS Energy Lett. 2017, 2, 1919−1936

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http://pubs.acs.org/journal/aelccp

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Figure 1. Schematic depicting strategies for integration of nanostructure-forming polymers in LiBs. In electrodes, polymers can bind cathode materials, act as the electrode, and/or behave as a template to enable high surface area constructs. In electrolytes, salt-doped and self-doped nanostructure-forming polymers can enable more mechanically and electrochemically stable LiBs as probed through experimental and theoretical/simulation investigations. Gyroid image adapted with permission from ref 95. Copyright 2009 American Chemical Society. Theory and Simulation left image adapted from ref 44 with permission. Copyright 2016 American Chemical Society. Theory and Simulation right image adapted from ref 49, with permission from Elsevier.

thermal requirements to produce a safe and efficient battery is a difficult engineering and materials challenge. Nanostructured polymeric materials, including block polymers (BPs), cross-linked networks, and BP/nanoparticle (NP) hybrid electrolytes, are uniquely suited for this task because they can incorporate multiple functionalities into one material. For example, competing constraints, such as high conductivity and high mechanical stability, can be achieved simultaneously using these nanostructure-forming macromolecular systems.3 More specifically, in the case of BPs (which are composed of two or more chemically distinct polymers, known as blocks, covalently bound together), if polymer blocks are thermodynamically incompatible, the BP can phase separate to create structures with domain spacings on the order of tens to hundreds of nanometers. This BP microphase separation results in the formation of various nanoscale morphologies, including lamellae, networks (e.g., gyroid), cylinders, and spheres, as determined primarily by the interplay between constituent volume fractions and the relative strengths of interblock interactions.6 For battery electrolyte applications, cylinder and network morphologies are highly desirable as they allow for interconnected two- and three-dimensional ion transport domains without additional processing.7,8 Furthermore, in comparison to ceramics, which require energy-intensive treatments to achieve required properties, BPs and nanostructured polymer systems typically can be fabricated with moderate thermal or solvent processing. With respect to LiB electrodes, device performance can be enhanced through the use of nanostructure-forming polymers

anode, which can penetrate the electrolyte, causing short circuiting and battery failure.4 There have been significant efforts to mitigate these hazards, with a prevalent approach being the replacement of organic solvents with solid-state electrolytes. Though solid-state electrolytes typically possess ionic conductivities lower than their liquid- or gel-based counterparts, they have greater thermal and electrochemical stability. For instance, it is suggested that lithium dendrite formation can be completely arrested if the shear modulus of the electrolyte (Gelectrolyte) is at least 1.8× the shear modulus of lithium metal; at room temperature, this condition requires Gelectrolyte = ∼6 GPa.5 However, improvements in electrolyte design, such as minimizing polarization of the electrolyte, also can stabilize LiBs by arresting dendrite nucleation.4 If ionic conductivity and stability are increased, exciting opportunities for batteries with much faster charging rates and longer lifetimes become attainable. While innovations in electrolyte materials can hasten the realization of these battery improvements, the electrode design also requires reconsideration to optimize battery performance, that is, the design of improved binders that can mechanically stabilize active materials, promote favorable electrode−electrolyte contacts, and increase electronic and ionic conductivity. By developing materials that can fundamentally decouple mechanical and thermal properties from ionic and electronic conductivity, high-performing and safer batteries become possible.3 The demonstration of a single material that meets the numerous and complex electrochemical, mechanical, and 1920


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is markedly lower than that of traditional liquid and gel electrolytes as a result of PEO’s relatively high melting temperature (∼60 °C). Additionally, the order−disorder transition temperature (TODT) of these doped PEO-based BPs typically is quite high, requiring substantial energy input or solvent content to melt or solution process these materials. Furthermore, the counterion can have a considerable impact on lithium ion transport as it is prone to countercurrent motion that limits the efficiency of the electrolyte, induces polarization in the electrolyte, and restricts the total current that can be carried through the battery.11 Finally, either long-range order of microphase-separated morphologies or formation of continuous transport pathways is desired to promote high conductivity through the electrolyte with minimal processing; however, in many instances, postprocessing is necessary to optimize conductivity and mechanical integrity in many cylinder or lamella-based systems.7,12 Herein, several recent strategies to overcome the limitations of older BP-based electrolytes are highlighted, including innovative experimental and theoretical efforts in both salt-doped and self-doped BPs, to increase the feasibility of these nanostructure-driven systems. New Salt-Doped Nanostructured Polymer Electrolytes. Though PS-b-PEO is the archetypal nanostructured LiB electrolyte, it is not easy to incorporate additional functionalities or directly modulate PS-b-PEO’s material properties. By using chemical synthesis to design BPs with new architectures and monomer segment distributions, enhanced lithium-ion motion and mechanical properties can be achieved in a manner that potentially outweighs any detriments associated with the increases in macromolecular complexity. Beyond PS-b-PEO, other more traditional polymers also have been considered for ion-conducting applications. For example, PS-b-poly(methyl methacrylate) (PS-b-PMMA) doped with lithium chloride showed complexation between the lithium and the PMMA.13 Additionally, poly(propylene oxide) [PPO]-based BPs have been proposed as an alternative to PEO-containing nanostructured materials, but the additional methyl group in PPO hinders ion conduction.14 Thus, PEO (and PEO analogues) has dominated development in recent years because of its ability to effectively solvate lithium ions, in comparison to other polymer families.3 In this section, several strategies are outlined that discuss the creation of novel BP or BP-inspired electrolytes in salt-doped polymer systems (Table 1 and Figure 2 for examples). As one example of nanostructure-forming polymer systems, bottlebrush block polymers (BBPs) consist of a polymerized backbone grafted with long polymer side chains with a blocklike distribution along the backbone, in an analogous fashion to a linear BP. For instance, Bates et al. synthesized poly[(norbornene-graf t-styrene)-block-(norbornene-graf t-ethylene oxide)-block-(norbornene-graf t-styrene)] BBPs and characterized the crystallinity, morphology, and ionic conductivity of these BBPs upon doping with lithium bis(trifluoromethane)sulfonimide (LiTFSI).15 The salt-doped BBPs had assembly behavior similar to that of a high molecular weight PS-b-PEO, forming hexagonally packed cylinders of PS in a PEO matrix.15 By maintaining an amorphous PEO domain across all working temperatures (at salt doping ratios above 10:1 [EO]/[Li]), the room-temperature ionic conductivity (∼2 × 10−5 S/cm) was much higher than that of similar volume fraction (though lower overall molecular weight) PS-b-PEO (∼4 × 10−6 S/cm).24 Because of the morphology, the modulus of the material was relatively low (∼104 Pa), which is potentially nonideal for

in three major forms: as an actual electrode, as an electrode binder, and as a template for nanostructured electrodes. As an electrode or a binder, it is desirable to include a combination of electronic and ionic conductivity, redox activity, and mechanical robustness. Nanostructured electrodes and binders may improve electrode properties, unlocking possibilities for flexible, adaptive, healable, or stretchable energy storage devices. Functional polymer systems may replace today’s current insulating binders [e.g., poly(vinylidene fluoride), PVDF], increasing electrode conductivity and allowing higher charging rates. Improved binders are especially important to facilitate the implementation of active electrode materials, such as silicon or tin, which have high energy and high capacity but suffer from severe volume expansion.9 As templates, nanostructure-forming polymers, such as BPs, with well-defined microphase separation can be used for patterning active materials, thereby generating porosity for enhanced mass transfer. By harnessing the inherent advantages of macromolecular assemblies and modular chemistries, both the electrode and the electrolyte of LiBs can be upgraded, enabling safer and more effective batteries for next-generation technologies. These strategies are outlined in Figure 1. Looking forward, to facilitate

