Lithium nickel manganese oxide of LiNi1/2Mn1/2O2, which is a one-to-one mixture of LiNiO2 and LiMnO2 in its chemical composition, has been prepared by an improved processing method. Cycle tests of a lithium cell with LiNi1/2Mn1/2O2 operated in voltages of 2.5-4.5 V shows about 200 mAh g(-1) of rechargeable capacity without noticeable loss during 30 cycles. The thermal behavior of fully charged LiNi1/2Mn1/2O2 is also examined by differential scanning calorimetry (DSC) and shows that the exothermic reaction of square(0.89)Li(0.11)Ni(1/2)Mn(1/2)O(2) (fully charged state under this experimental condition) is milder than that of squareNiO(2) or squareMn(2)O(4). The results on rate-capability tests of a Li/LiNi1/2Mn1/2O2 cell show that 85% of discharge capacity observed at low rate discharge can be stored and delivered at 1 h rate at present. (C) 2003 Elsevier Science B.V. All rights reserved.