Abstract
The demonstration of reversible anionic redox in Li-rich layered oxides has revitalized the search for 1 higher energy battery cathodes. To advance the fundamentals of this promising mechanism, we investigate herein the 2 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 3 (anionic) are the only redox-active sites. We reveal its charge compensation mechanism and local structural evolutions 4 by applying operando (and complementary ex situ) X-ray absorption spectroscopy (XAS). Among other local effects, 5 the anionic-oxidation-driven distortion of oxygen network around Ru atoms is thereby visualized. Oxidation of lattice 6 oxygen is also directly proven via hard-X-ray photoelectron spectroscopy (HAXPES). Furthermore, we demonstrate a 7 spectroscopy-driven visualization of electrochemical reaction paths, which enabled us to neatly decouple the individual 8 cationic-anionic dQ/dV contributions during cycling. We hence establish the redox and structural origins of all dQ/dV 9 features and demonstrate the vital role of anionic redox in hysteresis and kinetics. These fundamental insights about Li-10 rich systems are crucial for improving the existing anionic-redox-based cathodes and evaluating the ones being 11 discovered rapidly.