Excess Lithium in Transition Metal Layers of Epitaxially Grown Thin Film Cathodes of Li2MnO3 Leads to Rapid Loss of Covalency during First Battery Cycle

We have investigated the initial-cycle battery behavior of epitaxial thin films of Li2MnO3 cathodes by employing resonant inelastic X-ray scattering (RIXS) at the O K-edge and Mn L-3-edge. Thin films (25 nm thickness) with Li/Mn ratios of 2.06 (stoichiometric) and 2.27 (overstoichiometric) were epitaxially grown by pulsed laser deposition and electrochemically cycled as battery cathodes in a half-cell setup, stopped at potentials for full charge (delithiation) and complete discharge (relithiation) for X-ray analysis. No signatures of localized oxygen holes are found in O K-RIXS of the Li2MnO3 regardless of the Li/Mn ratio. The prepeak region of the O K-XAS shows a net increase (a net decrease) after delithiation (relithiation) for both material compositions. Moreover, signatures of additional phases are detected only for the overstoichiometric material upon relithiation. Mn L-RIXS of the overstoichiometric cathode material shows a progressive loss of charge transfer state intensity during the first battery cycle, revealing a more rapid loss of Mn-O covalency in the overstoichiometric material.