Electrochemical and ex situ X-ray study of Li-(Li0.2Ni0.2Mn0.6)O2 cathode material for Li secondary batteries

Electrochemical properties of Li(Li0.2Ni0.2Mn0.6)O-2, synthesized by a sol-gel method, were studied by galvanostatic cycling. When the Li/Li(Li0.2Ni0.2Mn0.6)O-2 cell was cycled at 2.0-4.6 V, the material showed a long, irreversible plateau at 4.5-4.6 V and exhibited very high initial charge capacity of 240 mAh/g, which is much larger than a theoretical value for the reaction of Li(Li0.2+Ni0.22+Mn0.64+)O-2 --> Li-0.6(Li0.2+Ni0.24+Mn0.64+)O-2 + 0.4Li (126 mAh/g). Discharge capacity of the material gradually increased from 155 at the first cycle to 205 mAh/g at the tenth cycle and was stabilized afterward. From the ex situ X-ray study, we found that Li ions in the transition metal layer were irreversibly extracted during charging at a voltage higher than 4.5 V; the diffraction peaks corresponding to Li in the transition metal layer were hardly observed in the cathode material after extended cycling. The Li/Li(Li0.2Ni0.2Mn0.6)O-2 cell that was cycled at 2.0-4.3 V showed stable but very small capacity (69 mAh/g) and Li in the transition metal layer remained almost intact. When the cell was initially charged to 4.6 V and then cycled at 2.0-4.3 V, the cathode material showed higher capacity (114 mAh/g) than the one cycled at 2.0-4.3 V. (C) 2003 The Electrochemical Society.