Cationic-Anionic Redox Chemistry in Multivalent Metal-Ion Batteries: Recent Advances, Reaction Mechanism, Advanced Characterization Techniques, and Prospects

Rechargeable multivalent metal-ion batteries (MMIBs) have garnered a surge of attention as competitive candidates for large-scale energy storage applications owing to their high capacity, abundant resources, and good security. However, their practical implementation is still stuck at the prototype stage, mainly plagued with the lack of suitable cathode materials capable of reversible insertion/extraction of multivalent ions and the intrinsically complicated redox reaction mechanism. Recently, anionic redox chemistry has shown to be an effective strategy to increase energy density, providing a new research direction for the next generation of high-energy rechargeable batteries. Unfortunately, anion redox chemistry has not received sufficient attention in MMIBs so far. Here, the fundamental principle and mechanism of anionic redox reactions in MMIBs are discussed and the recent advances regarding cathode materials based on cooperative cationic-anionic redox (CCAR) mechanism are summarized. Additionally, various advanced characterization techniques for studying the anionic redox process are highlighted, aiming to effectively illustrate the underlying reaction mechanism. Finally, challenges and perspectives for the future research on cationic-anionic redox chemistry in MMIBs are proposed. Insight into the significance of CCAR chemistry is provided here in MMIBs, presenting a new avenue for the development of high-energy-density cathode materials for MMIBs.