The structural changes of pristine and ZrO2-coated LiMn0.5Ni0.5O2 cathode materials were investigated by using in situ X-ray diffraction (XRD) during charging process. An obviously solid solution phase transition from a hexagonal structure (HI) to another hexagonal structure (H2) was observed during the charging process at a constant current of 0.3 mA in the potential range of 2.5-5.7 V. The second hexagonal structure has a shorter a-axis and a longer c-axis before the crystal collapse. Before the structure collapses the c-axis length increases to maximum and then significantly decreases to 14.1 angstrom. The c-axis length of the pristine and ZrO2-coated LiMn0.5Ni0.5O2 increases to the maximum at the charge capacity of 119.2 and 180.9 mAh g(-1), respectively. It can be concluded that the ZrO2 coating can strongly stabilize the crystal structure of the LiMn0.5Ni0.5O2 compound from the comparison of the lattice parameter variations between the pristine and the ZrO2-coated LiMn0.5Ni0.5O2 compounds upon charge. The potential fluctuation resulting from the decomposition of electrolytes starts at the charge capacity of around 200 and 260 mAh g(-1) for the pristine and ZrO2-coated LiMn0.5Ni0.5O2, respectively. It suggests that the ZrO2 coating layer can impede the reaction between the cathode material and electrolyte. (C) 2007 Elsevier B.V. All rights reserved.