Elucidation of transport mechanism and enhanced alkali ion transference numbers in mixed alkali metal–organic ionic molten salts

Abstract

Mixed salts of Ionic Liquids (ILs) and alkali metal salts, developed as electrolytes for lithium and sodium batteries, have shown a remarkable ability to facilitate high rate capability for lithium and sodium electrochemical cycling. It has been suggested that this may be due to a high alkali metal ion transference number at concentrations approaching 50 mol% Li⁺ or Na⁺, relative to lower concentrations. Computational investigations for two IL systems illustrate the formation of extended alkali–anion aggregates as the alkali metal ion concentration increases. This tends to favor the diffusion of alkali metal ions compared with other ionic species in electrolyte solutions; behavior that has recently been reported for Li⁺ in a phosphonium ionic liquid, thus an increasing alkali transference number. The mechanism of alkali metal ion diffusion via this extended coordination environment present at high concentrations is explained and compared to the dynamics at lower concentrations. Heterogeneous alkali metal ion dynamics are also evident and, somewhat counter-intuitively, it appears that the faster ions are those that are generally found clustered with the anions. Furthermore these fast alkali metal ions appear to correlate with fastest ionic liquid solvent ions.