Looking forward, to facilitate incorporation of nanostructure-forming soft materials into both the electrolyte and electrode components, it will be essential to understand the relevant structure−property−processing relationships and develop approaches that leverage the strengths of nanostructured polymer systems in energy storage materials. incorporation of nanostructure-forming soft materials into both the electrolyte and electrode components, it will be essential to understand the relevant structure−property−processing relationships and develop approaches that leverage the strengths of nanostructured polymer systems in energy storage materials. This Perspective will examine major advances in electrolytes and electrodes that unlock future directions toward the commercialization and widespread adoption of such nanostructured polymeric materials in LiB applications. Though the majority of this work will highlight BPs and other nanostructure-forming soft materials systems, we also will briefly discuss selected research efforts that are synergistic with self-assembling macromolecular designs. Electrolytes. Nanostructure-forming macromolecules are an exciting class of materials for LiB electrolytes because they permit the design of microphase-separated soft materials systems with addressable and tailorable properties, for example, by facilitating the decoupling of mechanical and thermal stability from ionic conductivity. For the case of BPs, the archetypal BP electrolyte is polystyrene-block-poly(ethylene oxide) [PS-b-PEO] doped with lithium salts, which shows reasonable conductivity at elevated temperatures and good electrochemical stability.10 However, PS-b-PEO-based materials also have some undesirable properties that have encouraged the development of a new generation of BP electrolytes. Most notably, the ionic conductivity, especially at room temperature, 1921


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Table 1. Example Salt-Doped Nanostructured Electrolytes

a

Ionic conductivity. 1922


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Figure 2. Schematics of several types of nanostructured salt-doped polymer electrolytes. Various architectures and monomer segment distributions of BPs alter properties and processability. Crosslinked and hybrid nanostructured systems offer improved mechanical or cycling stability while also maintaining desirable conductivities.

arresting lithium dendrite growth.15 However, with such a modular system,25 it is possible that a better balance between ionic conductivity and shear modulus could be attained by altering the morphology, thereby enhancing BBP feasibility in LiB electrolyte applications. Modifying the monomer segment density profile in BPs also has been leveraged with the goal of improving roomtemperature conductivity, maintaining assembly behavior, and resisting dendrite formation. Tapered block polymers (TBPs) are a class of modified BPs in which a gradient composition region is located between homogeneous blocks. The addition of the tapered region lowers the segregation strength and decreases the TODT while maintaining (or even enhancing) the ability to form cocontinuous network morphologies.26 In work from Epps and co-workers, normal, inverse, and nontapered TBPs composed of PS and poly(oligo-oxyethylene methacrylate) [POEM] were doped with lithium triflate to form a promising electrolyte material. The normal TBP formed a bicontinuous network (gyroid) when doped to lithium salt levels that maximized conductivity.16 Additionally, the TBP architecture altered nanoscale interfacial profiles to improve ion transport (Figure 3a); for example, a cylinder-forming normal TBP possessed a significantly higher conductivity (8 × 10−5 S/ cm at 90 °C) than the analogous nontapered BP (3 × 10−5 S/ cm at 90 °C).17 The increase in normal TBP conductivity was attributed to a significant reduction in the glass transition temperature (Tg) of the POEM block, which enhanced polymer chain motion. This motion could be modulated by both the tapered composition and overall taper fraction in the copolymer.17 These TBPs represent a powerful approach to generate more processable materials with greater inherent transport properties and synergistic cocontinuity of both the mechanical-stabilizing and conducting domains. To achieve improved mechanical properties and “lock in” morphologies upon addition of salt, various cross-linking

Figure 3. (a) TBP conductivities as a function of temperature. The normal TBP had the highest conductivity across all temperatures. Adapted with permission from ref 17, published by The Royal Society of Chemistry. (b) Cross-linked PEO-like electrolyte galvanostatic cycling tests. The cross-linked system showed enhanced resistance to dendrite formation in comparison to an unmodified PEO standard. Adapted with permission from ref 21. Copyright 2014 American Chemical Society.

strategies have been implemented in nanostructured polymer systems, as shown in Figure 2. In one example, a triblock polymer of poly[isoprene-block-(styrene-co-norbornenylethylstyrene)-block-ethylene oxide] was doped with the ionic liquid 1-ethyl-3-methyl-imidazolium bis(trifluoromethylsulfonyl)imide (EMITFSI) to impart conductivity.18 The BP selfassembled into an O70 network morphology that was fixed via metathesis cross-linking of the norbornene side chains.18 Upon cross-linking, the elastic modulus of the electrolyte increased from 104 to 108 Pa, with a measured conductivity of 2 × 10−5 S/cm at 90 °C (at 50 vol % EMITFSI relative to PEO).18 It is difficult to directly compare ionic liquid conductivity to lithiumion conductivity because of differences in ion size, ion−polymer interactions, and ion solvation, but this approach could represent a promising alternative in LiB materials. In another example, polymerization-induced phase separation was used to form a nanostructured, bicontinuous polymer electrolyte.19,27 A functionalized PEO was homogeneously mixed with styrene, divinylbenzene, and a solution of LiTFSI in 1-butyl-31923


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the need to generate novel monomers through synthetically intensive routes, though incorporating all of the desired properties into a single material remains a challenge. To mitigate this issue, new architectures and monomer segment distributions can provide alternatives that balance processability, ionic conductivity, and/or mechanical properties. Single-Ion Nanostructured Polymer Electrolytes. In a typical nanostructured polymer electrolyte, lithium salts must be doped into the polymer to impart suitable ionic conductivity.29 Both the cation and anion are able to diffuse through the electrolyte, causing salt depletion in the membrane and undesirable concentration polarization.4 If concentration polarization is minimized, higher currents can pass through the cell, enhancing performance. By chemically tethering the anion to the polymer, only lithium cations can diffuse, allowing all ion motion to be harnessed for electrochemical energy conversion in the LiB.30 The Li+ transference number (tLi+) corresponds to the relative amount of current attributed to lithium-ion motion, which also represents the current available to power a device. In typical PEO/LiTFSI electrolytes, tLi+ ≈ 0.2,31 and increasing the transference number toward unity has a similar effect to quintupling the conductivity while simultaneously improving the overall electrolyte performance by minimizing concentration polarization.31 However, the development of new, more synthetically complex polymers is necessary to make effective single-ion polymer electrolytes, in which lithium ionic conductivity is similar to that of salt-doped electrolytes described above (>10−4 S/cm at 90 °C), and desirable characteristics like thermal and mechanical stability are retained.

