Finding Harmony between Ions and Electrons: New Tools and

Perspective pubs.acs.org/cm

Cite This: Chem. Mater. 2017, 29, 8918-8931

Finding Harmony between Ions and Electrons: New Tools and Concepts for Emerging Energy Storage Materials† Kenneth Hernández-Burgos,‡,§ Zachary J. Barton,‡ and Joaquín Rodríguez-López*,‡,§ ‡

Department of Chemistry and §Beckman Institute for Advanced Science and Technology, University of Illinois at Urbana−Champaign, 600 South Mathews Avenue, Urbana, Illinois 61801, United States ABSTRACT: The study of charge transport within and between materials, including interfaces, is one of the most critical challenges facing electrochemistry due to its importance in determining rate and charge capacity in battery materials. In this perspective, we discuss examples of materials that reveal the delicate balance between structure, ion and electron transport, and chemically specific interactions that modulate reactivity during charge storage. We first address a new type of highly soluble redox-active polymers in which physical and electronic structure as well as interactions with the electrolyte determine their reaction mechanisms. Finding common ground with a variety of polymeric and 2D materials, we highlight new directions and strategies for enhancing electrochemical performance. Because of the importance in determining ion transfer mechanisms, we further explore ionically sensitive scanned probe methods that are helping to elucidate relationships between ion and redox reactivity. Examples of how these techniques are applied to ultrathin anodes and interfaces for ion batteries further underscore the importance of exploring the role of ion transfer and ionic interactions in emerging electrode materials. We hope that this perspective will provide new ways of approaching longstanding challenges in the field and provide key examples that encourage new directions in our understanding and control of the reactivity of energy storage materials.

1. INTRODUCTION Though some historical formulations of electrochemistry have considered electronic and ionic phenomena as separate, isolated topics,1,2 the modern search for high-performance energy storage materials is predicated on the interweaving of concepts and techniques for understanding reactive pathways involving simultaneous electronic and ionic processes. These pathways are often difficult to predict a priori due to the possibility of competing multistep processes and their sensitivity to changes in the local chemical environment.3,4 Consequently, finding harmony between discordant electronic and ionic processes requires detailed studies of specific steps of charge transport mechanisms within and between materials (Figure 1a). For example, our interests lie in redox-active polymers (Figure 1b) in which the accessibility of stored charge is strongly modulated by conformational changes, which determine pendant-topendant charge exchange,5−7 and in ultrathin electrodes that recreate battery interfaces (Figure 1c). In this perspective, we highlight examples of both materials, discussing how their operation depends on a delicate balance between structure, ionic8,9 and electronic transport, and chemically specific interactions. Although a variety of analytical and materials approaches reveal bulk transformations in great detail, new surface-sensitive analyses are required to better elucidate the impact of charge transfer at interfaces (Figure 1a).10−12 We

highlight scanning electrochemical microscopy (SECM) as an emerging analytical tool for simultaneously investigating electronic and ionic reactivity at operating interfaces (Figure 1d). This provides information about “lithium not going to the right place at the right time”13 in lithium-ion batteries and is applicable to a variety of emerging technologies. Rate and mechanistic limitations initiated by deficiencies and heterogeneities in ion transport following electron transfer are also pervasive in electrocatalysis,14 biological interfaces,15 and molecular redox systems.6,16 Elucidating the relationships between surface structure, interfacial electronic and ionic properties, and redox performance is essential for devising new strategies to improve energy storage materials.

2. STUDYING CHARGE MOBILITY AND PROPAGATION IN POLYMERIC MATERIALS 2.1. Modulating Charge Transfer in Redox-Active Polymers. Redox-active polymers (RAPs), i.e., macromolecules with a repeating unit consisting of a redox-active motif, adopt various conformations influenced by the relative strengths of long- and short-range electrostatic and dispersion interactions.16−21 These factors strongly influence their electron Received: May 31, 2017 Revised: August 13, 2017 Published: August 20, 2017

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This Perspective is part of the Up-and-Coming series. © 2017 American Chemical Society

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DOI: 10.1021/acs.chemmater.7b02243 Chem. Mater. 2017, 29, 8918−8931

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Chemistry of Materials

Figure 1. Superior energy storage requires finding harmony between electronic and ionic phenomena. (a) Interphases and interfaces are critical in facilitating charge transfer and compensation. Emerging systems: (b) ionic and electronic interactions strongly determine the pathways for charge propagation in redox-active polymers and their colloidal particles, (c) near-surface perturbations and thickness effect drive alternative pathways for ion intercalation in graphitic carbon, and (d) interfacial imaging of ionic processes elucidates the impact of heterogeneity.

Figure 2. Characterization of interpendant charge transfer on redox-active polymer (RAP) films and solutions. (a) Depiction of charge hopping mechanism RAP films. (b) Cyclic voltammograms at 20 mV/s for the second generation viologen-based RAP films in a solution containing 0.1 M supporting electrolyte. (c) UV−vis spectra for o-benzene polymer and m-benzene polymer (second generation RAPs) at a concentration of 0.06 mM. Bulk electrolysis cycles for (d) first generation viologen-based RAP and second generation polymers (e) o-benzene C4 polymer and (f) mbenzene C5 polymer. (a) Figure is adapted with permission from ref 17. Copyright 2016 Electrochemical Society. (b, c, e, f) Figures are adapted with permission from ref 16. Copyright 2016 American Chemical Society. (d) Figure is adapted with permission from ref 87. Copyright 2014 American Chemical Society.

electronically conductive backbone,31 electron conduction

transfer properties, such as their standard reduction potential (E0) and standard heterogeneous rate constant (k0), as well as their physical diffusion and intraparticle electron diffusion coefficients.6,22−25 Solvent identity and ionic strength determine the conformation of redox-active polyelectrolytes in solution and as adsorbed films.4,26,27 RAP films act as electron transfer mediators between the underlying electrode and solution species. 28 While RAPs can be built with an

also takes place via electron exchange reactions between neighboring pendants, enabling the use of nonconjugated backbone motifs.6,7,22,32 In such systems, the diffusion coefficient for electron transfer is described by the Dahms− Ruff model: 8919


