Understanding Li-ion thermodynamic and kinetic behaviors in concentrated electrolyte for the development of aqueous lithium-ion batteries

High-concentration aqueous electrolytes are attractive for deployments in future lithium-ion batteries due to high safety, environmental friendliness, and wide voltage window. It is of great significance to understand the Li-ion behaviors in high concentration conditions for both mechanistic studies and commercial applications. Herein, by analyzing cyclic voltammetry and voltage profiles using a customized single-particle model, we clarify the Li-ion thermodynamic and kinetic behaviors in aqueous electrolytes at various concentrations using LiFePO4 as the active electrode. With the increase of the electrolyte concentration, the equilibrium potentials of LiFePO4 shift to higher values, which is attributed to the increased Li-ion activity and activity coefficient induced by the formation of polymeric solution structure ((Li+(H2O)2)n) at high concentrations. To further quantify the interface reaction constants (k0) and the activation energy (Ea), theoretical simulations based upon experimental data are carried out, identifying that the sluggish Li-ion desolvation process is the main contributor to the slower interface kinetics in high concentration electrolytes. Other factors affecting the Li-ion interface process, including temperature, scan rate, and type of anion, are also evaluated here. These fundamental understandings are of great value to the development of high-concentration aqueous electrolyte, in a cost-effective, sustainable and efficient way.