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 alpha-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 Li-2[Mn-2]O-4 (F4(1)/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 Li-2[Mn-2]O-4 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 Li-y[Mn-2]O-4 spinel electrodes, when cycled between 4.6 and 2.0 V. The unexpected stability of the LixMnO2 electrodes (0.5 less than or equal to 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. (C) 1999 The Electrochemical Society. S0013-4651(98)04-084-1. All rights reserved.