Electrochemical behavior of Li intercalation processes into a Li[NixLi(1/3-2x/3)Mn(2/3-x/3)]O2 cathode

Li[NixLi(1/3 - 2x/3)Mn(2/3 - x/3)]O-2 (X = 0.17, 0.25, 0.33, 0.5) compounds are prepared by a simple combustion method. The Rietvelt analysis shows that these compounds could be classified as having the alpha-NaFeO2 structure. The initial charge-discharge and irreversible capacity increases with the decrease of x in Li[NixLi(1/3 - 2x/3)Mn(2/3 - x/3)]O-2. Indeed, Li[Ni0.50Mn0.50]O-2 compound shows relatively low initial discharge capacity of 200 mAh/g and large capacity loss during cycling, with Li[Ni0.17Li0.22Mn0.61]O-2, and Li[Ni0.25Li0.17Mn0.58]O-2 compounds exhibit high initial discharge capacity over 245 mAh/g and stable cycle performance in the voltage range of 4.8-2.0 V. On the other hand, XANES analysis shows that the oxidation state of Ni ion reversibly changes between Ni2+ and about Ni3+, while the oxidation state of Mn ion sustains Mn4+ during charge-discharge process. This result does not agree with the previously reported 'electrochemistry model' of Li[NixLi(1/3 - 2x/3)Mn(2/3 - x/3)]O-2, in which Ni ion changes between Ni2+ and NI4+. Based on these results, we modified oxidation-state change of Mn and Ni ion during charge-discharge process. (C) 2003 Elsevier Ltd. All rights reserved.