Effects of Li-Salt Concentration and Viscosity on Li-Ion Insertion/Extraction Reaction Kinetics at LiMn2O4 Electrode in Ether-Based Electrolyte Solutions

Abstract The desolvation of Li+ ion is generally considered to be the rate-determining step of Li+ insertion/extraction reactions at the negative and positive electrodes in the electrolyte solutions of Li-ion batteries (LIBs). However, specific factors that affect the charge-transfer kinetics at the electrode/electrolyte interface remain to be clearly understood. In this study, we investigated the interfacial charge-transfer reaction rate at LiMn2O4 thin-film electrodes in LiN(SO2CF3)2/monoglyme electrolytes. Our analysis revealed that the Li+ activity and electrolyte viscosity significantly affect the interfacial charge-transfer reaction rate. A higher Li-salt concentration enhances Li+ activity, accelerating the reaction rate. However, increasing the salt concentration beyond a certain level increases the electrolyte viscosity, which retards the interfacial reaction kinetics. A trade-off between these two factors results in the fastest reaction rate at an intermediate concentration (~1.8 mol L−1). In the electrolytes with higher LiN(SO2CF3)2 concentrations, the chemical potential of Li+ increases, which facilitates the desolvation of Li+. However, the activation energy of the electrolyte viscosity increases with increasing salt concentration. Consequently, the activation energy of the interfacial charge-transfer reaction reached the minimum at approximately 1.8 mol L−1. These insights into the factors that affect the Li+ insertion reaction kinetics can help design optimal electrolytes for high-power LIBs.