Electrochemical cyclability of orthorhombic LiMnO2 - Characterization of cycled materials

The electrochemical removal of lithium from orthorhombic LiMnO2 (o-LiMnO2) leads to a phase transition with a first plateau at about 3.7 V. This corresponded to the formation of a spinel-like material; a possible transition to a rhombohedral LixMnO2 phase was ruled out through structural and crystal-site energy considerations. Several electrochemical cycles were necessary to achieve a complete phase transformation; the smaller the crystallites/crystals the fewer the number of cycles needed. The capacity difference between large and small crystallite/crystal compounds was ascribed to kinetic reasons as shown by ex situ x-ray diffraction analyses and quasi-equilibrium electrochemical studies. Capacities as high as 200 Ah/kg were found for approximate to 0.3 mu m crystal size materials. Contrary to the spinel prepared at high temperature, the electrochemically obtained spinel-like phase cycled Very well in the 2.5 to 4.3 V range, suggesting structural differences between the two materials. An extended x-ray absorption fine structure study at the manganese K edge confirmed this observation through a marked difference between the manganese second neighbors for two compounds. This can be related to the orthorhombic-to-cubic phase transition itself and/or to the memory effect of the stacking faults originally present in o-LiMnO2.