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Chemistry of Materials k δ2 DE = EX C* 6

control or ν1/2 dependence during potentiodynamic sweeps. For example, very thick polymer films exhibit peak currents that scale linearly with the square root of the potential sweep rate in CV experiments (Figure 2b). In contrast, very thin polymer films often exhibit a peak current intensity that varies linearly with respect to potential sweep rate in their CV response. In cases where ionic transport within the film constitutes a bottleneck in reactivity, changing the supporting electrolyte concentration may lead to enhancements in Faradaic accessibility at a fixed concentration of redox-active species. In cases where electron transfer is the primary limiting factor, modifying the chemical structure or local environment of the redox-active center may optimize kEX.40−43 However, accurately interpreting measurements of polymer films is not always a straightforward process. For example, the blocking of ionic or electronic pathways in response to inhomogeneous charge propagation can lead to electrically isolated polymer islands, a phenomenon known as charge trapping.36,44−46 Charge trapping introduces artifacts in CV measurements that can be mistaken as evidence of kinetic irreversibility.36,46 Evidence of increased accessibility to the redox-active groups may be the result of morphological changes in the film that unblock access to species within it. Thus, a complete understanding of redox processes in polymers requires not only the deployment of electrochemical techniques but also of methods for investigating layer morphology, thickness, and the absolute redox-active center loading. The following examples highlight the role of each of the electronic and ionic interactions in RAPs. We investigated the electronic modulation of DE in a “second-generation” RAP (Figure 2c) by attaching two redoxactive pendants with well-defined interpendant distancing at each tether point, in contrast to the “first-generation” RAP with one redox-active motif per unit. In “second-generation” RAPs, reduction of either one of the viologen-based pendants produces an intervalence compound, a class of molecule capable of rapidly transferring electrons between distal regions through overlapping valence shells in different oxidation states rather than by a continuous path through the connecting atoms of the tether and support structure.16,48,47,49 The impact of this phenomenon on the kEX term describing DE (eq 1) was investigated by applying the Marcus−Hush formalism to the spectroelectrochemical data.16,50−54 The observation of a nearinfrared adsorption band associated with the formation of an intervalence state (Figure 2c) confirmed that kEX could be modulated by synthetic control of the spacer length separating cotethered redox-active pendants.16 Furthermore, the observed positive shift in the formal reduction potential for the polymers in solution and in a film (Figure 2b) indicated thermodynamic stabilization of the reduced radical product, which is primarily a consequence of the radical delocalization through π−π interactions between the cotethered aromatic pendants.16,55,56 However, it was evident from the peak current in CV and its dependence on ν1/2 that the redox process was still limited by diffusive transport. Since the o-benzene C4 polymer derivative offers approximately a 2-fold increase in redox-active site density and a 100-fold increase in kEX relative to its first generation analogue, it is unlikely that DE is limiting. Analysis of the film thickness and comparison to the number of redoxactive units accessed via electrochemical experiments revealed significant charge trapping. Therefore, we concluded that the observed limitations in Faradaic activity were attributable to ion diffusion into the film.

(1)

where DE is the electron diffusion coefficient, C* is the concentration of redox-active pendants, kEX is the self-exchange rate constant, and δ is the average distance between redoxactive centers.17,22,33This conduction mode is also referred to as charge hopping (Figure 2a),25,33 and in RAPs it involves the sequential transfer of electrons between proximal redox-active pendants in a coiled polymer without the need to follow the polymer backbone. Because the conformation and packing density of adsorbed polymer coils impact all three variables in eq 1, the speed and efficiency of charge hopping is affected by the local chemical environment.16,26,27 Additionally, changes in the state of charge in redox-active pendants impact intermolecular forces so greatly that asymmetry can emerge in the electron transfer coefficients as well as in the cathodic and anodic kinetic rate coefficients.16,22,25,33,34 Typical DE values for viologen-based and ferrocene-based RAPs are in the range of 10−12 to 10−9 cm2/s.17,22,155 In comparison, conducting polymers (CPs) such as polypyrrole show DE values in the order of 10−9 to 10−8 cm2/s.29,30,156 While CPs exhibit superior electronic transport compared to RAPs, they require rigid backbone structures that support large conjugation lengths, often making them very insoluble in water and common battery solvents. In contrast, RAPs offer a broader chemical versatility, conveniently incorporating a wide variety of redox designs via modular synthesis. RAPs can be designed as precipitated films in modified electrodes but also as highly soluble materials for flow batteries.72,74,87 While CPs exhibit pseudocapacitive charge/discharge profiles in batteries, RAPs show well-defined redox processes.34 Therefore, improving charge transport in RAPs and elucidating their common ground with CPs present opportunities to further the value of these materials for battery applications. A direct interpretation of the Dahms−Ruff model for electron transport stresses the importance of finding optimal conditions for interpendant electron transfer reactions. However, ionic species are responsible for neutralizing the newly formed charges upon electron transfer. That is, the overall electrochemical response of RAPs also depends on ionic transport phenomena to attain charge balance.16,17,22,35−38 These effects are further amplified with increasing physical dimensions and decreasing ionic permeability of a colloidal particle or film. Murray et al. discussed these phenomena in great detail while studying ferrocenated imidazolium ionic liquids.6 Because the physical diffusion of the polymer is so slow in such highly viscous media, charge transport that are dominated by DE and kEX were expected to be enhanced by increases in C*, since the frequency of close encounters between redox-active sites would presumably be increased. However, the relationship between the peak current and cyclic voltammetry (CV) potential sweep rate was unaffected by changes in Cpolymer, which suggested that the reaction rate was controlled by the physical rate of the counterion diffusion.6,39 In the case of polymer layers, their thickness and morphology also impact polymer performance. The diffusion coefficient of the limiting speciesions or electronscan usually be determined by transient electrochemical experiments, such as chronoamperometry or voltammetry, where the time dependencies are analyzed. The identification of diffusion as a limiting factor can be accomplished by either a t−1/2 dependence in potentiostatic 8920


