Differential Electrochemical Mass Spectrometry Study of the Interface of xLi2MnO3•(1-x)LiMO2 (M = Ni, Co, and Mn) Material as a Positive Electrode in Li-Ion Batteries

Lithium-rich mixed metal layered oxides constitute a large class of promising high-potential positive electrode materials in which higher specific charges are accessed only by activation of the Li2MnO3 domains. During the activation, oxygen is extracted from the oxide and evolves at the electrodeelectrolyte interface. Differential electrochemical mass spectrometry was employed to follow volatile species developed during cycling. Although typical Li-ion aprotic carbonate electrolytes already suffer from oxidative decomposition at high potentials, the presence of O-2 is here confirmed to enhance its reactivity. During the first cycle, O-2 and CO2 evolve and their respective amounts vary as a function of the cycling conditions. However, for ethylene carbonate-based electrolytes, the amount of O-2 and CO2 is found to be independent of the electrolyte composition. Moreover, X-ray photoelectron spectroscopy revealed that carbon-based components of the solid layers are dissolved between 3.0 and 4.0 V versus Li+/Li where no gas is evolving.