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

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 LiMn 2 O 4 thin-film electrodes in LiN(SO 2 CF 3 ) 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(SO 2 CF 3 ) 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.