methylimidazolium bis(trifluoromethylsulfonyl)imide (an ionic liquid). The styrene/divinylbenzene polymerized to form a chemically cross-linked electrolyte that incorporated the ionic liquid and lithium salt within the PEO domain. Despite minimal long-range order, both the PEO and cross-linked PS domains were mostly continuous, resulting in both high elastic modulus and ionic conductivity.19 For instance, over the course of the styrene/divinylbenzene polymerization, the modulus increased to ∼108 Pa, while the conductivity only dropped by a factor of two to 3 × 10−3 S/cm.19 Recently, these studies have been extended to systems without ionic liquids through the use of the plasticizer succinonitrile (SN), which should mitigate much of the counterion diffusion.20 Enhanced conductivities (>10−3 S/cm) and elastic modulus (3.5 × 108 Pa at 30 °C) were maintained, suggesting that the SN selectively plasticizes the PEO domains.20 Further studies are necessary to determine how the SN addition impacts lithium ion vs counterion motion. Elsewhere, cross-linked, crystallization-induced, nanostructured polymer electrolytes have shown the ability to resist dendrite formation despite a moderately low shear modulus (∼105 Pa) while retaining high conductivity (>10−4 S/cm) at room temperature.21 For example, Khurana et al. synthesized a material composed of a polyethylene backbone with PEO crosslinks, which was doped with LiTFSI and plasticized by low molecular weight dimethyl-PEO.21 By altering the molecular weight between cross-links and plasticizer loading, the total charge passed before cell failure (Cd) increased by over an order of magnitude in comparison to PS-b-PEO BP electrolytes, as shown in Figure 3b.21 Additionally, more crystalline backbones promoted longer cell lifetimes with minimal drop-offs in conductivity.28 Further studies are expected to determine the mechanism of the dendrite suppression and deconvolute mechanical from electrochemical stability in these systems. The combination of polymers and NPs to form hybrid nanostructured electrolytes also is a promising strategy for performance enhancement because of the mechanical properties intrinsic in many NP composites. In one example, silica NPs grafted with short-chain oligomeric PEO (500 g/mol) were cross-linked with poly(propylene oxide) (2000 g/mol) to form a nanocomposite.23 Upon soaking in a 1 M LiTFSI in propylene carbonate solution, the nanocomposite swelled into a nanostructured gel-like electrolyte. The resulting materials had ionic conductivities up to 6 × 10−3 S/cm at 50 °C, within an order of magnitude of the conductivity of the neat 1 M LiTFSI in propylene carbonate solution. Moreover, the nanostructured hybrid electrolytes had exceptional dendrite inhibition and cell cycling abilities.23 In another nanocomposite example, NPs were incorporated into BP electrolytes to enhance mechanical properties and cycling stability.22 TiO2 NPs were added to a symmetric, high molecular weight PS-b-PEO doped with LiTFSI.22 The morphology of the hybrid electrolytes was affected by the NP loading, in which higher loadings (above 11.5 wt %) homogenized the samples, disrupting the lamellar morphology of the neat BP. Ionic conductivities of the hybrid electrolyte were lower than the native BP electrolyte, but cell cycling and dendrite suppression were improved significantly. These results highlight the enhanced mechanical and electrochemical properties of hybrid electrolytes but suggest additional avenues for future study through investigations of the size, shape, or chemistry of the incorporated NPs on performance. In all of the systems described above, modifications to saltdoped electrolytes improved room-temperature conductivity, electrochemical stability, and/or mechanical properties without

The development of new, more synthetically complex polymers is necessary to make effective single-ion polymer electrolytes, in which lithium ionic conductivity is similar to that of saltdoped electrolytes described above (>10−4 S/cm at 90 °C), and desirable characteristics like thermal and mechanical stability are retained. For the case of BPs, one of the earliest examples of a singleion-conducting monomer segment incorporated into the LiB electrolyte was lithium methacrylate (LiMA), which was integrated in poly(lauryl methacrylate-ran-lithium methacrylate)-block-POEM [P(LMA-r-LiMA)-b-POEM] (Figure 4a).32 The ionic conductivity of these early single-ion BPs was moderate (∼10−5 S/cm at 100 °C), but the tLi+ was nearly 0.9.32 For these particular materials, the effect of counterion location in the BP was examined by comparing different BPs composed of the same monomer segments.32 It was found that incorporating the single-ion conductor into the lithium transporting block [PLMA-b-P(OEM-r-LiMA)] unexpectedly yielded the lowest conductivity in comparison to PLMA-bPLiMA-b-POEM or P(LMA-r-LiMA)-b-POEM, likely as a result of the lithium cation’s inability to disassociate from the anion.32 However, by delocalizing the charge of the anion (through the addition of a Lewis acid such as BF3), the PLMAb-P(OEM-r-LiMA) had similar conductivities to all other BPs (both those with and without BF3).32 Thus, two key design 1924


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Figure 4. Single-ion-conducting polymer electrolytes. (a) Covalently anion-tethered polymers. (b) Tensile stress of PSTFSILi-b-PEO-bPSTFSILi in comparison to PS-b-PEO-b-PS. Adapted by permission from Macmillan Publishers Ltd.: Nature Materials ref 33, copyright 2013. (c) Single-ion dopants such as “salty” NPs offer an alternative strategy. Adapted with permission from ref 36. Copyright 2017 American Chemical Society. (d) Conductivity of salty NP-doped PS-b-PEO. Adapted with permission from ref 36. Copyright 2017 American Chemical Society. (e) Associative anion-tethered polymers. (f) Conductivity of PEO-b-PDTOA. Adapted with permission from ref 37. Copyright 2015 American Chemical Society.

