Rate limiting activity of charge transfer during lithiation from ionic liquids

Abstract

Abstract Given the increased use of room temperature ionic liquid electrolytes in Li-ion batteries, due to their non-flammability and negligible volatility, this study evaluates the lithiation kinetics to understand and improve the rate performance of Li-ion batteries. Lithium titanate spinel is used as a model electrode and the electrolyte is composed of LiTFSI and TFSI-coordinated alkoxy-modified phosphonium ionic liquid. Based on the analysis of activation energies for each process, we report that the charge-transfer reaction at the electrode/electrolyte interface is the rate-limiting step for cell operation. This finding is further supported by the observation that a 50-fold decrease in charge-transfer resistance at higher temperatures leads to a significant performance improvement over that of a traditional organic electrolyte at room temperature. Charge-transfer resistance and electrolyte wetting on the electrode surface are critical processes for optimal battery performance, and such processes need to be included when designing new ionic liquids in order to exceed the power density obtained with the use of current carbonate-based electrolytes.