Understanding ion transport in comb-branched polymer electrolytes to optimize electrochemical properties



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Single-ion conducting polymer electrolytes (SICPEs) offer high lithium transference numbers and desirable physical properties while maintaining moderate conductivities. Bottlebrush and comb-branched copolymer electrolytes are a particular architecture that offer modularity and increased ion solvation. Despite this promise, ion transport in these systems is poorly understood. In this body of work, my collaborators and I leverage a highly tunable synthetic system to create SICPEs with varying anionic functionalities and compositions with molecular dynamics simulations to understand the nature of ion transport in these architectures. Differences in activation energy, ion-pair lifetimes and ion diffusivities show that lithium ions diffusion shifts from a vehicular mechanism to an ion-hopping mechanism and the ion content of the polymer backbone is increased. Single-ion conduction can be promoted by maximizing ionic aggregate percolation in these systems, which is influenced by the charge delocalization of the anionic moeity. Optimal SICPEs were used as macromolecular Lithium salts in blends with neutral polymer electrolytes, poly(ethylene oxide-co-allyl glycidyl ether) and poly(cyanoethyl glycidyl ether) to optimize conductivity as a function of salt loading and composition. These blends improved conductivity by up to two orders of magnitude beyond that of the neat SICPE and maintained high lithium transference numbers. Unlike studies conducted with small molecule salt, conductivity was observed to be mostly independent of glass transition temperature. Instead these blends are most strongly influenced by the overall Li⁺:Oxygen atom ratio, which exhibits optimal conductivity and activation energy near 0.05. These results prove the importance of several design criteria for polymer electrolytes: anion functionality, composition and neutral dielectric constant. In the appendices, other intermolecular forces were investigated in aqueous polymer solutions intended to control the growth of ice crystals in slurries near the freezing point of water, the synthesis and applications of poly(propargyl glycidyl ether), and the synthesis and perfomance of poly(allyl glycidyl ether) derived polyampholyte and antifreeze proteins are discussed.


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