A cation disordered Li2MnO3 with NaCl type structure is found to show good electrochemical performance. The initial charge profile of the disordered Li2MnO3 shows a plateau at 4.1 V, while the ordered Li2MnO3 shows a plateau at 4.4 V. The good electrochemical activity affects the subsequent electrochemical performance, i.e., its initial charge and discharge capacities are 410 mA h g(-1) and 320 mA h g(-1). An iodometry and an X-ray fluorescence reveals that Mn ion in Li2MnO3 reduces from 4+ to ca. 3.7+ accompanying some amount of O-2 release from Li2MnO3, during the disordering process. Such oxygen deficiency might be one of the key factors that improves the initial electrochemical activity of the Li2MnO3(4+). Ex-situ XRD reveals that the lattice size of the disordered Li2MnO3 changes by about 3% during cycling. (C) 2018 The Electrochemical Society.