Anionic Redox Regulated via Metal-Ligand Combinations in Layered Sulfides.

The increasing demand for energy storage is calling for improvements in cathode performance. In traditional layered cathodes, the higher energy of metal 3d over O 2p orbital results in one band cationic redox, capacity solely from cation can not meet the needs for higher energy density. Emerging anionic redox chemistry is promising to access higher capacity. In recent studies, low-lying O non-bonding 2p orbital was designed to activate one-band oxygen redox, but still accompanied by reversibility problems like oxygen loss, irreversible cation migration and voltage decay. Herein, by regulating metal-ligand energy level, both extra capacities provided by anionic redox and highly reversible anionic redox process were realized in NaCr1-y Vy S2 system. The simultaneous cationic and anionic redox of Cr/V and S was observed by in-situ X-ray absorption near edge structure (XANES). Under high d-p hybridization, the strong covalent interaction stabilized the electron holes on anions, prevented irreversible dimerization and cation migration, restrained voltage hysteresis and voltage decay. Our work provides a fundamental understanding of highly reversible anionic redox in layered compounds, and demonstrated the feasibility of anionic redox chemistry based on hybridized bands with d-p covalence. This article is protected by copyright. All rights reserved.