The mechanism of Li+ transport in an ether-based electrolyte

Abstract

Molecular dynamics (MD) simulations are performed on an ether-based electrolyte, composed of 1 M lithium bis(trifluoromethanesulphonyl)imide (LiTFSI) dissolved in the ether-based electrolyte of 1,3-dioxolane (DOL)/1,2-dimethoxyethane (DME) (v/v = 1 : 1), at temperatures of 298.15 K, 273.15 K, 253.15 K, 233.15 K, and 213.15 K, respectively. The simulation demonstrates that the solvated Li⁺ is much more strongly binding with DME than with DOL or TFSI⁻ and various coordination structures of Li⁺, including solvent separated Li⁺s (SSLis), contact ion pairs (CIPs), and aggregates (AGGs), co-exist in the electrolyte. The migration of Li⁺ is driven by a vehicular mechanism, in the sense that the strongly coordinating DME acts as a vehicle of Li⁺, which migrates with DME while exchanging the weakly coordinating DOL or TFSI⁻. At low temperature, the populations of CIPs and AGGs are much reduced and the dominant solvation structure is the SSLi consisting of Li⁺ coordinated by three DMEs. In this case, Li⁺ is trapped in the cage solely consisting of the strongly coordinating DME with low ionic conductivity, featuring a decrease in the partial transference number of the self-contribution of Li⁺ with decreasing temperature.