Session W47: Focus Session: Polymers for Energy Storage and Conversion - Fundamentals of Ion Transport

Decoupling of Ionic Transport from Segmental Relaxation in Polymer Electrolytes

Polymer electrolytes provide elegant solutions to many difficulties in battery technology. However, their relatively low ionic conductivity has become the bottleneck for developing batteries with higher power density, shorter charging time, and better operations at low temperatures. In this work, we present detailed studies of the relationship between ionic conductivity and segmental relaxation in a set of specially-designed polymer electrolytes with systematic variation in chain rigidity. Our analysis shows that the ionic conductivity indeed can be decoupled from segmental dynamics in rigid polymers and the strength of the decoupling correlates with the fragility, but not with the glass transition temperature. These results call for a revision of the current picture of ionic transport in polymer electrolytes. We relate the observed decoupling phenomenon to frustration in packing of rigid polymers, which also affects their fragility. The principles demonstrated in this study may provide an alternative approach to design of highly conductive materials: by incorporating relatively rigid chain structures, it is possible to develop a new class of solid polymer electrolytes with strongly decoupled ionic conductivity.   

Ion Dynamics in Solid-State Polymer Electrolyte Electrochemical Cells using \textit{in situ} Time-Resolved Infrared Spectroscopy

Understanding ion transport in solid-state polymer electrochemical cells is of great interest for the advancement of cell efficacy. However, currently there is limited experimental knowledge of ion transport on a molecular level. In this study, we report a new spectroelectrochemical experimental technique that provides \textit{in situ} molecular level detail about cation and anion transport of an ionic liquid in solid-state polymer electrolyte electrochemical cells. In situ time-resolved Fourier transform infrared attenuated total reflectance (FTIR-ATR) spectroscopy was utilized to measure the time dependent accumulation of ions at the cathode and anode interface under an applied potential. The results show that the cation and anion of the ionic liquid both transport and accumulate at the cathode under dry conditions, but only the cation accumulates at the cathode under humid conditions. This experiment was coupled with electrochemical impedance spectroscopy to simultaneously measure overall charge transport and cyclic voltammograms.   

Ion Transport in Polymerized Ionic Liquid Block and Random Copolymers

Polymerized ionic liquid (PIL) block copolymers, a new type of solid-state polymer electrolyte, are of interest for energy conversion and storage devices, such as fuel cells, batteries, supercapacitors, and solar cells. In this study, a series of PIL diblock and random copolymers with various PIL compositions were synthesized. These consisted of an IL monomer and a non-ionic monomer, 1-[(2-methacryloyloxy)ethyl]-3-butylimidazolium bis(trifluoromethanesulfonyl)imide (MEBIm-TFSI) and methyl methacrylate (MMA), and 1-[(2-acryloyloxy)ethyl]-3-butylimidazolium bis(trifluoromethanesulfonyl)imide (AEBIm-TFSI) and styrene (S), respectively, were synthesized. The anion conductivity (ion transport) and morphology were measured in all of the polymers with EIS, SAXS/WAXS, and TEM. Ion transport in block copolymers are significantly higher than random copolymers at the same PIL composition and are highly dependent on the block copolymer nanostructure. The relationship between ion transport mechanisms and the phase behavior of these materials will be discussed.   

Ion conduction in phosphonium-polysiloxane ionomers

Low Tg ionomers with phosphonium cations covalently attached as side chains have potential application in energy conversion and storage devices. For example, alkaline fuel cells rely on membranes that transport hydroxide anions and some advanced batteries rely on membranes transporting fluoride anions. To better understand ion conduction in phosphonium-polysiloxane ionomers, allyl tributyl phosphonium bromide monomer was synthesized and, along with a vinyl ethylene oxide monomer, attached to polymethylhydrosiloxane by hydrosilylation. These ionomers maintain low Tg $\approx $ -74 $^{\circ}$C with up to 10 mol{\%} phosphonium and are fully water soluble, allowing easy anion exchange and purification. We report dielectric spectroscopy results for these ionomers with a variety of counter-anions. Electrode polarization at low frequencies is analyzed to determine the number density of simultaneously conducting counter ions and their mobility. This analysis reveals higher mobility and lower activation energy for conducting anions that are larger and more diffuse, such as bis(trifluoromethane sulfonyl)imide, contributing to better performance as anion-conducting membranes.   

