Unraveling the structure and composition of Li4Mn2O4.5 (Li2O•Li0.667Mn1.333O2) electrodes for lithium batteries

This communication addresses the debate about the composition and structure of a lithium-rich manganese oxide electrode with a fully disordered Li-Mn-O rock salt component that was first reported as Li4Mn2O5 (Li2O•2LiMnO2), by Freire et al. in 2015. When prepared at 800 C, it has been determined that the formula of this compound can be designated more accurately as 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 during the initial charge reaction appears to include the oxidation of the manganese ions by oxygen released by the Li2O component between 4.5 and 4.6 V. In complete contrast, nickel-substituted electrodes, such as Li2O•2LiMn0.5Ni0.5O2 and Li2O•2LiMn0.475Ni0.475Co0.050O2, in which the manganese ions adopt a tetravalent state, have disordered rock salt components that are electrochemically inactive.