ABSTRACT Recently Armstrong and Bruce'reported a layered modification of lithium manganese oxide, LiMnO7, isostructural with LiCoO2. LiMnO1 obtained by ion exchange from ct-NaMnO1 synthesized in air is characterized by x-ray diffraction and by electrochemical insertion and extraction of lithium in a series of voltage ranges between 1.5 and 4.5 V relative to a lithium electrode. During cycling, voltage plateaus at 3.0 and 4.0 V vs. Li develop, indicating that the material is converted from its original layered structure to a spinel structure. This finding is confirmed by x-ray diffraction. Contrary to expectations based on thermodynamics, insertion of larger amounts of lithium leads to a more complete conversion. We suggest that a relatively high mobility of manganese leaves Li and Mn randomly distributed in the close-packed oxygen lattice after a deep discharge. This isotropic Mn distribution can relatively easily relax to the Mn distribution characteristic of spinels whereas the anisotropic distribution characteristic of layered structures is not reformed when excess lithium is extracted.

Infroduction With the introduction of the lithium-ion battery concept, the" voltage"(potential relative to a lithium electrode) of electrode materials has become an important design parameter. A high voltage of the positive electrode, preferably above 4 V vs. Li, is required in order to minimize the loss in energy density introduced by the nonzero voltage of the coke or graphite used as negative electrode. High voltages can be found in two groups of intercalation materials: in layered lithium transition metal dioxides (eg, LiCoO1 and LiNiO2) and in lithium transition metal spinels, especially LiMn2O4. These materials have" unusu-ally" high voltages, ie, voltages that are considerably higher than known from materials with similar compositions, but different structures. The high voltage of the layered oxides stems from a destabilization of the host structure caused by lithium extraction. This can be illustrated by the fact that lithium extraction from the ideally layered LiCoO2 occurs at a considerably higher voltage compared to lithium extraction from disordered LiCoO2 2 where interlayer transition metal ions reduce the electrostatic repulsion between negatively charged oxide layers. This destabilization eventually leads to irreversible structural breakdown if a critical limit of lithium extraction is exceeded. The high voltage of lithium extraction from the manganese spinel can be ascribed to the oxygen coordina-tion of lithium in this structure. The tight tetrahedral coordination offered by the spinel structure defines a particularly stable environment for lithium ions, resulting in higher potentials during lithium insertion or extraction than in materials with octahedral coordination. The tetrahedral sites in most other close-packed structures used as intercalation hosts share faces with octahedral sites occupied by transition metal ions. These sites are energetically disfavored for electrostatic reasons. The current efforts in the investigations on new electrode materials with high energy density are focused on alternative materials with similar or better performance compared to LiCoO2, 2 LiNiO2, or Li, 1. Mn2104. A point of special concern is to find materials with an initial lithium capacity larger than the, at best, 160 mAh/g offered by these materials. Cost and environmental issues are also of concern, making manganese-based materials much more attractive than eg, cobalt oxides. Orthorhombic LiMnO2 is such a mater-ial; it exists both in a low-temperature modification with an initial charge capacity close to 230 mAh/g, 8 and in a high-temperature modification with an initial charge capacity of 150 mAh/g. 11" 2 Upon …