Session Q21: Focus Session: Polymers for Energy Storage and Conversion III - Ion Transport

Probing Structural Changes in Poly(3-hexylthiophene) (P3HT) During Electrochemical Oxidation with In Situ X-ray Scattering

Mixtures of poly(3-hexylthiophene)-b-poly(ethylene oxide) (P3HT-b-PEO) block copolymer and lithium bis(trifluromethanesulfonyl) imide (LiTFSI) salt can microphase separate into electron (P3HT) and ion (PEO/LiTFSI) conducting domains. P3HT is a semicrystalline polymer with intrinsically semiconducting electronic properties. Electrochemical oxidation (doping) of the P3HT block provides the P3HT-b-PEO/LiTFSI mixtures with electronic conductivity suitable for lithium battery operation[1][2]. Due to the presence of the solid-state electrolyte (PEO/LiTFSI) in intimate contact with the microphase separated P3HT domains, electrochemical oxidation of P3HT can be performed entirely in the solid state; therefore, P3HT-b-PEO/LiTFSI provides a unique opportunity to study the structural changes in P3HT induced by oxidation. We use in situ x-ray scattering techniques to probe structural changes in P3HT during electrochemical oxidation and correlate these changes with previously observed enhancements in electron mobility[2]. [1] Javier, A. E., Patel, S. N., Hallinan, D. T., Srinivasan, V., Balsara, N. P., Angew. Chem. Int. Ed. Engl., 50, 9848-51 (2011). [2] Patel, S. N., Javier, A. E., Balsara, N. P. ACS Nano, 7, 6056-6068 (2013).   

Design of superionic polymers for energy storage applications

Replacing traditional liquid electrolytes by polymers will significantly improve electrical energy storage technologies. Despite the significant advantages for applications in electrochemical devices, the use of solid polymer electrolytes has been impeded by their poor ionic conductivity. By analyzing the relationship between ionic conductivity and segmental relaxation, we demonstrate that polyether-based solid electrolytes have intrinsic limitations for ionic transport at ambient and low temperatures, due to strongly coupled segmental and ion dynamics. On the other hand, the ionic conductivity in rigid polymer can be strongly decoupled from segmental relaxation, in terms of both temperature dependence and relative transport rate, thus providing a significant advantage over traditional polyether-based electrolytes. Our analysis emphasizes that decoupling of ionic transport from segmental dynamics is the key for macromolecular design of superionic polymers.   

Effects of Confinement and Interface on Ion Transport Properties of Block Copolymer Electrolytes

There is growing interest in blending polymers and ionic liquids (ILs) as a simple route to obtain solid-state ion conductors for a wide variety of electrochemical devices such as lithium batteries and fuel cells. Since the ion transport and mechanical properties of these materials are generally coupled, the development of IL-impregnated polymer electrolytes with improved ionic conductivity and optimized mechanical stability has lately been the subject of extensive studies employing diverse combinations of polymers and ILs. In this work, we present fascinating experimental insights into confinement- and interface-driven modulation of ion transport properties for block copolymer electrolytes. By varying the type of ILs, qualitatively similar lamellar morphology was identified, however, the highest conductivity was only achieved when ILs were confined within ionic domains with a sharp interface. In contrast, a high degree of intermixing of ionic and non-ionic domains at the interface resulted in a reduction by one order of magnitude in the conductivity owing to the creation of tortuous ion conduction pathways. This work suggests the future prospects for designing desired nanostructures as efficient ion conductors.   

Using Tapered Block Copolymers to Create Conducting Nanomaterials

Soft materials, such as polymers, colloids, surfactants, and liquid crystals, are a technologically important class of matter employed in a variety of applications. One sub-class of soft material, \textbf{block copolymers}, provides the opportunity to design materials with attractive chemical and mechanical properties based on the ability to assemble into periodic structures with nanoscale domain spacings. Several applications for block copolymers currently under investigation in my group include battery and fuel cell membranes, analytical separations membranes, nano-tool templates, precursors to electronic arrays, and drug delivery vehicles. One area of recent progress in the group focuses on the behavior of conventional block copolymer and tapered block copolymer systems for lithium battery membrane applications. We find that we can tune poly(styrene-$b$-ethylene oxide) diblock copolymer nanostructures by adjusting the lithium counterion and lithium salt concentration, as well as the taper volume fraction and composition. Additionally, we can estimate the effective interaction parameters ($\chi_{\mathrm{eff}})$ for the salt-doped copolymers to determine the overall influence of tapering on the energetics of copolymer assembly. These tapered materials allow us to design nanostructured membrane systems with increased conductivity and improved mechanical properties in ion transport devices.   