Ion Conduction in Polymerized Ionic Liquid Thin Films

The ion conductivity in thin films is typically assumed to be isotropic. We have developed methods to measure in-plane and through-plane ionic conductivity in thin homopolymer and block copolymers. Specifically, we are studying the conductivity in imidazolium-containing polymerized ionic liquids as a function of film thickness. These data will be compared to conductivity measurements in a triblock copolymer with a polymerized ionic liquid midblock.   

Cross-linking of Ordered Pluronic/Ionic Liquid Blends for Solid Polymer Electrolytes

Ion gels were fabricated by cross-linking PPO-PEO-PPO triblock copolymers swollen in a room temperature ionic liquid (IL). The copolymers are modified by esterification to replace the terminal hydroxyl endgroups with methacrylate endgroups. This allows the copolymer/IL blends to be cross-linked by a UV cure, forming a gel. The strong interaction of the IL with the PEO block suppresses PEO crystallization which is necessary for good ion conduction. In addition, the interaction between the IL and PEO is strongly selective for PEO, strengthening microphase separation. Despite this, the low molecular weight copolymers remain disordered in the melt even when blended with the IL. However, high molecular weight copolymers are capable of microphase separating into highly ordered block copolymer morphologies. This difference allows the effect of microphase separation on ion transport to be studied. The effect of block copolymer composition is also studied, by varying the PEO fraction of the copolymer. The resultant gels show high ionic conductivity and solid-like behavior, indicating that these materials may be effective as solid polymer electrolytes.   

Ionomer Design Principles for Single Ion-Conducting Energy Materials

Single-ion conducting ionomers with low glass transition temperature, high dielectric constant and containing bulky ions with diffuse charge, are needed for polymer membranes that transport small counterions. Overarching design principles emerging from quantum chemistry calculations suggest that diffuse charge can be attained from simple considerations of atomic electronegativity. For lithium or sodium batteries, perfluorinated tetraphenyl borate ionomers with solvating polar comonomers are proposed. For fluoride or hydroxide batteries and for iodide transporting solar cells, tetra-alkyl phosphonium ionomers with anion receptors are proposed. First attempts to construct such ionomers to test these ideas will be discussed, with results from dielectric spectroscopy to measure conductivity, dielectric constant and number density of simultaneously conducting ions.   

Ion Dynamics in Model Ionomer Melts as a Function of Polymer Architecture

Ionomers, polymers with a small fraction of covalently bound ionic groups, have potential advantages as solid, single ion conducting electrolytes in future batteries. However, the strong electrostatic interactions in these materials can make counterion diffusion unacceptably slow. Understanding how controllable molecular properties affect ionomer dynamics could spur design of improved materials. With this goal, we perform molecular dynamics simulations of ionomers of various architectures and evaluate their dynamic behavior. Our model of coarse-grained polymers with explicit counterions captures the fundamental physics while allowing access to the long time and length scales relevant to ionomer melts. The simulated structure factors reproduce the trends found in experimental scattering of recently synthesized ionomers with controlled precise or pseudorandom spacing of charged groups along the chain. We calculate counterion diffusion constants and other dynamic properties, which can change significantly depending on the location and spacing of charges along the chain. Randomly spaced materials can have slower or faster dynamics than periodically spaced materials, depending on whether the sequence is completely random or pseudorandom, which will be discussed.   

Ion Conduction and Dielectric Response of Imidazolium-based Single-ion Conductors

We synthesized ionomers with imidazolium cations covalently attached as side groups with various ionic liquid counter-anions. Since these ionic polymers are single-ion conductors that are potentially useful for ionic actuators, it is of great interest to understand structure-property relations, such as the effect of different counterions and different imidazolium pendant structures, including tail and side chain lengths. Conductivities and dielectric properties of a range of monomers and polymers containing ionic liquid moieties are compared. The effects of counterions and side chain length are clearly observed in the T$_{g}$ and ionic conductivity: larger anions and/or longer side chains lead to lower T$_{g}$ and higher conductivity than smaller anions and/or shorter side chains. However, if the tail becomes too long (12 carbons) it facilitates - ion aggregation with a significantly lower dielectric constant and lower mobility for the conducting ions. Our study of counter-anions and polymer structural variations leads to insight regarding optimal design of imidazolium single-ion conductors for facile ion transport.   

