Electrochemical behavior of Li intercalation processes into a Li[Ni xLi (1/3−2 x/3) Mn (2/3− x/3) ]O 2 cathode

Abstract

Li[Ni x Li (1/3−2 x/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 α-NaFeO 2 structure. The initial charge–discharge and irreversible capacity increases with the decrease of x in Li[Ni x Li (1/3−2 x/3) Mn (2/3− x/3) ]O 2. Indeed, Li[Ni 0.50Mn 0.50]O 2 compound shows relatively low initial discharge capacity of 200 mAh/g and large capacity loss during cycling, with Li[Ni 0.17Li 0.22Mn 0.61]O 2 and Li[Ni 0.25Li 0.17Mn 0. 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 Ni 2+ and about Ni 3+, while the oxidation state of Mn ion sustains Mn 4+ during charge–discharge process. This result does not agree with the previously reported ‘electrochemistry model’ of Li[Ni x Li (1/3−2x/3)Mn (2/3−x/3)]O 2, in which Ni ion changes between Ni 2+ and NI 4+. Based on these results, we modified oxidation-state change of Mn and Ni ion during charge–discharge process.