As an alternative to PSTFSILi-b-PEO-b-PSTFSI and PSTFSILi-b-PEO in which the anion is covalently bonded to a bulky aromatic constituent, Porcarelli et al. synthesized POEM-block-poly([lithium 1-(3-(methacryloyloxy)-propylsulfonyl)-1-(trifluoromethylsulfonyl)imide] (POEM-b-PLiMTFSI).34 The glass transition temperature of the POEM rose from −62 to 0.6 °C as the volume fraction of PLiMTFSI increased, primarily as a result of mixing of lithium ions into the POEM block.34 The BP with the lowest Tg showed the highest conductivity (∼10−5 S/cm at 60 °C),34 consistent with the TBPs described earlier.17 Mechanical testing and cell cycling will be useful to evaluate lithium dendrite suppression in this POEM-b-PLiMTFSI system, but the high tLi+ values (0.83) and high electrochemical stability (up to 4.5 V vs Li+/Li) foreshadow this material as a promising option for electrolyte applications.34 In another example, Long and co-workers synthesized PS-b-P[STFSILi-co-di(ethylene glycol)methyl ether methacrylate]-b-PS in an attempt to incorporate the immobilized anion into the ion-conducting block.35 Microphase

principles could be gleaned: delocalization of the anion and separation of the counterion from the ion-transporting domain improve dissociation, which increases the ionic conductivity of the system. These tenets were applied to the design of a single-ion variation on the common PS-b-PEO-b-PS electrolyte by chemically modifying the PS blocks with a TFSI anion to form PSTFSILi-b-PEO-b-PSTFSILi (Figure 4b).33 The mechanical performance via tensile tests showed that the singleion BP had a significantly improved (almost an order of magnitude) maximum tensile stress in comparison to a PS-bPEO-b-PS doped with LiTFSI,33 suggesting that there was substantial mixing between blocks facilitated by the affinity of lithium ions and PEO. The conductivity of these materials was moderate (∼10−5 S/cm), but tLi+ was above 0.85 (see Table 2), leading to an increased electrochemical stability window (stable up to at least 5 V vs Li+/Li).33 Though higher conductivities are desired, the marked improvements in mechanical behavior and electrochemical stability indicate promise. 1925


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Table 2. Example Single-Ion Nanostructured Electrolytes

a

Ionic conductivity.

While the above systems have substantial promise, other approaches have been reported to circumvent some of the synthetic hurdles in the more traditional BP-based “singlecomponent”, single-ion materials. For example, the addition of single-ion polymer-grafted NPs to a BP matrix is a new route in LiBs to achieve microphase-separated hybrid electrolytes. Balsara and co-workers synthesized polyhedral oligomeric

separation was demonstrated on length scales typical of BP assembly. Furthermore, a storage modulus of greater than 1 GPa was maintained up to 70 °C, making this a promising material for lower-temperature applications.35 The conductivity of the electrolytes was about 8 × 10−6 S/cm at 90 °C; however, further studies can assess the transference number and the possibilities for these BP electrolytes in applications. 1926


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conductivity, tLi+, electrochemical stability, and other relevant properties. Theory and simulation can help deconvolute the numerous factors in these experimental systems to enable both better scientific understanding of various systems and to improve designs for effective nanostructured polymer electrolytes. Furthermore, by linking theory and simulation to experimental results, important and translatable thermodynamic and kinetic phenomena can be extracted. For soft electrolyte materials, some properties of interest include charge interactions (both between polymers and ions and between multiple ions), chain conformations, salt and polymer density distributions, diffusion coefficients, and physical inhomogeneities, characterized by polarization and tLi+. Much like the experimental systems that they describe, theoretical frameworks for nanostructured polymer electrolytes also are complex. With respect to BPs, early theories treated the electrolytes as polyelectrolytes (Li+ in PEO) bound to a neutral block (PS), ignoring the salt anions; however, this framework predicted enhanced miscibility between the blocks upon addition of salt, which contradicts the majority of experimental results. Nakamura et al. used an alternate approach, in which the anions were considered to be in the PEO domain and could interact with the PEO-Li+.40 Significant improvements in the agreement between the theoretical and experimental results, such as order−disorder transitions, were achieved across various lithium salts and salt doping ratios.40 One notable finding of the Nakamura work was that ion pair formation was insignificant up to [EO]/[Li] = 10:1 for all salts considered.40 This finding provides a useful experimental benchmark for new techniques, such as magic-angle spinning nuclear magnetic resonance spectroscopy, to probe bound vs unbound lithium. The above theoretical framework was leveraged via selfconsistent field theory to study the ordered lamellar phase of a salt-doped PS-b-PEO.41 Understanding ordered phase thermodynamics is essential to predicting Li+, anion, and polymer density distributions, as well as the dependence of domain spacing and TODT on salt concentration. Salt and polymer density distributions tend to be difficult to probe experimentally,42 and domain spacing and TODT as a function of salt concentration have been widely reported; therefore, if these parameters can be connected through theory, insights can be gained in ion-containing electrolytes. For example, in the strong segregation regime, which is typical of most practical BP electrolytes, the ion distribution uniformly followed the PEO density distribution with no salt in the PS domains.41 The domain spacing and TODT data tracked experimental data,43 lending further credence to the theoretical framework.41 Simulations connected to theory provide an additional platform for extracting valuable experimental comparisons. For example, a coarse-grained simulation model with explicit Coulomb interactions was used to probe a PS-b-PEO-like BP doped with lithium salt.44 At low salt concentrations ([EO]/ [Li] > 10:1), the ion association and solvent dilution effects were relatively balanced, such that TODT increased upon further addition of salt; conversely, at high salt concentrations ([EO]/ [Li] < 10:1), solvent dilution of polymer in salt dominated; therefore, TODT decreased as salt was added.44 This salt concentration demarcator was similar to that found in Nakamura et al. for ion pair formation.40,44 The density distributions of the PEO block, anion, and lithium ion all followed similar trends, in which the ion concentration profiles were linked to the PEO block distribution.44

