Transport in Superconcentrated LiPF₆ and LiBF₄/Propylene Carbonate Electrolytes

Superconcentrated electrolytes for lithium-ion batteries have shown promise in circumventing certain limitations of conventional carbonate electrolytes at lower concentrations while introducing new challenges such as decreased conductivity. We use molecular dynamics simulations with diffusion and residence time analyses to elucidate the main modes of transport of LiPF6 and LiBF4 in propylene carbonate at concentrations ranging from 1 to 3 M. Notably, we find that the Li+ mode of diffusion with respect to its surrounding propylene carbonate solvation shell is a mix of vehicular and structural diffusion at all studied concentrations, exhibiting a small increase toward structural diffusion in the superconcentrated regimes. Furthermore, and important for future strategies toward improved conductivity, we find that the Li+ ions associated with PF6– anions move in an increasingly vehicular manner as the salt concentration is increased, while the Li+ ions associated with BF4– anions move in an increasingly structural manner.