Lithium-ion energy storage devices rely heavily on electrolytes comprised of linear and cyclic carbonate molecules. While ethylene carbonate (EC) is ubiquitous as a solvent in rechargeable batteries, its close relative propylene carbonate (PC) is unsuitable because it destroys the electrode surface during the first charge cycle. Observations from studies of LiPF6 dissolved in EC and PC have been used to rationalize the behavior of mixtures of EC and PC. It has been suggested that the electrochemical behavior can be traced to the electrolyte solvation structure. In specific, the presence of PC in the solvation shell of lithium ions favors the reaction mechanism responsible for ruining reversible charge transport, while the presence of EC favors the formation of a stable surface film. Thus, substantial interest has arisen around the question of whether Li favors EC or PC in ternary electrolytes and the implications for energy storage. Few studies have been reported to address this question with molecular resolution, so we present a classical molecular dynamics study of the preference of lithium ions for EC and PC in the bulk solution phase. In order to investigate the competitive solvation of lithium, LAMMPS simulations utilizing a previously successful force field were executed on pure EC and PC separately, mixtures of each with LiPF6, and at different salt concentrations. In general it is found that EC favors ion separation while PC favors contact ion pair formation, in agreement with expectations based on dielectric constant. Given recent discussions of charge scaling to account for solvent polarization with simple two-body models, we also consider the impact of charge scaling on solvation structure. In both cases, charge scaling favors greater ion association and closer agreement with experimentally determined diffusion coefficients. Finally, mixtures of EC:PC and the same lithium salt were considered using the charge scaled model. While a slight preference for EC was determined, the effect is quite small and implies that in the bulk phase the solvation shell closely tracks with the composition of the electrolyte. Ongoing work considers the role of the electrode interface or a vacuum interface on distorting these results from these bulk studies.

Implicit polarization and co-solvent preference: Modeling EC and PC solvation in lithium electrolytes

2019 Spring National Meeting

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.