silsesquioxane (POSS) cores with PSTFSILi chains covalently attached, which selectively segregated into the PEO phase of a PS-b-PEO BP, effectively doping the BP with the “salty” NPs (see Figure 4c,d).36 In this composite electrolyte, the NPs exhibited some behaviors similar to those of small-molecule salts: adjusting the dopant ratios induced order−order transitions from lamellae to cylinders, intermediate salt doping ratios maximized the conductivity, and the electrochemical stability window was similar. However, tLi+ = 0.98 in the salty NP hybrid (as opposed to tLi+ ≈ 0.1−0.3 for salt-doped PS-bPEO).36 In comparison to other single-ion BPs, the hybrid system is very versatile because the NPs can, in principle, be used in any salt-dopable BP system to impart single-ion characteristics. Though most single-ion polymers use a chemical approach to covalently link the lithium salt anion to the polymer, noncovalent associations of the polymer with a doped anion can function similarly to create a BP electrolyte with single-ion character.37 One example of this strategy is PEO-blockpolydithiooxamide (PEO-b-PDTOA), as shown in Figure 4e.37 PDTOA, like PS, has a high Tg, resulting in desirable mechanical and thermal stability; additionally, the thioamide group on the polymer can form hydrogen bonds with anions to restrict anion motion. These pseudo-single-ion BPs had ionic conductivities of up to 7.0 × 10−4 S/cm at 90 °C (Figure 4f),37 which were over an order of magnitude larger than those of many traditional single-ion conductors. Though the pseudosingle-ion BPs performed better than conventional salt-doped PS-b-PEOs with respect to transference number, they still had a lower tLi+ (0.67) than true single-ion polymers.37 In a similar vein, perfluoropolyethers (PFPEs) have been reported to behave as pseudo-single-ion conductors when doped with LiTFSI (see Figure 4e), reaching tLi+ = 0.91.31,38 Like most single-ion conductors, the conductivities were on the order of 10−5 S/cm.38 While the PFPEs were liquid-like, these materials could potentially be improved upon incorporation into nanostructured polymer systems via blending or copolymerization with other ion-conducting polymers. Nonetheless, such pseudo-single-ion BP methodologies demonstrate an exciting alternative to the conventional anion-tethering approaches to incorporate single-ion character with potentially improved ionic conductivity. The inclusion of single-ion conductors into nanostructured polymer electrolyte systems has potential to reduce polarization in the LiB electrolyte, enabling enhanced electrochemical stability and increased power density. Both covalently attached polyanions and polymers that associate with anions produce viable single-ion behavior, but the conductivity of these electrolytes must be improved to compete with the saltdoped alternatives. Further comprehension of existing systems, particularly with respect to ion dissociation, chain dynamics, and interblock mixing, will lead to more informed and optimized designs.39 Finally, new chemistries that improve conductivity will increase the feasibility of single-ion nanostructured polymer electrolytes in LiBs. Nanostructured Polymer Electrolyte Systems Interfacing with Theory and Simulation. Given the inherent complexity of nanostructured polymer electrolyte systems, it is often difficult to isolate the major physical effects that control transport and ultimately performance in conducting nanomaterials through experimental studies. For example, there are many possible combinations of polymers and salts, polymer architectures, and other additives (homopolymers, NPs, etc.) that can enhance 1927


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have been investigated. Interactions between NPs and polymers and between NPs and lithium salts are two of the key factors that must be considered. For example, NPs with more acidic surfaces have been reported to increase the ionic conductivity in comparison to those with a more neutral surface.52,53 Similarly, NP size was shown to impact the conductivity of hybrid electrolytes, with smaller fillers being much more effective at improving ion conductivity.54 Further studies will continue to better characterize the relationships in more complex nanocomposite and hybrid electrolyte systems with respect to their ion conductivity, mechanical properties, and electrolyte performance, but the above examples clearly foreshadow the importance of the theory and simulations approaches. Forming links between theory, simulation, and experiment is essential to correlate different findings across a wide parameter space to maximize performance and stability. Theory and simulation can be connected to experimental data but also are indispensable in garnering information that is difficult to probe experimentally. Additional studies in single-ion and pseudosingle-ion nanostructured electrolytes will be tremendously valuable to elucidate the interactions between tethered anions, lithium ions, and polymer chains to increase the conductivity. Furthermore, extending theory to consider species such as ion pairs and triplets, and quantifying their prevalence in polymer electrolytes, is critical. As the experimental frameworks move forward, concomitant developments in theory and simulation also are necessary to meet the growing challenges in materials design. Polymer-Enabled Electrodes. Next-generation LiB electrodes necessitate new materials, such as nanostructure-forming polymers, to enhance their performance to reach the abovementioned DOE goals. Current LiB electrodes contain a polymer that binds the active materials together (see Figure 5a), which provides a logical focus area for improving electrode conductivity or mechanical flexibility. In comparison to polymer electrolytes, which primarily require ionic conductivity and electrochemical/mechanical stability, polymers for electrode applications require additional characteristics such as electronic conductivity, mechanical compatibility with electrode additives, and resistance to dissolution in the battery electrolyte. The electroactive material, most often LiCoO2 or LiFePO4, instead could be a redox-active polymer, which opens avenues for all-plastic or flexible, stretchable batteries (Figure 5b). However, to use a nanostructured polymer material in the final battery electrode, it must withstand the highly oxidizing or reducing conditions experienced at the electrode and maintain its redox activity. In another approach, microphase selfassembly can be harnessed to nanopattern electroactive materials with sacrificial templating such that they facilitate highly porous electrode formation (Figure 5c). This section will discuss major challenges and advances in electrode binders, redox-active polymers for battery electrodes, and electrode nanotemplating. Polymeric Electrode Binders. Polymeric binders are ideal components to provide mechanical stability in the active materials of battery electrodes during cycling. However, traditional binders [e.g., PVDF and carboxyl methyl cellulose (CMC)] encounter technical challenges when applied in nextgeneration high-capacity electrodes (e.g., Si55 and Sn56). For example, silicon anodes using PVDF as a binder showed poor cyclability57 because of volume expansion and pulverization of the silicon. This challenge motivates the search for new

In addition to polymer and salt distributions, thermodynamic transitions, and domain spacings, theory and simulation can be leveraged to extract important ion transport characteristics. Previous experimental work had demonstrated that in homopolymers increases in molecular weight led to reduced conductivity, with a plateau above a homopolymer molecular weight of 1 kg/mol,45 whereas in BPs, increases in conducting block molecular weight led to increased conductivity, with a plateau above a BP molecular weight of 100 kg/mol.46 Ganesan et al. examined the conductivity behavior in homopolymers and BPs with course-grained simulations.47 The simulation results matched the experimental trends in both systems. In the homopolymer electrolytes, conductivity decreases were due only to the differences in diffusivity of the ions (which decreased with increasing molecular weight) because the charge carrier concentration was independent of molecular weight.47 In the BP electrolytes, both the charge carrier concentration and ionic diffusivity increased with increasing molecular weight, leading to higher conductivity.47 This qualitative agreement with experiment provides opportunities for further refinement of the simulations. For instance, using multiscale simulation strategies has enhanced understanding of complex multicomponent nanostructured electrolyte systems, such as PS-bPEO doped with lithium hexafluorophosphate (LiPF6) salts.48 Heterogeneity in the electrolyte, particularly in the distribution of anions and cations, was noted, which would be otherwise lost in more coarse-grained approaches,48 and this additional information is critical to rationalizing ion transport in nanostructured polymer electrolytes. Further multiscale simulation efforts are attempting to understand electrolyte processes occurring on multiple time and length scales. With the number of different experimental single-ion polymers, which all currently have modest conductivities, there is a need to understand how electrolytes behave with a bound anion. In this vein, Molecular Dynamics simulations were performed on PSTFSILi-b-PEO-b-PSTFSILi,49 inspired by the above-mentioned experimental work from Armand and co-workers.33 Significant mixing was noted between the PSTFSILi and PEO blocks, and ion diffusion was slowed by cross-linking of the PSTFSILi chains in the PEO matrix, such that higher self-doping ratios led to lower conductivities.49 The ability to decouple chain dynamics, self-doping ratios, and morphology via simulation is important in the design of new single-ion conductors with improved lithium-ion conductivity. By linking experiment with theory and simulation, new insights can be gained with more synergistic activities. For example, using scanning transmission electron microscopy images of microtomed bulk lamellar-forming PS-b-PEO doped with LiTFSI, a novel method was developed to simulate the local ion transport over small areas (1 μm2).50 Annealed (low density of grain boundaries) and unannealed (high density of grain boundaries) samples were imaged, and conductivity simulations were run on the processed images to compare the local and bulk conductivity of a single material.50,51 Concurrent simulation-derived and experiment-derived morphology factors were obtained for both the annealed and unannealed samples.50 Though there were significant differences between the relative conductivities,50 this effort hints at future opportunities (and possible challenges) in marrying experimental and simulation/ theory studies. Though many of the efforts in theory and simulation have focused on polymer and BP materials, other nanostructured electrolyte systems, such as NP-containing polymer systems, 1928


