In this study, we will show how the oxygen release depends on the Li2MnO3 content of the material and how it affects the actual voltage fading of the material. Thus, we compared overlithiated NCMs (x Li2MnO3 center dot (1-x) LiMeO2 ; Me = Ni, Co, Mn) with x = 0.33, 0.42 and 0.50, focusing on oxygen release and electrochemical performance. We could show that the oxygen release differs vastly for the materials, while voltage fading is similar, which leads to the conclusion that the oxygen release is a chemical material degradation, occurring at the surface, while voltage fading is a bulk issue of these materials. We could prove this hypothesis by HRTEM, showing a surface layer, which is dependent on the amount of oxygen released in the first cycles and leads to an increase of the charge-transfer resistance of these materials. Furthermore, we could quantitatively deconvolute capacity contributions from bulk and surface regions by dQ/dV analysis and correlate them to the oxygen loss. As a last step, we compared the gassing to the base NCM (LiMeO2 , Me = Ni, Co, Mn), showing that surface degradation follows a similar reaction pathway and can be easily modulated by controlling the amount of Li2MnO3. (C) The Author(s) 2018. Published by ECS.