Charge–discharge mechanism and capacity degradation of Co-substituted Li5AlO4 during cycling

High-performance advanced and futuristic products require batteries with higher energy densities than those of conventional batteries. Anion-redox active materials have high specific capacities, but they exhibit low life-cycle performance. In this study, the charge–discharge mechanism and the primary factors contributing to the capacity degradation of Co-substituted Li5AlO4 (CSLA) during charge–discharge cycling were investigated. X-ray absorption spectroscopy (XAS) was performed to determine the change in the electronic state of CSLA during the charge–discharge process. The changes in the XAS peak intensities were attributed to the formation and consumption of peroxide and superoxide, as observed in the O K-edge X-ray absorption near-edge structure (XANES) spectra of CSLA obtained over the first ten charge–discharge cycles. The results indicated that the redox reaction involving the formation and consumption of peroxide and superoxide allows for the charge compensation of CSLA. Furthermore, the intensity of the O K-edge XANES peak attributed to the formation and consumption of peroxide decreased with the increasing number of charge–discharge cycles of CSLA, which correlated with the capacity degradation of CSLA. This study provides new insights into the charge–discharge and capacity degradation mechanisms of anion-redox cathode materials. •Study of charge–discharge and capacity degradation of Co-substituted Li5AlO4.•Partial structural change around Al is caused by oxygen redox reaction.•Charge compensation is due to formation/annihilation of superoxide and peroxide.•Oxide–superoxide reaction is reversible; oxide–peroxide reaction is irreversible.•Decrease in capacity of Co-substituted Li5AlO4 caused by peroxide decomposition.