Connection between Lithium Coordination and Lithium Diffusion in [Pyr12O1 ][FTFSI] Ionic Liquid Electrolytes

Abstract

The use of highly concentrated ionic liquid-based electrolytes results in improved rate capability and capacity retention at 20 °C compared to Li⁺ -dilute systems in Li-metal and Li-ion cells. This work explores the connection between the bulk electrolyte properties and the molecular organization to provide insight into the concentration dependence of the Li⁺ transport mechanisms. Below 30 mol %, the Li⁺ -containing species are primarily smaller complexes (one Li⁺ cation) and the Li⁺ ion transport is mostly derived from the vehicular transport. Above 30 mol %, where the viscosity is substantially higher and the conductivity lower, the Li⁺ -containing species are a mix of small and large complexes (one and more than one Li⁺ cation, respectively). The overall conduction mechanism likely changes to favor structural diffusion through the exchange of anions in the first Li⁺ solvation shell. The good rate performance is likely directly influenced by the presence of larger Li⁺ complexes, which promote Li⁺ -ion transport (as opposed to Li⁺ -complex transport) and increase the Li⁺ availability at the electrode.

Keywords: batteries; density functional calculations; electrolytes; ionic liquids; lithium diffusion.