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

The success of polyethylene oxide (PEO) in solid-state polymer electrolytes for lithium-ion batteries is well established. In order to understand this success and to explore possible better alternatives for salt-electrolyte mixtures for battery systems, we study various ions at different concentrations in poly(ether-acetal) (P(nEO-mMO) where EO is ethoxyl and MO is methoxyl) electrolytes to deepen the understanding of the effect of local chemical structure on ion transport. Advanced molecular dynamics (MD) techniques like free energy sampling using newly developed, tailored interaction potentials have helped elucidate the various coordination environments of ions and the energetics of transport in these systems. Using cleverly chosen collective variables, we gain insight into the competition between cation-anion pairing and coordination by the different polymers at various concentrations, the relative stabilities of inter vs intra chain coordination environments and its effect on glass transition temperature and the energetics as the ions move through and between chains.<br/>At an equivalent reduced temperature with respect to the polymer-specific glass transition temperature, for Li salt-polymer systems, two-dimensional free-energy plots reveal the existence of multiple coordination environments for the lithium (Li) ions in these systems and their relative stabilities. Furthermore, we observe that the Li ion movement in PEO follows a serial, stepwise pathway when moving from one coordination state to another, whereas this happens in a more continuous and concerted fashion in a polyacetal such as poly(1,3-dioxalane) (P(EO-MO)). The implication is that inter-conversion between coordination states of the Li-ions may be easier in P(EO-MO). Free-energy calculations also show that Li coordination by multiple chains is in fact energetically more favourable in P(EO-MO) unlike PEO. We hence rationalize the observed higher increase in glass transition temperature (T_g) with salt loading in polyacetals as due to predominantly intermolecular Li-ion coordination involving multiple polymer chains, rather than just mostly one chain for PEO-based electrolytes. This interchain coupling in the polyacetals, resulting in the higher T_g, works against any gains due to variations in Li-ion coordination that might enhance transport processes over PEO. Smart choice of collective variables also allows us to explore the energetics of the various solvation environments as ions move along and between chains in these systems and hence gain much deeper insight into ion transport. We also explore how transport and the various coordination environments are affected when we switch to systems with other monovalent ions (Na⁺) or multivalent ions (Mg²⁺). Finally, with the insight we have gained into the different coordination environments and the energetics of transport in these different ion-polymer systems at various concentrations, we suggest ideas for better design of polymers with improved transport properties for battery electrolyte materials.