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Chemistry of Materials The electrochemistry of the second-generation polymers underscored the need to match electronic and ionic improvements at short and long ranges. We found that decreasing interpendant distance, and therefore increasing kEX, beyond a limiting value caused the formation of radical anions to outrun the diffusion of counterions to balance the charge. The resulting imbalanced electronic interactions within the RAPs impacted their capacity and reversibility. In fact, the bulk electrolysis of the second generation polymers yielded contrasting results between them and when compared with their first generation analogues (Figure 2d−f). While the obenzene C4 polymer with its shorter interpendant distances afforded superior electronic communication between redoxactive pendants, its electrochemical charging process was completely chemically irreversible (Figure 2e). On the other hand, the m-benzene C5 polymer, which has a smaller kEX, yielded a highly reversible bulk electrolysis. In fact, the cycling profile of the m-benzene C5 RAP becomes even more symmetrical over time, unlike its first generation analogue (Figure 2d,f).6,16,62 A stark difference in electrochemical behavior was obtained by a small substitutional difference at the molecular level, which affected the ability of electrons to transfer within the polymer structure while enabling ions to respond reversibly to the sites undergoing charge transfer. While our study allowed us to leverage the self-exchange phenomenon to improve the rate of electron transfer throughout the RAP, a main lesson for energy storage remains: hypotheses pertaining microscopic electronic enhancements are incomplete without considering the role of ions as key reactants in the chemical equation. 2.2. Electrolyte and Solvent Effects. We now turn to the impact of electrolyte and solvent interactions on RAPs and other multidimensional architectures for energy storage.35,57,58 Solvation and ion exchange effects on the conformation of redox polymeric matrices such as hydrogels are well known in applications for actuators and sensors.59−62 For energy storage, we are mainly interested in understanding the impact of these factors on the redox potentials and mobility of species. These interactions can be broadly classified as resulting from either specific chemical interactions or nonspecific electrostatic interactions. The importance of these interactions is highlighted by the differences in the reactivity of poly(nitrostyrene) (PNS) in the presence of tetrabutylammonium (TBA+), lithium (Li+), and potassium (K+) (Figure 3a). PNS displays reversible redox behavior in the presence of TBA+ but irreversible reduction in the presence of Li+ due to strong ionic coordination with the radical anions formed by the reduction of the redox-active pendants.35,57,63 However, K+, which is intermediate in size and polarizability to TBA+ and Li+, exhibits intermediate behavior. Reduced PNS radical anions do form ion pairs with K+, as evinced by shifts in the reduction potential in K+-containing electrolytes, but, in contrast to Li+-containing electrolytes, the PNS remains chemically reversible. In fact, the peak reduction and oxidation current densities in the presence of K+ are greater than those observed in the presence of TBA+ (Figure 3a). Thus, the identity of the supporting electrolyte can have a significant impact on the thermodynamics and reversibility of redox-active polymers. These chemical interactions between redox-active species and electrolyte ions are also evident in the behavior of the second-generation viologen dimer polymers (Figure 2): superior charge transfer is attainable when redox processes do not produce electrostatic barriers that hinder ionic movement.

Figure 3. Impact of electrolyte and solvent interactions on RAP reactivity. (a) CVs in nonaqueous solutions containing 5 mM PNS and various electrolytes (TBA+, K+, and Li+). (b) CVs in solutions consisting of 5 mM redox-active polymer and various concentrations of tetraglyme (TG) in acetonitrile (ACN). (c) Solvent-dependent CVs for a 135 nm RAC monolayer submerged in solutions of 0.1 M LiBF4 (in organic solvents) and 0.1 M KCl (in water) in a 1:1 ratio by volume. The solvents tested were acetonitrile (ACN), N,Ndimethylformamide (DMF), water, propylene carbonate (PC), and tetraglyme (TG). All CVs were performed at 20 mV/s. Inset is a SEM image of the RAC monolayer. (d) Plot of peak current from part c against the inverse of viscosity. (a) Figures are adapted with permission from ref 35. Copyright 2016 Royal Society of Chemistry. (c, d) Figures are adapted with permission from ref 72. Copyright 2016 American Chemical Society.

If these barriers are insurmountable, as in the case of the obenzene C4 polymer or the PNS-Li+ ion pairs, undesirable charge trapping occurs. These interactions are not exclusive to RAPs, as charge trapping can also be observed with carbonylbased 2D polymers in some electrolyte systems.57,63 Similar to the PNS case, strong ionic interactions stabilize the charge of the reduced form of carbonyl-based organic molecules, resulting in increased reactivity and positive shifts in their reduction potentials.63,64 DeBlase et al. demonstrated that the radical anions and dianions formed by the reduction of naphthalene diimide-based porous polymer networks exhibited different degrees of interaction with small radii cations. This was evidenced by large shifts in their reduction potentials of up to 710 mV and 460 mV for Mg2+ and Li+, respectively, in contrast to only 120 mV for K+ and no interaction with TBA+. However, these interactions were not observed for the naphthalene diimide monomers in solution, thus suggesting a synergistic effect resulting from the electrostatic interaction and the material structure, characterized by a densely packed redoxactive network. Nonetheless, strongly interacting ions led to decreased k0 values and lower ion mobility within the polymer host.57 Altogether, these observations in viologen, PNS, and carbonyl-based 2D polymers lead to the same conclusion that ions cannot be considered mere spectator species in RAPs. Developing experimental and theoretical frameworks for identifying the solvent and electrolyte combinations that optimize the performance of RAPs presents an area of opportunity. For example, in a poor solvent, the electrochemical properties of a polymer are difficult to study because the coils tend to crash out of solution. In Figure 3b, adding tetraglyme (TG) to the solution of viologen RAP results in increased adsorption on the electrode. The use of a solvent 8921


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the electrochemical reactivity and adsorption behavior of viologen-based RAPs and RACs are sensitive not only to the identity of the solvent but also to the identity and concentration of the supporting electrolyte.7 Beyond their value as potential battery materials, RAPs and RACs provide a suitable analytical platform on which to dissect specific aspects of redox and ionic reactivity for systems of higher chemical and dimensional complexity (Figure 4).

