Kinetic Regulation of Anionic Redox Reaction Voltage by Metastable Over-Lithiated Surface Shells Formation for...

Anionic redox reaction (ARR) has emerged as a pivotal mechanism to support next-generation of high-energy-density batteries. While extensive efforts have focused on elucidating the origin of ARR-induced additional capacity, ARR operating voltage constitutes the limiting factor for its practical applications. Particularly, voltage hysteresis between charge-discharge brings poor energy efficiency. This work investigates key factors determining ARR operating voltage in Li_1.17Ti_0.58Ni_0.25O₂ (LTNO), a unique model system enabled by its well-isolated ARR plateau around 2.0 V. By combining in situ XRD, TEM, sXAS, and RIXS, the kinetic regulation of both transition metal redox and ARR voltages is validated. For the very first time, depth-resolved Li 1s XPS and Li-K sXAS are combined to trace Li distribution profile and occupation sites. A metastable over-lithiated shell forms on the LTNO surface, where the extra surface Li-ions occupy tetrahedral coordination. Such a surface shell exerts a large energy barrier for Li-ion transfer and leads to an extremely low ARR plateau. Furthermore, the kinetics-regulated ARR voltage may work in other cathodes, where voltage decay in Li-rich layered cathode can be kinetically dominated in long-term cycle. These findings provide new insights into understanding the ARR operating mechanism, and guidelines for optimizing ARR performance can be proposed via facilitating Li transfer kinetics.