Unraveling the Structure and Composition of Li₄Mn₂O4.5 (Li₂O•Li0.667Mn1.333O₂)Electrodes for Lithium Batteries Using a High-Temperature Synthesis Approach

This paper addresses the debate about the composition and structure of a lithium-rich manganese oxide electrode with a fully disordered rock salt component, Li4Mn2O5 (or Li2O•2LiMnO2), first reported by Freire et al. in 2016; it is typically prepared by a high-energy ball milling procedure. It has now been demonstrated that, when prepared at 800°C, the formula of this compound is Li4Mn2O4.5, alternatively Li2O•Li0.667Mn1.333O2, or close thereto. The cubic, disordered Li0.667Mn1.333O2 (or Li0.333Mn0.667O) rock salt component, in which the manganese ions adopt an average oxidation state of 2.5+, transforms to a clearly-defined spinel configuration during electrochemical cycling. The electrochemical activation process that occurs during the initial charge reaction includes the oxidation of the manganese ions by oxygen released by the Li2O component between 4.5 and 4.6 V. In complete contrast, nickel- and nickel-cobalt-substituted electrodes, such as Li2O•2LiMn0.5Ni0.5O2 (Li4MnNiO5) and Li2O•2LiMn0.475Ni0.475Co0.050O2 (Li4Mn0.95Ni0.95Co0.10O5), in which the manganese ions adopt a tetravalent state, have completely disordered rock salt components that are electrochemically inactive.