with a low dielectric constant results in the precipitation of the highly charged RAP as evidenced by the transition in CV behavior from a diffusion-controlled response (0% TG), to a mix of diffusion-controlled and surface-bound behavior (5% TG), and then to a completely surface-bound response (20% TG) for the same RAP solution (Figure 3b). This simple experiment demonstrates that polymer reactivity can be modified by taking advantage of attractive or repulsive interactions between the different components in the electrolyte. However, a complete discussion of the effects and conformations of RAPs in solutions and in films is not straightforward and requires a description of chain stretching, optimized torsion angles, entropy change of the solution in response to polymer conformation changes, image forces, line charge density, and bridging attractions.18,65−67 Besides solvent−solute interactions, the conformation of polymers in solution depends on the mechanical flexibility of the chain(s).68 Mathematical descriptions of how polyelectrolyte chain stiffness varies with supporting electrolyte concentration have been laid out in some detail previously.18,69 Additionally, the sensitivity of polyelectrolyte compaction behavior to supporting electrolyte concentration in solution and at surfaces has been delineated previously,70 and investigations of polymer adsorption considering polymer chain length and permeability to electrolyte have also been described.19,20,67 This wealth of information is the result of over 50 years of work in polyelectrolyte theory. In contrast, investigations linking the redox behavior of RAPs and their physical properties are just emerging. We foresee that electrochemical investigations of highly soluble RAPs with systematic variations of their charge density, structure, and interactions with electrolytes will open new avenues of study in materials for energy storage.34,71−74 2.3. Aiding Charge Transport in 3-D Polymer Architectures. Our group recently introduced redox-active colloids (RACs), dispersions of nanoporous cross-linked poly(vinylbenzyl chloride) particles functionalized with redoxactive pendants, e.g., ethyl viologen or ferrocene.72 These spherical nanoparticles offer greater mechanical stability, energy density, and long-term Coulombic efficiency than their RAP analogues in nonaqueous redox flow cells. However, designing successful RACs requires an understanding of how particle size, porosity, and chemical structure impact their reaction mechanisms. The versatile format of RACs allowed us to investigate their CV response when deposited as well-ordered monolayers on conducting substrates (Figure 3c). Due to their homogeneous packing density and uniform thickness, RAC films are far more amenable to electroanalytical determinations of charge transport rates than RAP films, which typically exhibit wider variations in film thickness and redox-active pendant density.155 We foresee that the controllable synthesis of RAP thin films22,164,165 and monodisperse particles72,166 will gain more relevance in the elucidation of charge transfer and transport on nanoelectrochemical151,161,167,168 systems. CVs of monolayer RAC films (Figure 3c) submerged in a variety of solvent systems exhibited a linear correlation between peak current intensity and solution viscosity (Figure 3d). We attribute this trend primarily to the differences in electrolyte diffusion in the different solvents. However, since the application of Walden’s rule to account for differences in solution viscosity does not eliminate the trend observed between peak current and solution dielectric constant,75 we also suspect differences in solvated ion radii and particle wetting behavior played important roles.18,21,76 Our studies suggest that

Figure 4. Bridging concepts of ionic and electronic mobility in complex architectures. (a) Left, A schematic highlighting the morphological changes of conducting polymers (CPs) under the presence of ethylene glycol. Right, Changes in electronic and ionic conductivity as a function of ethylene glycol content. (b) Left, A schematic representation of CPs inside covalent organic framework (COF) pores and, right, electrochemical performance of the CP−COF composite. (c) Left, Artist’s representation of the Li1.2Ni0.2Mn0.6O2 coated with CP, including the charge propagation mechanism and the SEM of the coated particle. Right, Device performance showing an enhancement in the presence of CPs. (a) Figures are adapted with permission from ref 96. Copyright 2016 Creative Commons Attribution 4.0 International License (http://creativecommons.org/ licenses/by/4.0/). (b) Figures are adapted with permission from ref 58. Copyright 2016 American Chemical Society, Author Choice and Editors’ Choice. (c) Figures are adapted with permission from ref 99. Copyright 2016 American Chemical Society.

Obtaining high charge mobility within a polymer matrix is vital for maximizing electrochemical performance.29,30 A successful strategy for enhancing internal charge mobility is to create conductive paths connecting redox-active centers, not only by interpendant charge hopping14 but also through the use of conducting polymers (CPs).77 Similarly to RAPs, the charge diffusion can be measured using different electrochemical methods such as CV, electrochemical impedance spectroscopy, and chronoamperometric methods.29,30,78 Unlike the backbones used in the RAPs described in previous sections, CPs 8922


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PEDOT as a CP for wiring, using electrodeposition within the porous COF structure (Figure 4b) and effectively enabling access to the full redox capacity of COF films with thicknesses on the order of several micrometers. In these studies PEDOT improved not only the conductivity of the system but also the rate capability (C-rate up to 100C), cyclability (up to 10000 cycles), and accessibility (by 40-fold) of this COF in acidic media. Similarly, CPs have been used to enhance the performance of electrode materials for lithium ion batteries (LIBs). Specifically, many materials that are suitable for Li+ insertion suffer from low intrinsic conductivity and difficulties in establishing successful interparticle contact within composite electrodes.5,84,85,94,99−101 For example, Wu et al. investigated the use of Li-rich cathode materials with an adversely reacting interface that led to undesirable side processes at high voltages, ultimately displaying poor cycling performance and rate capability.99,102 For their studies, Li1.2Ni0.2Mn0.6O2 was coated with PEDOT:PSS demonstrating up to 51.6% greater capacity retention compared to uncoated samples (Figure 4c).99 In this application, CPs are convenient since they are less dense than traditional conducting additives and also act as interface stabilizers that prevent side reactions that deteriorate the cathode material.99 Additionally, CPs can serve as redox-active anchors and trapping media.85,95,103 For example, Yang et al. were able to improve the performance of lithium−sulfur batteries by using PEDOT:PSS to prevent the sulfur dissolution while allowing charge transport throughout the film.95 These examples demonstrate how differential ion and electron transport in CPs can be leveraged to optimize the performance of energy storage materials. These systems have significantly different chemical properties and structures than RAPs but can be investigated by the same general strategies for elucidating redox and ion dynamics.

