Diagnosis of Mechanisms of Kinetics in Lithium-Rich Manganese-Based Cathode Materials

Abstract The rate-determining step (RDS), which determines rate capability in lithium-rich manganese-based oxide (LRMO) cathode materials, remains unclear. In this work, we establish a quantitative diagnostic method for the RDS in LRMOs using in-situ electrochemical impedance spectroscopy (EIS) and linear sweep voltammetry (LSV) at different states of charge (SOC) and C-rates. From our results, the SOC determines the intrinsic bottleneck of the RDS, while the C-rate drives the transition from single-step to mixed control. Upon discharging to 4.4 V, the RDS is dominated by electron transfer at the interface accompanied by layered-to-spinel phase transition, whereas it is strongly influenced by solid-state diffusion in the bulk with depletion of Li vacancies arising from Jahn–Teller distortion when discharging to 3.0 V. Furthermore, Li+ diffusion, instead of charge transfer, plays a major part in the RDS of LRMOs when the C-rates are increased. These findings establish quantitative criteria for designing future cathode materials.