Electrochemical Cyclability of Orthorhombic LiMnO2 : Characterization of Cycled Materials

The electrochemical removal of lithium from orthorhombic 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 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 ≈0.3 μm 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 .