Effectively enhancing ion diffusion in superconcentrated ionic liquid electrolytes using co-solvent additives

The incorporation of high salt concentrations in ionic liquid (IL) electrolytes, forming superconcentrated ionic liquids, has been shown to improve Li-ion transference numbers and enhance cycling stability against lithium metal anodes. However, this benefit comes at the cost of significantly increased viscosity and reduced ionic conductivity due to the formation of large ion aggregates. To optimize conductivity further, a co-solvent can be introduced at an optimal concentration to enhance ion transport while preserving superior interfacial stability. The effectiveness of this approach depends on the solvent as it affects ion diffusion to varying degrees. This computational study examines how co-solvents can effectively enhance metal ion diffusion in superconcentrated ionic liquids by comparing two widely used organic solvents. We found that the key lies in their ability to effectively participate in Li solvation shells, disrupting the large Li-anion aggregates. Our results show that anion exchange in a Li(anion)_<i>x</i>(solvent)_<i>y</i> hybrid solvation shell occurs more rapidly than in a Li (anion)_<i>z</i> solvation shell, facilitating Li diffusion through a structural diffusion mechanism. A co-solvent with a high donor number exhibits a stronger affinity for lithium ions, which is identified as a crucial factor in enhancing ion diffusion. This work provides valuable insights to guide the design of superconcentrated ionic liquid electrolytes for lithium-metal battery development.