Li[Ni x Li ð1=3 2 2x=3Þ Mn ð2=3 2 x=3Þ ]O 2 (X ¼ 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 a-NaFeO 2 structure. The initial charge-discharge and irreversible capacity increases with the decrease of x in Li[Ni x Li ð1=3 2 2x=3Þ Mn ð2=3 2 x=3Þ ]O 2 . Indeed, Li[Ni 0.50 Mn 0.50 ]O 2 compound shows relatively low initial discharge capacity of 200 mAh/g and large capacity loss during cycling, with Li[Ni 0.17 Li 0.22 Mn 0.61 ]O 2 and Li[Ni 0.25 Li 0.17 Mn 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 2þ and about Ni 3þ , while the oxidation state of Mn ion sustains Mn 4þ during chargedischarge process. This result does not agree with the previously reported 'electrochemistry model' of Li[Ni x Li (1/322x/3)- Mn (2/32x/3) ]O 2 , in which Ni ion changes between Ni 2þ and NI 4þ . Based on these results, we modified oxidation-state change of Mn and Ni ion during charge -discharge process.