The increasing demand for energy storage is calling for improvements in cathode performance. In traditional layered cathodes, the higher energy of the metal 3d over the O 2p orbital results in one-band cationic redox; capacity solely from cations cannot meet the needs for higher energy density. Emerging anionic redox chemistry is promising to access higher capacity. In recent studies, the low-lying O nonbonding 2p orbital was designed to activate one-band oxygen redox, but they are still accompanied by reversibility problems like oxygen loss, irreversible cation migration, and voltage decay. Herein, by regulating the metal-ligand energy level, both extra capacities provided by anionic redox and highly reversible anionic redox process are realized in NaCr1- y Vy S2 system. The simultaneous cationic and anionic redox of Cr/V and S is observed by in situ X-ray absorption near edge structure (XANES). Under high d-p hybridization, the strong covalent interaction stabilizes the holes on the anions, prevents irreversible dimerization and cation migration, and restrains voltage hysteresis and voltage decay. The work provides a fundamental understanding of highly reversible anionic redox in layered compounds, and demonstrates the feasibility of anionic redox chemistry based on hybridized bands with d-p covalence.
Keywords: anionic redox; d-p covalent interactions; energy bands; layered cathodes; metal-ligand combination.
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