integrate conjugated systems capable of transporting charge on delocalized molecular orbitals. Doping triggered by a redox reaction generates electron transport paths that give them conductivity values similar to those of semiconductors or semimetals, but only over a narrow potential window. Doping in these CPs has two forms: n-doped, in which the polymer is reduced and cations then enter it to balance the charge, and pdoped, in which the polymer is oxidized and anions then enter it to balance the charge.77,79−81 As long as they are separated by an insulating region in which no redox processes occur, ndoped and p-doped regions may exist separately within the same CP.27,77 Since the electrochemical window can be tuned by changing the heteroatom or substituents in the monomer structure,27,82 CPs have found uses in a variety of applications ranging from flexible electronics, sensors, electrocatalysis, and energy storage (Figure 4).5,27,58,77,83−87 CPs are compatible with RAP structures, and their integration offers exciting prospects for enhancing their reactivity by improving electronic interconnectivity. Recent examples include the wiring of redoxactive motifs in RAP layers that otherwise exhibit charge trapping, such as N,N,N’,N’-tetramethyl-p-phenylenediamine, or suboptimal reversibility, such as quinone groups.31,88−90 However, the broad applicability of these strategies for energy storage materials requires matching the window of conductivity of the CP with the redox potential of the RAP pendant and consequently knowledge of how to improve both components simultaneously.88−91 Strategies for enhancing charge transport within CPs can also be applied in RAPs.5,27,58,77,80,92−95 However, the use of CPs in conjunction with RAPs is not free from complications: many CPs contain rigid chains and conjugated coplanar π-systems, but these features so limit their solubility that the CP microstructure must be optimized to retain solvent accessibility. A very popular polymer blend that integrates the conducting polymer poly(3,4-ehyleneioxythiophene) (PEDOT) and polystyrene sulfonate (PSS) as a solubilizing ionomer (i.e., an ion conducting polymer), PEDOT:PSS, has been used extensively as a conveniently processable charge transport material (Figure 4a).96 The polymer preparation process gives control over the size and distribution of domains enriched with either PEDOT or PSS, which in turn affects electronic conductivity and ionic mobility within the final material. One way to adjust the domain sizes and charge transport pathways is to add a small fractional volume of cosolvent, e.g., ethylene glycol (Figure 4a). The movement of ions within PEDOT:PSS CPs is mostly controlled by the electrolyte accessibility and the domain sizes.96,154 Tailoring the chemical structure of the polymer blend at the nano- to mesoscale can lead to application-specific optimization of the transport of ions, electrons, and holes. Overall, these concepts resonate alike in CPs and RAPs, thus underscoring the broad impact of elucidating strategies that simultaneously enhance electronic and ionic transport. Solvation also plays an important role in unleashing the full potential of RAPs and the CPs wiring them. Recently, Dichtel et al. introduced a new redox-active β-ketoenamine-linked covalent organic framework (COF) that exhibits reversible electrochemical processes from its anthraquinone subunits.97 Although the COF exhibited excellent resistance to acidic degradation, the extremely low accessibility (2.5%) of the active material was a challenge.97,98 A stark contrast in accessibility was observed when replacing the acidic medium by acetonitrile, as 98% of the redox-active units were utilized when shifting to the organic solvent.98 Improvements were later sought using

3. STUDY OF ION REACTIVITY AT INTERPHASES The operation of battery electrodes involves a plethora of coupled ionic and redox processes that exhibit spatial, temporal, and chemical heterogeneity. Most modern LIBs utilize a graphitic anode material to accept guest Li ions during the charge process.84 In these materials, Li+ inserts reversibly into the galleries between graphene sheets.104−109 This process is referred to as “intercalation”, and its kinetics depend strongly on the balance between elastic strain caused by the deformation of the graphitic planes and the strong attractive electrostatic interactions between intercalated species.110 During intercalation, concurrent processes create interfacial structures between the electrolyte and the host. This solid electrolyte interphase (SEI) results from solvent and electrolyte breakdown reactions at the electrode surface, and its electrochemical and mechanical properties are integral to battery operation.111−113 The SEI characteristics depend not only on the electrode material (e.g., graphitic carbon, Sn, Li, LiCoO2, or LiFePO4) but also the cycling conditions.69 The electronic properties of the host material determine the capacity retention and stability of the SEI. Likewise, the ionic properties of the SEI determine performance parameters such as the maximum usable cycling rate and the required overpotential. Therefore, a comprehensive picture of the evolution of redox and ionic processes is key to creating superior batteries. 3.1. Scanned Probe Methods for the Characterization of Ionic Fluxes. The multifarious relationship between SEI properties and battery electrode performance has been investigated by a host of complementary techniques, including 8923


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Figure 5. Scanning probe methods for the detection of spatially heterogeneous redox and ionic processes at battery interfaces. (a) SECM maps of ferrocene (Fc) feedback indicating SEI reformation during lithiation of a Si thin film. Supplementary AFM data confirmed the structural changes observed by SECM. Reproduced from Ventosa et al. (ref 126)published by The Royal Society of Chemistry. (b) SICM data showing Cu film roughening with lithiation. Reproduced from Lipson et al. (ref 129) with permission. (c) SECCM data showing local charge/discharge of an individual active particle. Reproduced from Takahashi et al. (ref 132) with permission. (d) Hg-based SECM data showing colocalized electronic and ionic activity through oxidation of singly reduced ethyl viologen (left) and amalgamation of lithium (right), respectively. The substrate was an ∼120 μm Au electrode (indicated by a black dashed line) surrounded by PTFE, a proxy for battery materials. Reproduced from Barton and Rodrı ́guezLópez (ref 134) with permission. Copyright 2014 American Chemical Society.

conductivity.137,138 For example, SICM was first used to monitor the nanoscale roughening of a metal anode surface upon cycling (Figure 5b).129 In SECCM, the scanning probe is a quasi-reference counter electrode, not a working electrode, and electrolyte is only present in its pipette housing.139 The electric circuit for the cell is only complete when the probe is brought into such close proximity to a sample that the liquid meniscus protruding from the probe orifice makes contact with the substrate. The establishment and severance of electrical connection (as well as changes in conductivity) while repositioning the probe allows for rapid determination of topography without relying on the activation or kinetics of electrochemical processes.140 Scanning probe measurements are distance dependent, but each electrochemical measurement in SECCM is made with the same tip−substrate separation, so decoupled maps of topography and activity can be generated simultaneously (Figure 5c). And, because the electrochemical cell is limited to the footprint of the probe, spatially localized CVs and galvanostatic cycling measurements can be made with a large electroactive substrate serving as the sole working electrode. Though studies of electrochemical reactivity by SECCM have yet to be reported in nonaqueous conditions representing those found in commercial battery systems, this strategy of studying isolated reactive hot spots has the potential

