Activation and stabilization mechanisms of anionic redox for Li storage applications: Joint experimental and theoretical study on Li2TiO3–LiMnO2 binary system

Abstract: A binary system of Li₂TiO₃–LiMnO₂ is systematically examined by joint experimental and theoretical studies as electrode materials for Li storage applications. Increase in a fraction of Li₂TiO₃ effectively activates anionic redox, and thus holes are reversibly formed on oxygen by electrochemical oxidation. Such holes are energetically stabilized through π-type interaction with Mn t_2g orbital as suggested by theoretical calculation. However, excess enrichment of Li₂TiO₃ fractions in this binary system results in the oxygen loss as an irreversible process on delithiation because of a non-bonding character for Ti–O bonds coupled with the formation of O–O dimers, which are chemically and electrochemically unstable species. Additionally, detailed electrochemical study clearly shows that Li migration kinetics is relatively slow, presumably coupled with low electronic conductivity. Nevertheless, nanosizing of primary particles is an effective strategy to overcome this limitation. The nanosized sample prepared by mechanical milling delivers a large reversible capacity, ∼300?mA?h?g⁻¹, even at room temperature and shows much improved capacity retention. Formation and stabilization of holes for the nanosized sample are also directly evidenced by soft X-ray absorption spectroscopy. From these results, factors affecting the reversibility of anionic redox as emerging new chemistry and its possibility for energy storage applications are discussed in more details.