Decoupling Cationic–Anionic Redox Processes in a Model Li-Rich Cathode via Operando X-ray Absorption Spectroscopy

The demonstration of reversible anionic redox in Li-rich layered oxides has revitalized the search for higher energy battery cathodes. To advance the fundamentals of this promising mechanism, we investigate herein the cationic–anionic redox processes in Li 2 Ru 0.75 Sn 0.25 O 3 —a model Li-rich layered cathode in which Ru (cationic) and O (anionic) are the only redox-active sites. We reveal its charge compensation mechanism and local structural evolutions by applying operando (and complementary ex situ ) X-ray absorption spectroscopy (XAS). Among other local effects, the anionic-oxidation-driven distortion of the oxygen network around Ru atoms is thereby visualized. Oxidation of lattice oxygen is also directly proven via hard X-ray photoelectron spectroscopy (HAXPES). Furthermore, we demonstrate a spectroscopy-driven visualization of electrochemical reaction paths, which enabled us to neatly decouple the individual cationic–anionic dQ / dV contributions during cycling. We hence establish the redox and structural origins of all dQ / dV features and demonstrate the vital role of anionic redox in hysteresis and kinetics. These fundamental insights about Li-rich systems are crucial for improving the existing anionic-redox-based cathodes and evaluating the ones being discovered rapidly.