Mechanisms Underlying Ionic Mobilities in Nanocomposite Polymer Electrolytes

Recently, a number of experiments have demonstrated that addition of ceramics with nanoscale dimensions can lead to substantial improvements in the low temperature conductivity of the polymeric materials. However, the origin of such behaviors, and more generally, the manner by which nanoscale fillers impact the ion mobilities remain unresolved. In this communication, we report the results of atomistic molecular dynamics simulations which used multibody polarizable force-fields to study lithium ion diffusivities in an amorphous poly(ethylene-oxide) (PEO) melt containing well-dispersed TiO$_{2} $ nanoparticles. We observed that the lithium ion diffusivities decrease with increased particle loading. Our analysis suggests that the ion mobilities are correlated to the nanoparticle-induced changes in the polymer segmental dynamics. Interestingly, the changes in polymer segmental dynamics were seen to be related to the nanoparticle's influence on the polymer conformational features. Overall, our results indicate that addition of nanoparticle fillers modify polymer conformations and the polymer segmental dynamics, and thereby influence the ion mobilities of polymer electrolytes.   

Characterization of PEO-b-P(STFSILi) as a Single-Ion Block Copolymer Electrolyte for Lithium Batteries

Block copolymers containing a poly(ethylene oxide) (PEO) ion-conducting block and a polystyrene (PS) structural block mixed with lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) salt have been studied in the past as solid electrolytes for lithium batteries. However, transport of these ionic species result in concentration gradients during battery operation, and the energy expended by this process is inefficient. In other words, these electrolyte systems have low cation transference numbers. A single-ion block copolymer electrolyte has been synthesized where the TFSI anion of LiTFSI is covalently bound to the PS backbone. Li can dissociate from the immobilized anion, enabling the conduction of Li ions with a theoretical transference number of unity. AC impedance spectroscopy and small angle X-ray scattering are used to determine charge transport and morphological properties of these PEO-b-P(STFSILi) block copolymers over a range of molecular weights.   

Morphology and Ionic Associations in Polyphosphazene Ionomers

Polyphosphazene ionomers with short chain poly(ethylene oxide) (PEO) moieties, bound ammonium cations, and free iodide anions were previously synthesized and their conductivity was studied through dielectric relaxation spectroscopy (DRS). Polyphosphazenes provide interesting conductive materials to study because of their low glass transition temperature and unique inorganic backbone. Non-ionic (poly[bis(methoxyethoxy)phosphazene] (MEEP) and two high ion content phosphazene ionomers were studied by multiple angle X-ray scattering (MAXS). Room temperature scattering shows the polymers are completely amorphous. Two peaks are observed in non-ionic MEEP and correspond to the phosphazene backbone-backbone spacing and to the amorphous halo of PEO. When longer ion-containing pendants are incorporated, an increase in the spacing is observed. A third peak is observed in the ionic systems and is interpreted as the average spacing between ions. The average ion separation closely matches the spacing of monomers and suggests the ions are not aggregated but stay in isolated pairs even at high ion content. Although there is no observable ionic aggregation, the conducting ion concentration remains low due to strong cation-anion associations.   

Dynamics and morphologies of course-grained ionomer melts under an external electric field

Ionomers have been identified as potential solid electrolytes in battery applications. However, these systems are hindered by strong electrostatic interactions that can lead to ionic aggregation, making ion diffusion very slow. To develop a molecular understanding of how the ion transport depends on the system morphology and ultimately the ionomer chemistry, we perform molecular dynamics simulations. We apply an external electric field to a melt of course-grained polymers with charged groups along or pendant to the backbone, and explicit counterions. We observe ionic aggregate morphologies that merge and whose shape anisotropy, depending on the strength of the field, may decrease or increase (along the field direction). We also quantify the dependence of the drift mobility of the ions on the aggregate morphologies and the field strength.   

Ion distributions in electrolyte confined by multiple dielectric interfaces

The distribution of ions at dielectric interfaces between liquids characterized by different dielectric permittivities is crucial to nanoscale assembly processes in many biological and synthetic materials such as cell membranes, colloids and oil-water emulsions. The knowledge of ionic structure of these systems is also exploited in energy storage devices such as double-layer super-capacitors. The presence of multiple dielectric interfaces often complicates computing the desired ionic distributions via simulations or theory. Here, we use coarse-grained models to compute the ionic distributions in a system of electrolyte confined by two planar dielectric interfaces using Car-Parrinello molecular dynamics simulations and liquid state theory. We compute the density profiles for various electrolyte concentrations, stoichiometric ratios and dielectric contrasts. The explanations for the trends in these profiles and discuss their effects on the behavior of the confined charged fluid are also presented.   