FTIR,114 Raman spectroscopy,115,116 spectroscopic ellipsometry,117 X-ray techniques,118,119 TEM,120 electrochemical impedance spectroscopy (EIS),93 and electrochemical quartz crystal microbalance121 (EQCMB). These techniques provide access to a wealth of useful information but are unable to completely answer remaining unknown aspects of SEI growth and aging, such as spatiotemporal changes in ionic permeability and charge trapping during cycling. This is in part due to challenges not only in achieving sensitivity to local perturbations but also in deploying these techniques in situ. Consequently, present knowledge of heterogeneous ion transfer processes at dynamic SEIs is limited. We believe that questions regarding the evolution of redox and ionic aspects of interfacial reactivity are better addressed by emerging in situ scanning probe measurements, such as STM,122 SECM,123−128 scanning ion conductance microsopy (SICM),129−131 scanning electrochemical cell microscopy (SECCM),132,133 and Hg-based SECM,134 which access information both when and where it is needed (Figure 5).135,136 SECM typically assesses local redox reactivity through electrochemical feedback of a mediator and has been used to detect the healing of a mechanically damaged SEI during lithiation of a Si anode (Figure 5a).126 SICM is unable to investigate redox reactivity but instead accesses topographic information through chemically nonspecific changes in 8924


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Figure 6. Exploration of coupled electronic and ionic processes at interfaces. (a) Substrate CVs reveal the appearance of Li-ion intercalation stages at few-layer graphene (FLG) in response to increasing gallery number. CVs and related bulk electrochemical methods remain powerful tools for elucidating aspects of energy storage processes. Adapted from Hui et al. (ref 125) with permission. Copyright 2016 American Chemical Society. (b) SECM feedback maps over patterned multilayer graphene (MLG) demonstrate a loss of electrical conductivity associated with SEI formation at Li+ insertion potentials at graphitic interfaces. Adapted from Hui et al. (ref 125) with permission. Copyright 2016 American Chemical Society. (c) Hgbased SECM operating by the same principles of redox competition mode revealing the onset of ion uptake, distinguishing ionic processes from SEI aging seen in substrate CVs. Adapted from Barton and Rodrı ́guez-López (ref 142, copyright 2017 American Chemical Society, and Barton et al. (ref 143), copyright 2017 Elsevier, with permission. (d) Hg-based measurements over an electrified PNS film can access details of the interplay between electronic reactivity and the transport of multiple alkali ions, providing a deeper understanding of redox-active polymer film operation. Reproduced from Burgess et al. (ref 35)published by The Royal Society of Chemistry.

enabling the measurement of ionic reactivity in realistic alkali ion battery environments. When combined with substrate CV measurements, CV-SECM experiments performed with Hg disc-wells can isolate the contribution of ionic fluxes to the overall substrate response, discriminating between parasitic processes involved in SEI growth and ion staging processes involved in energy storage at graphitic materials. Hg disc-well probes are ideal for investigating graphitic energy storage materials, where the ionic permeability of the ever-changing SEI is absolutely central to the cycling performance yet poorly understood.143 3.2. Alkali Metal Intercalation into Multilayer Graphene and Polymers. To isolate specific interactions that are masked in ensemble measurements with disordered graphitic particles, much work has been done to investigate Li+ intercalation dynamics at ordered thin materials, such as highly oriented pyrolytic graphite (HOPG), multilayer graphene (MLG), and few-layer graphene (FLG).106,107,125,144 During intercalation reactions on these materials, enrichment of Li+ content within the graphitic host leads to island growth and migration between galleries. Thermodynamic and kinetic stabilization effects lead to well-defined transitions from 4, to 3, to 2, then to 1 graphitic plane(s) separating consecutive intercalant islands. This changing island plane separation is referred to as “staging”, with stage IV, III, II, and I indicating the number of intermediary graphitic planes between filled galleries, with larger numerals representing more dilute phases. Fewer electrostatic interactions between intercalating ions in adjacent galleries result in voltammetric peaks for diluted phases occurring at more positive potentials. We hypothesized that the number of these voltammetric features would be highly dependent on the number of graphene layers in ultrathin FLG electrodes. Slow-scan CVs at etched FLG revealed a profound impact on the intercalation process (Figure 6a), with monolayer graphene void of peaks due to the lack of galleries

to transform the analytical toolbox for addressing fundamental questions at operating energy storage material interfaces. While the electronic aspects of charge transfer are readily accessible by electroanalytical techniques, new strategies are required for understanding ionic processes within materials and across interfaces. Hg-based SECM excels in obtaining ionspecific information and isolating contributions to the formation and aging of the SEI from the overall response of a battery electrode material.132 This is achieved by electrochemical detection of alkali ion gradients through reversible amalgamation and stripping reactions at specific reacting sites. Because alkali-ion gradients are the motive force for alkali-ion battery operation, the ion-specific measurements obtainable by Hg-based probes offer a direct, unobscured view of localized battery material performance. Our laboratory first demonstrated the utility of these direct, linearly responsive signals at Hg sphere-cap ultramicroelectrodes (Figure 6a) over a battery material proxy.134 After demonstrating for the first time colocalized electronic and ion-specific signals in a nonaqueous system, we further improved the sensing platform by implementing CV-based strategies. By executing independent CV measurements between SECM motor movements, Hg-based probes can generate hyperdimensional CV-SECM data sets with kinetic, thermodynamic, temporal, and spatial information.141 Furthermore, this strategy of CV-SECM decouples measurements from probe movements, which all but eliminates the danger of signal distortions from forced convective transport during data collection. In order to capitalize on the ability to move the probe rapidly between measurements, we devised a reproducible protocol for fabricating a robust probe geometry: the Hg disc-well,142 which consists of a level pool of Hg confined to a glass-walled cavity (Figure 6c). Besides offering greater mechanical stability than Hg sphere-caps, Hg disc-well electrodes allow access to more concentrated alkali electrolytes, 8925


