Free Energy Sampling to Explore Ion Solvation Environments and Understand Transport and Glass Transition in Solid State Electrolytes for Battery Materials

Abstract

Molecular dynamics simulations using newly developed, accurate interaction potentials have helped elucidate possible mechanisms for various transport properties in systems of lithium salts dissolved in poly(ether-acetals) ( P(nEO-mMO) where EO is ethoxyl and MO is methoxyl ) that are prime candidates for solid state polymer electrolytes. The presence of a second coordination shell around the Li ion, observed in polymers that have MO groups, seems to distort the first coordination shell, creating a more open cage structure that may help facilitate transport in these polymers. Furthermore, this distorted coordination may also assist in the formation of a lower coordination transition state while the Li ions jump from one cage to another during diffusion. Free energy calculations further elucidate the relative stability of these different Li-ether coordination environments and the possible pathways to move between them (See figure 1 for example).Studies on poly(ether-acetals) also tell us that polymers with low glass transition temperature (Tg) are not necessarily the best foundation for high transport electrolytes as addition of salt may disproportionately increase Tg and reduce transport at comparable absolute temperatures. At battery working temperatures, PEO still exhibits superior ionic conductivity as compared to other polymers in this series due a smaller increase in glass transition with salt loading. Hence it is very important for practical applications to understand how salt loading affects the glass transition temperatures in these polymers and how we can tune it to our advantage. It was observed that the softness of the polymer chain controlled by the strength of the dihedral angles is an important parameter that has a significant effect on the glass transition and hence provide insight and tune further improved potentials. We will show the associated correlations of glass transition temperature in these systems with microscopic chemical details like the polymer backbone chemistry, ion clustering and coordination by extensively studying the trends in these different systems with extensive statistics from long time molecular dynamics simulations at different temperatures and compositions and understand transport at different timescales and help design new polymers electrolytes with improved transport at battery working temperatures. Figure 1: Pathways:2D Free energy analysis of Solvation of Li cation and TFSI anion in PEO and PEOMO. Coordination w/r to oxygens of ether/acetyl and oxygens of TFSI anions are used as collective variables a)Serial Process: PEO shows distinct stepwise pathways to move from one coordination state to another (arrows for guide showing TFSI forming a bond in the first step and then the polymer bond breaking in the second step b)Concerted Process: PEOMO shows a more direct path that implies bond breaking and forming happens in a more concerted way that allows for easier cation transport from one cage to another Figure 1