Effect of matrix crystallinity on the ionic conductivity in microstructured block copolymer solid electrolytes

Polyethylene oxide (PEO)-based block copolymers have been studied extensively for use as solid electrolytes for rechargeable lithium metal batteries. Previous work has concentrated on block copolymers containing an amorphous second block, such as polystyrene, for which the modulus is sufficiently high to resist growth of dendrites that would lead to short circuiting. In this work, we instead focus on using semicrystalline polyethylene as the mechanically robust component. Polyethylene-polyethylene oxide (EEO) block copolymers doped with lithium bis(trifluoromethanesulfone) imide (LiTFSI) were characterized using AC impedance spectroscopy over a range of temperature, molecular weight, and composition values in order to determine the effect of crystallinity in the structural microphase on the conductivity of this material.   

Solvation Effects on Counterion Transport in Single-Ion Conducting Ionomers

Ionomers with short ethylene oxide side chains are synthesized by free radical polymerization, to systematically test effects of solvating anion and cation, including directly substituting the ion attached to the polymer with its counterion. Dielectric relaxation spectroscopy is used to measure the conductivity, dielectric constant and segmental relaxations in these ionomers and the electrode polarization at very low frequencies is used to assess the number density of simultaneously conducting ions and their mobility. Conductivity and conducting ion content are larger for polyanions than their corresponding polycation because the counterion can be more effectively solvated by the ether oxygens. Changing ester linkages to amide linkages in polycations boosts conductivity and conducting ion content by solvating the anionic counterion. Such findings point a clear path toward design of superior single-ion conductors.   

Correlation between superionic behavior and ion aggregation in PEO-based single-ion conductors

PEO-based ionomers as solid polymer electrolytes offer the advantage of preventing reverse polarization in batteries by covalently bonding the anion to the PEO backbone. These ionomers form ion aggregates, which reduces the polymer mobility. Since ion transport is coupled to polymer dynamics, these systems have low conductivity. We therefore need a mechanism that decouples ion conduction and PEO dynamics to improve conductivity. We investigate these conduction mechanisms using different models in MD simulations of PEO based benzene sulfonate ionomers. The study shows that these ionomers are capable of showing superionic behavior. The geometry of benzene rings helps aligning the anions, assisting in the formation of chain-like aggregates. The simulations show that the chain-like aggregates result in ionomer conductivity greater than its self-diffusion limits. The superionic behavior is attributed to a charge transfer between two chain ends (conduction sites): a cation hopping to one chain end and the cation at the other end hopping to a nearby site. This allows long range positive charge transfer while the cations only move locally. The results suggest that the superionic behavior depends on the length and lifetime of the chain aggregates. While a long chain reduces the overall number of conduction sites, a short chain prevents long range charge transfer. If the lifetime of an aggregate is shorter than the hopping time for cations, the hopping will not occur and the self-diffusion dominates conductivity.   

Simulation of Ionic Aggregation and Ion Dynamics in Model Ionomers

Ionomers, polymers containing a small fraction of covalently bound ionic groups, are of interest as possible electrolytes in batteries. A single-ion conducting polymer electrolyte would be safer and have higher efficiency than the currently-used liquid electrolytes. However, to date ionomeric materials do not have sufficiently high conductivities for practical application. This is most likely because the ions tend to form aggregates, leading to slow ion transport. A key question is therefore how molecular structure affects the ionic aggregation and ion dynamics. To probe these structure-property relationships, we have performed molecular simulations of a set of recently synthesized poly(ethylene-co-acrylic acid) copolymers and ionomers, with a focus on the morphology of the ionic aggregates. The ionomers have a precise, constant spacing of charged groups, making them ideal for direct comparisons with simulations. {\em Ab initio} calculations give insight into the expected coordination of cations with fragments of the ionomers. All-atom molecular dynamics (MD) simulations of the ionomer melt show aggregation of the ionic groups into extended string-like clusters. An extensive set of coarse-grained molecular dynamics simulations extend the results to longer times and larger length scales. The structure factors calculated from the MD simulations compare favorably with x-ray scattering data. Furthermore, the simulations give a detailed picture of the sizes, shapes, and composition of the ionic aggregates, and how they depend on polymer architecture. Implications for ion transport will be discussed. [Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.]