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species. To achieve this, new techniques that are capable of correlating the reactivities of coupled redox and ionic processes, quantitatively and with microscopic insight, are required. The dependencies between physical structure, ionic and electronic transport, and chemically specific interactions in energy storage materials can be studied by various electroanalytical techniques, but unique information may be gleaned through ion-sensitive scanned probe techniques. These have already been successfully deployed to study SEI growth and aging at operating battery materials, revealing spatiotemporal changes in ionic permeability and charge trapping during cycling. Hg-based SECM methods have also revealed how differential incorporation of cations in RAP films affects redox activity. Now that ion-sensitive techniques are emerging for correlating redox and ion processes in energy storage materials, we think the future will also see more complex coupling schemes that further incorporate structural information. Examples of these techniques will include Raman and infrared spectroscopy, transmission electron microscopy, and X-ray techniques.49,74,140−142,157,158 Taking Raman spectroscopy as an example, the capabilities of this technique have been used to study the staging mechanism of alkali ions into graphitic electrodes115,146,148 and the evolution of structural stress under operation-induced strain.147 Raman and microelectrode techniques have been used successfully to analyze dynamic processes on battery materials with micrometric resolution.159 Looking ahead on combining spectroscopy with redox and ion-sensitive scanned probe methods, a main challenge will be to match their spatial, temporal, and chemical resolution for generating quantitative models that simultaneously correlate current with spectral data at ever decreasing scales. Nanoelectrode SECM measurements and the coupling of Raman spectroscopy to SECM may be useful to explore ionic and redox processes with sub-micrometer resolution, making it useful to address heterogeneities that impact battery operation and the evolution of interfacial structures.11,142−145,149 The advent of superresolution probe spectroscopy150,152,160 and single-nanoparticle electrochemical161 interrogation suggest exciting directions for interrogating nanomaterials quantitatively. In combination with new nanofabrication techniques, coupled spectroelectrochemical approaches at the nanoscale will help elucidate ionic phenomena such as those described in this perspective for fewlayer graphene, at SEIs, or even at the atomic scale concerning ion insertion and (de-)solvation sites in diverse energy storage systems.153,162,163 We foresee that these techniques will continue to provide insights into charge mobility effects to overcome design challenges and for achieving superior energy storage materials.

and an increasing number of layers evincing more peaks with distinct features, progressively merging into bulk graphite-like behavior.125 Our results are consistent with the fact that Li+ diffusion in graphitic materials is anisotropic, exhibiting much greater mobility between graphene sheets than perpendicular to them,145 and also varies with the extent of intercalation due to attractive interactions between intercalated species.146 SECM feedback maps of surface redox reactivity confirmed the growth of the electrically resistive yet ionically conductive SEI over a patterned FLG substrate following Li+ intercalation and deintercalation (Figure 6b). Together with the alkali-ion measurement capabilities of Hg-disc wells (Figure 6c), these SECM strategies suggest exciting directions in the correlation of localized electronic and ionic properties during SEI formation on energy storage materials.119 The broad applicability of ion-sensitive probes for the study of battery materials is evidenced by its applicability to polymer electrodes. Previously described in Figure 3a, the electrolyte identity can affect the electrochemical performance of polymers in solution and as films (Figure 6d).35 In the case of a PNS film deposited on an electrode, the reduction process is followed by an influx of ions into the film. As shown in Figure 6d, a Hg probe positioned close to a PNS film shows a decrease in stripping current in response to increasingly negative substrate potentials. This behavior is expected as the film competes with the Hg probe for the same alkali-ion supply, thus showing that the local ion concentration decreases as a result of PNS reduction. This technique is particularly useful for studying the flux of ions into polymers since the stripping signal can be correlated to any charge trapping effects.35,134,135 Furthermore, making use of its potential-resolved selectivity toward alkali ions, these measurements demonstrate that differences in reactivity in the PNS films are not due to exclusion of either ion from the reacting film. We foresee that the use of scanning electrochemical probe techniques will enable an unprecedented understanding of the delicate balance of ionic and electronic reactivity required for better performing energy storage materials.

4. OUTLOOK In this perspective, we have discussed the importance of elucidating the impact that redox, electronic, and ionic factors have in the performance of materials for energy storage. We have highlighted intersecting trends found among different classes of redox and conductive polymers and ion intercalation materials and their interfaces. In all these cases, charge mobility is a deciding factor in material performance. For the case of soluble RAPs, the geometry, structure (interpendant distance), electrolyte identity and concentration, and solvent choice modulate charge accessibility and mobility. We have discussed examples in which CPs can enhance charge mobility, leading to improved rate capability and charge capacity. Designing experiments to optimize ion or electron transfer parameters is necessary to pursue increased materials performance, but the strategies for enhancing redox/electronic and ionic mobility do not always coincide. We highlighted parallelisms between materials where densely packed redox centers and tight ion pairs led to decreased performance instead of increased energy density. On the other hand, we also discussed cases where unexpected synergies between electrostatic interactions and host structure resulted in more attractive redox potentials for nanoconfined RAPs. An underlying topic in this perspective is the need to view ions beyond the role of mere spectator

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AUTHOR INFORMATION

Corresponding Author

*(J.R.-L.) E-mail: [email protected]. Phone: 217-300-2754. ORCID

Kenneth Hernández-Burgos: 0000-0002-4644-784X Zachary J. Barton: 0000-0001-6856-7634 Joaquín Rodríguez-López: 0000-0003-4346-4668 Notes

The authors declare no competing financial interest. Biographies Kenneth Hernández-Burgos earned a bachelor’s degree in chemistry from the University of Puerto Rico at Rı ́o Piedras (UPR-RP) in 2010. During his time at UPR-RP, he was a Minority Access for Research 8926


