As a positive electrode material for Li-ion batteries, xLi(2)MnO(3)center dot(1 - x)LiMO2 (M = Mn, Ni, Co) boasts various advantages compared to LiCoO2, such as higher specific charge (similar to 250 Ah/kg) and lower cobalt content. However, the oxygen release reaction at similar to 4.5 V vs. Li+/Li is not fully understood. xLi(2)MnO(3)center dot(1 - x)LiMO2 has been characterized by a combination of complementary methods, including electrochemical cycling at different temperatures, differential electrochemical mass spectrometry (DEMS), in situ pressure measurement, and in situ temperature measurement. The reaction at similar to 4.5 V vs. Li+/Li leads to the exothermic release of oxygen and a significant pressure build-up in the cell. Despite this, the electrochemical performance of the material is very promising. Finally, it is shown that the cells should be operated close to room temperature. (C) 2013 Elsevier Ltd. All rights reserved.