Effect of salt concentration on the solubility, ion-dynamics, and transport properties of dissolved vanadium ions in lithium-ion battery electrolytes: Generalized solubility limit approach (Part II).

In this article, we study the transport properties of superconcentrated electrolytes using Molecular Dynamics simulations, which have been shown experimentally to retard elemental dissolution in vanadium containing cathode materials. Five compositions between one and seven molar lithium bis(trifluoromethanesulfonyl)imide in 1,3-Dioxolane and 1,2-Dimethoxyethane solvent mixture are studied using non-polarizable Optimized Potentials for Liquid Simulations - All Atom force field. The simulated physico-chemical properties such as ionic conductivity, self-diffusion coefficients, and density are observed to match well with the results obtained through experiments. Radial Distribution Function analysis reveals a strong co-ordination between salt anions and vanadium cations as the electrolyte transitions from a salt-in-solvent type to solvent-in-salt type electrolyte. A high anion content in the first solvation shell of vanadium cations is observed for solvent-in-salt type electrolytes, through ion-clustering calculations. Solvation free energy calculations using Free Energy Perturbation method indicate that the active material dissolution should be retarded by using superconcentrated electrolytes. Ion-dynamics of the clusters reveal that vanadium cation transport occurs against its concentration gradient due to strong coulombic interactions with the salt anions in superconcentrated electrolytes. The improvement in the cycleability of several vanadium containing cathode materials provides a robust proof for the theoretical framework described in this manuscript.