Free energy sampling to understand the effect of local ion coordination in polymer electrolytes on transport properties

In order to understand the success of polyethylene oxide (PEO) for lithium-ion batteries and explore possible better alternatives for salt-polymer mixtures, we study various ions at different concentrations in poly(ether-acetal) electrolytes [P(nEO-mMO), where EO is ethoxyl and MO is methoxyl]. Free-energy sampling, using metadynamics with tailored interaction potentials, elucidates the various coordination environments of ions and the energetic pathways for ion transport in these systems. Using cleverly chosen collective variables, we gain insight into: (1) the competition between cation-anion pairing and coordination by the different polymers at various concentrations, (2) the relative stabilities of single- vs. multi-chain coordination environments and (3) the impact of multi-chain coordination on the glass transition temperature and associated ion transport. We also use advanced data-mining approaches to classify the diverse set of configuration environments buried in these simulations, gaining a deeper understanding of local coordination environments that might easily have been missed with manual inspection. Armed with this insight we suggest strategies for electrolyte design to improve ion transport in battery applications.