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“beyond-PVDF” binders that can retain or enhance the electroactive material performance. These beyond-PVDF binders should possess: (1) mechanical adhesion and flexibility with tolerance to large volume changes of the active material during cycling, (2) electronic conductivity to reduce or eliminate the need for conductive additives, and (3) high ionic conductivity to facilitate ion transfer. Nanostructure-

to modularly incorporate many of these properties in a single material. Notably, Yang and co-workers reported a graft copolymer binder, PVDF-graft-poly(tert-butyl acrylate) (PVDF-g-PtBA), for silicon electrodes.58 The Si anode with PVDF-g-PtBA had better cyclability (84% of the initial capacity after 50 cycles) relative to Si with PVDF (34% after the same cycle number).58 Wei and co-workers also demonstrated a poly(acrylic acid sodium)-graf t-CMC (NaPAA-g-CMC) copolymer as a binder for Si anodes.59 The NaPAA-g-CMC copolymer binder exhibited 80% capacity retention, whereas CMC, PAA, and a NaPAA/CMC mixture showed 39, 46, and 43% retention, respectively.59 Several studies have probed BPs as binders for traditional active materials (e.g., LiFePO460 and V2O561,62). These works suggest that BP binders could be used favorably with Si and Sn electrodes. Balsara and co-workers demonstrated a poly(3hexylthiophene)-block-PEO (P3HT-b-PEO) binder that conducted both electrons (P3HT) and ions (PEO). The electronic

Nanostructure-forming polymers, such as BPs and graft polymers, are potentially suited for electrode binders because of their ability to modularly incorporate many of these properties in a single material. forming polymers, such as BPs and graft polymers, are potentially suited for electrode binders because of their ability

Figure 5. (a) Nanostructure-forming polymer binders, with poly(3-hexylthiophene)-block-poly(ethylene oxide) (P3HT-b-PEO) as an example. (b) Redox-active polymer-based electrodes, with 2,2,6,6-tetramethylpiperidinyloxy-4-yl methacrylate)-block-PS (PTMA-b-PS) and multiblock copolymer poly((napthalenediimide)-block-polyether)x. Reproduced from ref 78 with permission of The Royal Society of Chemistry. (c) Schematic diagrams of a PEO-b-PS-based structure-directing agent. Reprinted with permission from ref 87. Copyright 2014 American Chemical Society. 1929


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Table 3. Performance Characteristics of Example Redox-Active Polymers for LiBs

a

Calculated based on the total mass of polymers. bTwo-electron redox reaction. cPoly{[N,N′-bis(2-octyldodecyl)-1,4,5,8-naphthalenedicarboximide2,6-diyl]-alt-5,5′-(2,2′bithiophene)}.

and ionic conductivities were as high as 10−2 and 10−4 S/cm, respectively; however, the high ionic conductivity was achieved only well above the melting temperature of the PEO domains.63 It was reported that the P3HT-b-PEO binder improved the capacity of LiFePO4 to near-theoretical values (about 140

mAh/g) without carbon additives.60 The multifunctional BP was able to replace both the PVDF binder and carbon electrode components, resulting in more efficient use of the active material. In recent work, Lutkenhaus and co-workers described flexible and carbon-free V2O5 electrodes using P3HT-b-PEO 1930


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Figure 6. (a) Schematic diagram of P3HT-b-PEO/V2O5 electrode preparation. Reprinted with permission from ref 62. Copyright 2016 American Chemical Society. (b) Digital images of V2O5 electrode containing 5 wt % P3HT-b-PEO (P5) in flexure. Reproduced from ref 61 with permission of Nature Publishing Group. (c) Cycling properties of V2O5, PEO/V2O5, P3HT/V2O5, PEO+P3HT/V2O5, P3HT-b-PEO/ V2O5, and PVDF/V2O5 electrodes at various C rates. Reprinted with permission from ref 62. Copyright 2016 American Chemical Society. (d) Cyclic voltammetry of V2O5 electrode containing 5 wt % P3HT-b-PEO. The capacitive (P3HT) and intercalation (V2O5) charge storage contributions to the total current (black) are blue and red, respectively. The scan rate is 1 mV/s. Reprinted with permission from ref 62. Copyright 2016 American Chemical Society.

binders.61,62 V2O5 undergoes volume expansion and pulverization during cycling, but the addition of P3HT-b-PEO binder at loadings of only 5−10 wt % prevented these undesired processes, leading to enhanced long-term cycling and capacity.61 Furthermore, it was found that the BP binder greatly enhanced the mechanical properties of the electrode. For example, the tensile toughness of the BP-containing V2O5 electrode (293 kJ/m3) was higher than that of reduced graphene oxide paper electrodes (178 kJ/m3); see also Figure 6a,b.61 To understand the role of nanostructuring as a result of the BP approach, V 2 O 5 electrodes of similar binder composition were made using a blend of P3HT and PEO homopolymers. These electrodes had significantly worse capacities in the range of 40−60 mAh/g at 1 C rate, (Figure 6c,d) vs 120−150 mAh/g for the electrode with P3HT-b-PEO as a result of severe macroscopic phase separation in the blend system.62 Two notable patents include other work on BP binders.64,65 Nakamura et al. reported PVDF-block-poly(hexafluoropropylene) binders for LiCoO2 electrodes with enhanced cycle stability.64 Elsewhere, Jannasch et al. presented PEO-grafted star BP binders containing two polyisopreneyl arms and two arms having inner blocks of hydrogenated polybutadiene and outer blocks of polystyrene.65 These disclosures indicate that nanostructure-forming binders can enhance capacity and cyclability of active electrode materials. In the future, these polymeric binders should be investigated for high-capacity Si and Sn electrodes, which display much more extreme volume expansion and thus have harsher constraints. Redox-Active Polymer-Based Electrodes. If one or multiple units of the nanostructure-forming polymer are redox-active, then the polymer may be used directly as a cathode or anode. Redoxactive polymers66,67 have several advantages, including rapid redox reactions, tunable and versatile redox properties through synthetic functionalization, solvent-based processing (e.g., printing or patterning), and mechanical flexibility for bendable or stretchable battery systems.68 However, there are still significant challenges to the adoption of redox-active polymers as electrodes, such as low capacity, low conductivity, and undesirable dissolution in the electrolyte.67 These obstacles

