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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. [ABSTRACT FROM AUTHOR]

Rechargeable magnesium batteries (RMBs) based on metal Mg anodes have shown great potential owing to the abundant natural resources, high volumetric capacity, and low safety hazard. Nevertheless, the development of RMBs is hampered by the sluggish kinetics of Mg2+ diffusion and the limited cyclability of cathode materials. Herein, nonstoichiometric copper selenide (Cu2–xSe) are synthesized via a solution‐based method and exploited as a durable cathode material based on ionic displacement mechanism for RMBs. The copper ions in the Se2− based sub‐lattices are reversibly exchanged by Mg2+ ions without causing lattice collapse. Owing to the same face‐centered cubic Se2− sub‐lattices and similar unit cell size of Cu2–xSe and MgSe, the energy barrier for lattice reconstruction during cycling processes is very low, significantly improving the rate performance, structural stability, and cycle life of the Cu2–xSe cathode. Moreover, metal Cu is in situ generated during discharging, thus greatly facilitating electron transport. Comprehensive characterizations confirm that the Cu2–xSe cathode undergoes reversible copper ion extrusion/reinjection during the discharge−charge steps. This work suggests the great potential for exploring high‐performance electrode materials based on ionic displacement mechanism for advanced multivalent‐ion secondary batteries. [ABSTRACT FROM AUTHOR]

• In situ electrolyte-induced polymerization of 1-aminoanthraquinone (1-AAQ) and 1,5-diaminoanthraquinone (1,5-DAAQ). • The PDAAQ cathode shows a high specific capacity, high-rate capability, and stable cyclability over 8000 cycles at 2 A g-1. • A hybrid charge storage mechanism based on Mg2+ cation insertion/extraction and anion doping/de-doping was revealed. • This strategy has universality for other aniline-based organic cathode materials. Organic electrodes are promising as next-generation energy storage materials owing to their diverse structures, low mass, and environmental friendliness. Nevertheless, the dissolution and degradation of organic active species in electrolytes are remaining obstacles to their authentic commercialization. Herein, we report an instantaneous in-situ upgrading strategy to convert multi-site organic cathode materials, e.g., 1-aminoanthraquinone (1-AAQ) and 1,5-diaminoanthraquinone (1,5-DAAQ), into poly(1-aminoanthraquinone) (PAAQ) and poly(1,5-diaminoanthraquinone) (PDAAQ) simply by dropping the electrolyte during battery assembly without extra operations. The remarkable chemistry is essentially a chemical polymerization process of redox-active organic monomers triggered by the electrolyte containing magnesium bis(hexamethyldisilazide) (Mg(HMDS) 2) as an initiator. Notably, due to the π-conjugated polymer chain with inhibited dissolution, high conductivity, and improved stability, the as-obtained PDAAQ cathode delivered a high specific capacity (254 mAh/g at 100 mA/g), superior rate performance (83 mAh/g at 2000 mA/g), and excellent cycling stability for over 8000 cycles accompanied by an average capacity decay of only 0.0026 % per cycle. Detailed characterizations further explore the kinetic behaviors and verify a reversible hybrid cation–anion co-redox mechanism of PDDAQ cathode, which involves reversible Mg2+ cation insertion/extraction on AQ units and anion doping/de-doping reactions on the PANI backbones. Density functional theory (DFT) computations demonstrate the optimized structure of the discharged products and indicate that Mg2+ preferably interact with two carbonyls oxygen atoms and nitrogen atom for both PAAQ and PDAAQ. This study demonstrates an intriguing in-situ chemical-polymerization synthetic approach for preparing multi-site organic electrode materials that compromise structure optimization and synthetic efforts, offering great promise for high-performance and sustainable secondary batteries. [ABSTRACT FROM AUTHOR]