(Invited) Reaction Mechanism of Lithium Excess Layered Manganese Oxides Studied By Thin-Film Model Electrodes and Operando Measurements

Abstract

The reaction mechanism of lithium excess manganese oxide, Li2MnO3, with layered rock-salt structure was examined using model thin film electrodes with liquid or solid electrolytes. The epitaxial or polycrystalline films with 30 nm-thick with various compositions and morphologies were fabricated using pulsed laser deposition and sputtering methods. These electrodes showed charge-discharge capacity ranging from 150 to 300 mAh g-1with different degradation characteristics depending on the compositions and morphologies. The reaction mechanisms were studied by in-situ and operando methods using surface X-ray diffraction and HAXPES analyses. The Li2MnO3shows a transformation to a high-capacity phase after the activation process by the first charge and the degradation during the subsequent cycling depends on the sample stoichiometry and morphology. The transition to the high capacity phase is related the phase change from the O3 to O2 type, together with the interlayer cation disordering on the Li/Mn layer. The activation process is related to the oxidation of oxygen and a formation new oxygen species together with the reduction of the manganese from tetra-valent state. The activation process and the reversible reactions of the lithium-rich layered rock-salt materials are elucidated.