Exploring the ion solvation environments in solid-state polymer electrolytes at different concentrations through free-energy sampling.*

A major obstacle to improving the performance of Li ion solid state polymer electrolyte batteries is that the exact mechanism of ion conduction in such systems is not well understood. A deeper understanding of solvation from atomistic simulations would prove to be a great stride towards overcoming this challenge. Unfortunately, systematic differences were observed in local forces predicted by various classical force fields in literature and ab-initio results for the system of interest (PEO/LiTFSI). Hence, parameters in the GAFF potential were modified to predict various structural features like bond length, angles and dihedrals in closer agreement with ab-initio estimates, resulting in significant improvements in the local interactions between atoms. These accurate classical force fields were then applied to the study of ion solvation cages and transport in these electrolytes at different ion concentrations. Accelerated molecular dynamics and free-energy sampling techniques were employed to explore the distribution of solvation cages within these electrolytes, their variation with concentration and hopping between them as a possible mechanism of transport. These insights will go a long way towards understanding the mechanism of ion solvation and conduction.