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(7) Burgess, M.; Moore, J. S.; Rodríguez-López, J. Redox Active Polymers as Soluble Nanomaterials for Energy Storage. Acc. Chem. Res. 2016, 49 (11), 2649−2657. (8) Xue, Z.; He, D.; Xie, X. Poly(ethylene oxide)-Based Electrolytes for Lithium-Ion Batteries. J. Mater. Chem. A 2015, 3 (38), 19218− 19253. (9) Golodnitsky, D.; Strauss, E.; Peled, E.; Greenbaum, S. Review On Order and Disorder in Polymer Electrolytes. J. Electrochem. Soc. 2015, 162 (14), A2551−A2566. (10) Verma, P.; Maire, P.; Novák, P. A Review of the Features and Analyses of the Solid Electrolyte Interphase in Li-Ion Batteries. Electrochim. Acta 2010, 55 (22), 6332−6341. (11) Shapiro, D. A.; Yu, Y.-S.; Tyliszczak, T.; Cabana, J.; Celestre, R.; Chao, W.; Kaznatcheev, K.; Kilcoyne, A. L. D.; Maia, F.; Marchesini, S.; Meng, Y. S.; Warwick, T.; Yang, L. L.; Padmore, H. A. Chemical Composition Mapping with Nanometre Resolution by Soft X-Ray Microscopy. Nat. Photonics 2014, 8 (10), 765−769. (12) De Jesus, L. R.; Horrocks, G. A.; Liang, Y.; Parija, A.; Jaye, C.; Wangoh, L.; Wang, J.; Fischer, D. A.; Piper, L. F. J.; Prendergast, D.; Banerjee, S. Mapping Polaronic States and Lithiation Gradients in Individual V2O5 Nanowires. Nat. Commun. 2016, 7, 12022−10031. (13) Harris, S. J.; Lu, P. Effects of InhomogeneitiesNanoscale to Mesoscaleon the Durability of Li-Ion Batteries. J. Phys. Chem. C 2013, 117 (13), 6481−6492. (14) Rolison, D. R. Catalytic Nanoarchitecturesthe Importance of Nothing and the Unimportance of Periodicity. Science 2003, 299 (5613), 1698−1701. (15) MacKinnon, R.; Cohen, S. L.; Kuo, A.; Lee, A.; Chait, B. T. Structural Conservation in Prokaryotic and Eukaryotic Potassium Channels. Science 1998, 280 (5360), 106−109. (16) Burgess, M.; Chenard, E.; Hernandez-Burgos, K.; Nagarjuna, G.; Assary, R. S.; Hui, J.; Moore, J. S.; Rodrı ́guez-López, J. Impact of Backbone Tether Length and Structure on the Electrochemical Performance of Viologen Redox Active Polymers. Chem. Mater. 2016, 28 (20), 7362−7374. (17) Burgess, M.; Hernández-Burgos, K.; Simpson, B. H.; Lichtenstein, T.; Avetian, S.; Nagarjuna, G.; Cheng, K. J.; Moore, J. S.; Rodríguez-López, J. Scanning Electrochemical Microscopy and Hydrodynamic Voltammetry Investigation of Charge Transfer Mechanisms on Redox Active Polymers. J. Electrochem. Soc. 2016, 163 (4), H3006−H3013. (18) Dobrynin, A. V.; Rubinstein, M. Theory of Polyelectrolytes in Solutions and at Surfaces. Prog. Polym. Sci. 2005, 30 (11), 1049−1118. (19) Xie, F.; Lu, H.; Nylander, T.; Wågberg, L.; Forsman, J. Theoretical and Experimental Investigations of Polyelectrolyte Adsorption Dependence on Molecular Weight. Langmuir 2016, 32 (23), 5721−5730. (20) Scheutjens, J. M. H. M.; Fleer, G. J. Statistical Theory of the Adsorption of Interacting Chain Molecules. 1. Partition Function, Segment Density Distribution, and Adsorption Isotherms. J. Phys. Chem. 1979, 83 (12), 1619−1635. (21) Szilagyi, I.; Trefalt, G.; Tiraferri, A.; Maroni, P.; Borkovec, M. Polyelectrolyte Adsorption, Interparticle Forces, and Colloidal Aggregation. Soft Matter 2014, 10 (15), 2479−2502. (22) Dalton, E. F.; Murray, R. W. Viologen(2+/1+) and Viologen(1+/0) Electron-Self-Exchange Reactions in a Redox Polymer. J. Phys. Chem. 1991, 95 (16), 6383−6389. (23) Facci, J.; Murray, R. W. Kinetics of Charge Transport through Polycationic Polymer Films on Electrodes Containing Various Concentrations of Fe(CN)63− and of IrCl63−. J. Electroanal. Chem. Interfacial Electrochem. 1981, 124 (1), 339−343. (24) Blauch, D. N.; Saveant, J. M. Dynamics of Electron Hopping in Assemblies of Redox Centers - Percolation and Diffusion. J. Am. Chem. Soc. 1992, 114 (9), 3323−3332. (25) Buttry, D. A.; Saveant, J. M.; Anson, F. C. Enhancement of Charge-Transport Rates by Redox Cross-Reactions Between Reactants Incorporated in Nafion Coatings. J. Phys. Chem. 1984, 88 (14), 3086− 3091.

Careers fellow and worked under the supervision of Prof. Ana R. Guadalupe. He earned his Ph.D. in the department of chemistry and chemical biology at Cornell University in Prof. Héctor D. Abruña’s lab in which he worked on the design and characterization of organic electrode materials for electrical energy storage applications. As a postdoctoral fellow in the Rodrı ́guez-López lab, he works in the characterization of redox-active polymers. Zachary J. Barton obtained a bachelor’s degree in biochemistry from Wheaton College in Illinois, where he studied the dynamics of αhemolysin pore formation through fluorescence cross-correlation spectroscopy (FCCS) under the supervision of Prof. Daniel Burden. He is presently a graduate fellow of the National Science Foundation in the Rodrı ́guez-López laboratory at the University of Illinois at Urbana−Champaign, where he develops new electroanalytical tools based on scanning electrochemical microscopy (SECM) to map ionic gradients with chemical specificity over energy storage materials. Joaquı ́n Rodrı ́guez-Loṕ ez is originally from Mexico, where he obtained a degree in chemistry from Tecnológico de Monterrey working under the supervision of Prof. Marcelo Videa. He obtained a Ph.D. at the University of Texas at Austin with Prof. Allen J. Bard and did a postdoctoral stay with Prof. Héctor D. Abruña at Cornell University. His group in the chemistry department at the University of Illinois at Urbana−Champaign advances electrochemical characterization strategies and imaging methods for novel energy nanomaterials.

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ACKNOWLEDGMENTS K.H.-B. and J.R.-L. acknowledge financial support from the Joint Center for Energy Storage Research (JCESR), an energy innovation hub funded by the Department of Energy, Office of Science, Basic Energy Sciences. K.H.-B. gratefully acknowledges the Beckman Institute Postdoctoral Fellowship at the University of Illinois at Urbana−Champaign and the additional funding provided by the Arnold and Mabel Beckman Foundation. Z.J.B. acknowledges the support of the National Science Foundation Graduate Research Fellowship Program (DGE-1144245). Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation. J.R.-L. acknowledges additional support from Alfred P. Sloan Foundation Fellowship. The authors thank Elena C. Montoto and Luxi Shen for assistance with editing.

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