may be addressed synthetically, possibly by introducing an insoluble or cross-linkable unit into the redox-active system. Conjugated polymers, which are both redox-active and conductive, have been explored as a potential option for decades,69 but they have suffered from low capacity (often 30− 50% of their theoretical value), insufficient conductivity, poor mass transport, and high self-discharge. Two promising alternative chemistries, organic radical polymers and polydiimides (PNDIs), are highlighted below and in Table 3. Gohy and co-workers reported 2,2,6,6-tetramethylpiperidinyloxy-4-yl methacrylate)-block-PS (PTMA-b-PS)-based cathodes.73,78,79 The TMA repeat unit contained a rapidly reversible redox-active nitroxide radical, which stores charge by a process in which the nitroxide unit is doped by an anion from the surrounding electrolyte (e.g., PF6−). The BP was used because PTMA homopolymer dissolves into the electrolyte upon cycling, causing capacity fade.80 Thus, to enhance long-term materials performance, the Gohy group paired the PTMA block with a nonsoluble PS block to reduce cathode dissolution.78 Cylindrical and lamellar nanostructured PTMA-b-PS electrodes with good capacity retention were demonstrated without carbon additives.78 In the presence of carbon nanotubes (CNTs), the PS block also anchored to the CNT wall to prevent electrode dissolution.79 However, because of the nonactive PS unit, the overall capacity was compromised (15 vs 111 mAh/g for PTMA alone).79 As one alternative, Nishide and co-workers demonstrated PTMA-block-poly(glycidyl methacrylate) electrodes.74 The epoxide groups formed a chemically immobilized coating, providing 100% capacity retention over 500 cycles without dissolution.74 This BP was employed in a supercapacitor configuration but could be translatable to battery-type operations. As another option, diimide units have gained popularity as a result of their ability to store multiple lithium ions (doping level > 1).66 PNDI BPs bearing complementary functional blocks have resulted in further improvements in battery performance relative to PNDI homopolymers. For example, Hernandez et al. demonstrated P(NDI-block-polyether)x multiblock polymer electrodes with 15 wt % carbon black that had a discharge 1931


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in multifunctional electrodes, such as stretchable or flexible battery or capacitor systems. These examples highlight the versatility of nanoscale polymer templating, in which many different electrode materials may be engineered into nanostructured active constructs of defined morphology, enhancing electrode surface area and also providing a well-controlled morphology for fundamental study. Future work may consider expanding the BP chemistries and architectures available for templating to widen the range of precursor−polymer combinations and morphologies. To this end, simultaneous progress on both precursors and polymers can be leveraged to form well-ordered morphologies over the entire thickness of the electrode. Conclusions and Future Directions. Nanostructure-forming polymers can be leveraged as valuable tools for fabricating new LiB electrolyte and electrode materials. In the electrolyte, with careful choice of components, the polymer can act as both a lithium-ion conductor and a mechanical barrier to prevent short circuits caused by lithium dendrites or other contaminants. Two common polymeric constructs are salt-doped and singleion-containing macromolecules. For the case of salt-doped polymers, new macromolecular architectures, monomer segment distributions, and cross-linking schemes are improving electrolyte properties with minimal increases in chemical complexity, enhancing battery cycling stability, separator modulus, and ionic conductivity. In the single-ion approach, synthetic efforts can be employed to immobilize the anion in the polymer either covalently, electrostatically, or with other interactions. These single-ion systems have enhanced electrochemical stability and cell cycling behavior; however, the materials synthesis typically is more intensive, and the conductivity of the resulting materials normally is reduced somewhat in comparison to the salt-doped counterparts. Theory and simulation can be harnessed to connect readily probed experimental results to difficult-to-predict thermodynamic and transport phenomena, which subsequently can be employed in the design of future experimental systems. In electrodes, nanostructured polymers have possible roles as binders, electrodes, and nanotemplates. Improved binders are desirable to overcome severe volume expansion issues present in many otherwise-promising active materials, such as Sn or Si, and furthermore, polymeric binders are an ideal medium to incorporate new properties into an electrode material, such as flexibility or stretchability. Though electroactive polymers as electrodes are still an emerging technology, recent efforts with organic radical polymers and PNDIs have shown promise toward higher retention of capacity upon cycling and increased doping levels. Templating with self-assembling polymers is a more established process in many other industries, such as nanomembrane fabrication and nanolithography, and should readily translate to LiB electrode fabrication to develop high surface area electrodes. Looking forward, there are enormous opportunities for continued advances in nanostructured materials for LiB applications. With respect to electrolytes, enhanced understanding of current systems through systematic studies of salt and polymer distributions, in situ cycling studies with neutron or X-ray scattering, and connecting results with theory and simulation will provide new guiding principles to facilitate more strategic design of next-generation electrolytes. In all cases, further investigations are necessary to establish fundamental relationships between solid-state electrolytes and the anode and cathode (to determine electrochemical stability, cell lifetimes,

