Relationship between surface chemistry and electrochemical behavior of LiNi1/2Mn1/2O2 positive electrode in a lithium-ion battery

The formation and the evolution of lithium-containing species on the surface of grains of a layered 4 V material such as LiNi1/2Mn1/2O2 along the electrochemical cycling have been followed using 7Li MAS NMR, electrochemical impedance spectroscopy (EIS) and XPS. Materials displaying different specific surface areas and stored in different atmospheres have been investigated in order to study the influence of the surface/volume ratio and the influence of the initial surface state, respectively. It is shown that the presence of an initial interphase of Li2CO3 influences the electrochemical behavior of the electrode, emphasizing the importance of the history of the electrode prior cycling. 7Li MAS NMR experiments performed upon cycling indicate the formation of interphase species in reduction and their partial removal in oxidation, indicating the dynamic character of the interphase upon cycling. Combined NMR, EIS and XPS experiments show the strong influence of the electrode/electrolyte interphase evolution on the electrochemical performance. Such results lead us to draw conclusions on the optimal storage conditions of layered 4 V materials for Li-ion batteries such as LiNi1/2Mn1/2O2.