Chronoamperometric Analyses of Lithium Insertion/Extraction Kinetics of Lithium Nickel Manganese Oxide Using the Diluted Electrode Method

Understanding the electrode kinetics of Li insertion materials used in Li-ion batteries is important for deducing the power capabilities of the batteries. However, examining the inherent reaction kinetics of these materials is difficult because Li-ion diffusion in the electrolyte within electrode pores is the rate-determining step in conventional electrodes, which mainly comprise active materials. Herein, the Li insertion kinetics of Li[Ni1/2Mn3/2]O4 is analyzed using the diluted electrode method. Results of charge/discharge tests performed at a constant voltage (chronoamperometry) reveal that both, charging (Li extraction) and discharging (Li insertion) in the diluted electrode (Li[Ni1/2Mn3/2]O4 content: 10 wt%), are much faster than those in conventional electrodes (88 wt%). The diluted electrode delivers half the capacity of Li[Ni1/2Mn3/2]O4 in only 20 and 200 s for charging and discharging, respectively, at high overvoltages. This indicates that the reaction kinetics is asymmetric between the charging and discharging processes; charging occurs approximately 10 times faster than discharging. Based on the overvoltage and temperature dependences of the reaction kinetics, phenomenological kinetic equations for the Li insertion/extraction of Li[Ni1/2Mn3/2]O4 are determined. The results of this study not only provide important knowledge for establishing the Li insertion kinetics but are also useful for understanding the power capabilities of all-solid-state batteries.