capacity of 170 mAh/g and excellent stability over 100 cycles.76 This improvement was attributed to the ionic conductivity as a result of the ether oxygen groups. Yao and co-workers reported a π-conjugated poly(diimide-alt-BT) polymer (poly{[N,N′bis(2-octyldodecyl)-1,4,5,8-naphthalenedicarboximide-2,6diyl]-alt-5,5′-(2,2′bithiophene)}) electrode with a doping level of up to 2.0. These electrodes exhibited high rate capability (100 C rate), near-theoretical capacity (54.2 mAh/g), and 96% capacity retention after 3000 cycles.77 The above examples suggest that both organic radical and diimide polymers can achieve very high doping levels, well beyond that of common conjugated polymers. While this phenomenon can lead to high capacity, further development is needed to realize practical redox-active polymeric electrodes, particularly in the areas of reducing solubility and enhancing electronic conductivity. Nanostructured Electrode Templates. Nanostructure-forming polymers have been implemented to prepare high surface area electrodes by leveraging the tailorable pore size and pore structure, enabled by macromolecules such as BPs, to control electroactive area and ion diffusion. For templating applications, one of the polymer domains selectively uptakes a precursor that is transformed into the desired electroactive material, after which the soft nanotemplate is removed. The resulting material is a high surface area electrode that mimics the original morphology of the parent BP system. Nanostructuring through BP self-assembly mitigates problems with volume expansion during cycling,81 increases active material utilization,82 and overcomes diffusion limitations, which impact both energy and power density.81 Furthermore, nanostructured template electrodes may be leveraged as a platform to analyze structural changes in operando or during cycling.83−85 With respect to nanotemplating, several efforts are highlighted. Tolbert and co-workers reported mesoporous manganese oxide templating with poly(ethylene-alt-propylene)block-PEO via an evaporation-induced self-assembly (EISA) process.81,86 The templated manganese oxide film had twice the stored charge in comparison to an untemplated analogue.86 Jo et al. fabricated nanostructured TiNb2O7 anodes using PS-bPEO as the structure-directing agent.87 The nanostructured TiNb2O7 electrode was reported to have 27% higher capacity with better rate capability vs bulk TiNb2O7.87 This performance enhancement was attributed to smaller crystallites (∼15 nm), higher surface area, and larger pore sizes (∼40 nm) in the nanotemplated system.87 Kawai and co-workers generated a 3D bicontinuous electrode consisting of a LiCoO2 active cathode material and a Li7La3Zr2O12 electrolyte for all-solid-state LiBs by calcination of the PS-block-poly(4-vinylpyridine)-precursor nanocomposites.82 The 3D electrode morphology led to an active material utilization of 98%. Elsewhere, Wiesner and coworkers demonstrated the one-pot synthesis of nanostructured TiO2/carbon composite anodes using polyisoprene-blockPEO,88 and Hwang et al. reported Sn-embedded carbon/silica electrodes generated using PS-b-PEO.89 In the Sn case, the BP morphology depended on the Sn precursor loading, which directed the formation of Sn nanowires or spherical Sn NPs. Finally, the polymer can be left behind as a mechanical support, as opposed to being removed as described in the cases above. Mui et al. demonstrated nanostructured electrodes with CNT and gold NPs in a bicontinuous gyroid POEM-b-PMMA matrix.90 Lee et al. recently reported flexible and porous CNTembedded polydimethylsiloxane (PDMS) electrodes by using PDMS-b-PMMA.91 These examples foreshadow opportunities 1932


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and cyclability).92,93 Adoption of new polymerization strategies, such as sequence-controlled polymerizations, can create precise materials to more fully understand lithium conduction mechanisms and optimize performance in typically heterogeneous electrolytes. New lithium salts or NPs, among other additives, will be essential to accelerate conductivity improvements in electrolyte systems to move toward faster-charging LiBs; however, scalable methods to enable selective transport of lithium ions (as opposed to anions) also are ripe for additional significant effort. Enhancing conductivity of single-ion nanostructured polymers should be a priority, which likely will require new synthetic designs of monomers that can function as both cation transporters and anion “detainers”. Furthermore, improved dissociation of the lithium ion from the immobilized anion will help to make these materials more viable. Finally, inspired by the Materials Genome Initiative,94 continued efforts in theory and simulation to streamline materials discovery will guide experimental efforts as material complexity (anion, molecular architecture, monomer segment distribution, morphology, etc.) increases. In electrodes, polymer binders are the most accessible application as there exists a tremendous opportunity to tailordesign new polymer binders that include features of conductivity and mechanical adhesion. Unique mechanical properties, including flexibility and elasticity, are possible with the inclusion of nanostructure-forming polymers in both binders and electrodes, which will empower new applications, such as biometrics, organic electronics, and smart textiles. Future work on electroactive polymer electrodes should target improvements in the reversible capacity of the polymer, which can be achieved by various means such as including monomer units with multielectron transfer capabilities and the development of polymers capable of high reversible doping levels and high conductivity over the entire voltage window. However, because conductivity and doping are intimately linked and multielectron transfer can lead to undesirable side reactions, these conditions often are difficult to satisfy simultaneously. Recent promising work with organic radical polymers and PNDIs reports rapid and reversible electrochemistry, but the low conductivity of these systems still remains a major challenge. For electrode nanotemplating using soft materials, a significant focus should be on obtaining architectures commensurate with standard electrode thicknesses (50−100 μm), which is complicated by the ability to maintain uniform patterning as a result of mass transfer of metal oxide precursors into macromolecular systems, such as BPs. Finally, there is an urgent need for theory and simulation efforts targeting electroactive polymer systems. For these applications, a single modeling approach is unable to address morphology, conductivity, and electrochemical reactions on the time scales and length scales of interest. Thus, concerted multiscale modeling studies, from quantum to microscopic length scales, are necessary to make the challenges of electroactive polymer design more tractable. In short, the shift of LiBs toward inclusion of polymers in all battery components is a prudent route that can accommodate the numerous constraints of next-generation devices. Nanostructure-forming polymers, including BPs, can address many current problems with respect to thermal and electrochemical stability and provide new opportunities in flexible and stretchable systems, which is difficult to envision with traditional battery materials. Fundamental understanding of the underlying structure−property−processing relationships is

essential to facilitating the design of next-generation materials that can enhance battery performance and accelerate commercialization of polymer-containing LiBs. By developing competitive materials, incorporation of polymers in LiBs can become more widespread, leading to safer, more reliable, more efficient, and lower-cost batteries.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (J.L.L.). *E-mail: [email protected] (T.H.E.). ORCID

Jodie L. Lutkenhaus: 0000-0002-2613-6016 Thomas H. Epps III: 0000-0002-2513-0966 Notes

The authors declare no competing financial interest. Biographies Melody A. Morris is currently pursuing her Ph.D. in Chemical Engineering at the University of Delaware under the guidance of Prof. Thomas H. Epps, III. Her research focuses on the synthesis and characterization of novel block polymer electrolytes for applications in lithium-ion batteries. Hyosung An is a Kwanjeong Educational Foundation fellow currently pursuing his Ph.D. in Chemical Engineering at Texas A&M University under the guidance of Prof. Jodie L. Lutkenhaus. His research focuses on organic/inorganic hybrid electrodes for lithium-ion batteries. Jodie L. Lutkenhaus is the William and Ruth Neely Faculty Fellow and an Associate Professor of the Artie McFerrin Department of Chemical Engineering with a courtesy appointment in the Department of Materials Science & Engineering at Texas A&M University. Her research examines the electrochemistry of redox-active polymers, selfassembly of polyelectrolytes, corrosion, and functional thin films and coatings. Thomas H. Epps, III is the Thomas and Kipp Gutshall Professor of Chemical and Biomolecular Engineering and a Professor of Materials Science and Engineering, with an affiliated appointment in Biomedical Engineering at the University of Delaware. His research focuses on the design, synthesis, and characterization of nanostructured soft materials and biobased systems in bulk, thin film, and solution environments for lithographic templating, separation membrane, ion-transport, thermoplastic elastomer, and therapeutics applications.

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ACKNOWLEDGMENTS

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REFERENCES

M.A.M. and T.H.E. thank DOE BES (DE-SC0014458) for financial support during the writing of this manuscript. T.H.E. also thanks the Thomas & Kipp Gutshall Professorship for financial support. H.A. and J.L.L. acknowledge financial support from DOE BES (DE-SC0014006) and NSF CBET-1336716 during the writing of this manuscript. H.A. also thanks the Kwanjeong Educational Foundation.

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