As a positive electrode material for Li-ion batteries, xLi2MnO3·(1− x)LiMO2 (M=Mn, Ni, Co) boasts various advantages compared to LiCoO2, such as higher specific charge (∼250Ah/kg) and lower cobalt content. However, the oxygen release reaction at ∼4.5V vs. Li+/Li is not fully understood. xLi2MnO3·(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 ∼4.5V 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.