Connection between lithium coordination and lithium diffusion in [Pyr12O1][FTFSI] ionic liquid electrolytes

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.