Exploring Anionic Redox Chemistry of Battery Cathodes with Resonant Inelastic X-Ray Scattering.

High-capacity and reversible cathodes are important for sustainable energy development. Layered oxides present promising options, achieving a high capacity while maintaining manageable production costs. Despite the redox activity of transition metal primarily contributes to the capacity, anionic redox offers additional potential, as evidenced by the subtle voltage plateau at high voltage. However, challenges remain, particularly concerning the reversibility of anionic redox, resulting in voltage fade, hysteresis, and the formation of undesired oxygen species. Resonant inelastic X-ray scattering (RIXS) captures not only the charge transfer processes between oxygen ligands and transition metals but also the formation of molecular oxygen, elucidating the chemical transformations of oxygen during electrochemical cycles. Mapping capability allows for the generation of spectral patterns with quantitative measures that surpass those achievable by other characterization techniques. Though early reports on its advantages, a comprehensive discussion of RIXS and its perspectives on anionic redox processes in cathode materials remains absent but necessary. This perspective aims to provide a systematic overview of RIXS, emphasizing its unique contributions to cathode material research. It also serves as a valuable reference for energy researchers and RIXS practitioners, featuring the advancements and future possibilities of this powerful technique.