Surface Nonbonding Oxygen State Regulation for Reversible Anionic Redox Chemistry in Li-Rich Mn-Based Layered Oxides.

Activated by the Li-O-Li configuration with nonbonding O 2p state (lO), anionic redox reaction (ARR) in Li-rich layered oxides (LLOs) contributes to additional capacity but exhibits significant irreversibility, leading to severe surface oxygen loss. Herein, surface nonbonding oxygen state (SNBOS) is regulated by the integrated surface structure engineering to suppress surface oxygen loss and enhance the reversibility of ARR. On the outermost layer, the conversion of layered structure into a LiLaO layer and spinel phase structure eliminates lO, thereby preventing the activation of ARR and suppressing side reactions between electrolyte and oxidized oxygen ions. Besides, by introducing 5d metal La on the near surface, the energy of lO is decreased corresponding to the increased charge-transfer gap Δ and the d-d coulomb repulsion term U is reduced, making U/2 decrease close to Δ and enhancing ARR reversibility. Furthermore, it is demonstrated that the oxidized oxygen of the modified sample cannot become O gas and escape, but rather exists more in the form of high-valence oxygen dimer anions , reducing the reaction depth of surface ARR and inhibiting oxygen loss. Therefore, the designed material demonstrates outstanding cycling stability and kinetics performance.