How does crystalline structure affect ionic conductivity in solid polymer electrolyte?

Solid polymer electrolytes (SPEs) have drawn intensive attention due to their potential applications in all-solid-state lithium batteries. Ion conduction in this system is generally considered to be confined in the amorphous polymer/ion phase and through segmental motion assisted hopping. In poly(ethylene oxide) (PEO) based SPEs, the crystalline nature of the polymer complicates the ion transport behavior. Herein we quantitatively show that the effect of polymer crystallinity on ion transport is two-fold: one is structural (tortuosity) and the other is dynamic (tethered chain confinement). We decouple these two effects by designing, and fabricating a model polymer single crystal electrolyte system with controlled crystal structure, size, crystallinity and orientation. The tortuosity effect results in a high conductivity anisotropy (10$^{2}$ -10$^{3})$ from directions parallel and transverse to PEO crystal lamellae. On the other hand, the dynamic effect is negligible at relatively high ion content, suggesting that semicrystalline polymer is a valid system for practical polymer electrolytes design.   

Decoupling of ionic conductivity from structural dynamics in polymerized ionic liquids

Charge transport and structural dynamics in low molecular weight and polymerized 1-vinyl-3-pentylimidazolium bis(trifluoromethylsulfonyl)imide ionic liquids (ILs) are investigated by a combination of broadband dielectric spectroscopy, dynamic mechanical spectroscopy and differential scanning calorimetry. While the dc conductivity and fluidity exhibit practically identical temperature dependence for the non-polymerized IL, a significant decoupling of ionic conduction from structural dynamics is observed for the polymerized IL. In addition, the dc conductivity of the polymerized IL is found exceed that of its molecular counterpart by four orders of magnitude at their respective calorimetric glass transition temperatures. This is attributed to the unusually high mobility of the anions even at lower temperatures when the structural dynamics is significantly slowed down. A simple physical explanation of the possible origin of the remarkable decoupling of ionic conductivity from structural dynamics is proposed.   

Conductivity and Stability of Photopolymerized Polymer Electrolyte Network

A melt-processing window has been identified within the wide isotropic region of the phase diagram of ternary blends consisting of poly (ethylene glycol diacrylate) (PEGDA), tetraethylene glycol dimethyl ether (TEGDME) and lithium bis(trifluoromethane) sulfonamide (LiTFSI). Upon UV-crosslinking of PEGDA in the isotropic window, the polymer electrolyte membrane (PEM) network thus formed is completely transparent and remains in the single phase without undergoing polymerization-induced phase separation or polymerization-induced crystallization. These PEM networks are solid albeit flexible and light-weight with safety and space saving attributes. The ionic conductivity as determined by AC impedance spectroscopy exhibited very high room-temperature ionic conductivity on the order of $\sim$10$^{-3}$ S/cm in several compositions, viz., 10/45/45, 20/40/40 and 30/35/35 PEGDA/TEGDME/LiTFSI networks. Cyclic voltammetry measurement of these solid-state PEM networks revealed excellent electrochemical stability against lithium reference electrode. The above study has been extended to the anode (graphite) and cathode (LiFePO$_{4}$) half-cell configurations with lithium as counter electrode. Charge/discharge cycling behavior of these half cells will be discussed.   

Holographic polymerization for highly conductive robust electrolyte membranes

The roles of nanostructure and confinement for ion mass transport in polymer electrochemical applications are key for improving the diffusion characteristics and mechanical robustness of solid electrolyte membranes. The challenges in fabricating highly controlled model systems are largely responsible for the interdependent ambiguities between nanostructures and the corresponding ion transport behavior. In this work, holographic polymer electrolyte membrane volume gratings comprised of alternating layers of robust cross-linked polymer resin and electrolyte, with an average d-spacing of 180 nm, were fabricated using holographic polymerization. These one-dimensional confinement structures were used to quantitatively study the anisotropic ionic conductivity properties and correlate this behavior to nano-confinement and phase mixing. Anisotropies greater than 5000 have been observed, and conductivities approaching 10$^{\mathrm{-4}}$ S/cm in robust freestanding films. In this case, the cross-linked resin serves as both load-bearing scaffold layers and as an electrolyte blending agent. These membranes serve as a platform in next generation nanostructured blend systems with enhanced mechanical properties for electrochemical applications.