Structural Characterization of Layered LiMnO2 Electrodes by Electron Diffraction and Lattice Imaging

Electrode materials of the formula LiMnO2 were prepared by an ion-exchange process from α-NaMnO2 and were examined by X-ray diffraction and convergent-beam electron diffraction to determine crystal structure and phase purity. Electron diffraction data showed that a typical parent LiMnO2 sample consisted predominantly of layered LiMnO2 (C2/m) with lithiated-spinel Li2Mn2O4 (F41/ddm) present in minor concentration; orthorhombic LiMnO2 (Pmmm) was detected in trace quantity. An orientation relationship between the monoclinic layered LiMnO2 and tetragonal lithiated-spinel Li2Mn2O4 was obtained from an analysis of single-crystal electron diffraction data. Electron diffraction analyses of three electrochemically cycled LixMnO2 (0 < x < 1) samples showed that the parent compound was unstable to repeated lithium intercalation/deintercalation and transformed to a spinel-type structure, consistent with previous reports. These LixMnO2 electrodes, like those derived from orthorhombic-LiMnO2, show greater stability compared to standard LiyMn2O4 spinel electrodes, when cycled between 4.6 and 2.0 V. The unexpected stability of the LixMnO2 electrodes (0.5≤x < 1.0) at 3 V is attributed to the presence of Li1+zMn2-zO4 spinel domains (0 < z < 0.33) in a composite electrode structure.