Kinetics analysis of the electro-catalyzed degradation of high potential LiNi0,5Mn1,5O4 active materials

P. Dumaz, C.Rossignol, A.Mantoux, N.Sergent, R.Bouchet
One approach to increase the energy density of Li-ion batteries is to use high potential cathode material like LiNi0·5Mn1·5O4 (LNMO). However, it suffers from low coulombic efficiency, self-discharge and poor cyclability in carbonates-based electrolytes. Many mechanisms to explain degradation such as HF generation, surface catalytic activity and transition metals dissolution have been suggested to explain these behaviors. By comparison with a non-fluorinated environment, we demonstrated that hydrofluoric acid is not the main reason of capacity loss. A comparison of electrolyte degradation on model thin-film and composite electrodes proved that electrolyte oxidation is catalyzed on the active material surface of LNMO and not on the carbon. A Tafel like behavior of the electrolyte oxidation was obtained thanks to the measure of the steady state current at different potentials. The low coulombic efficiency is essentially related to the self-discharge mechanism. Finally, the capacity fading has been quantitatively correlated to the electrolyte oxidation: at 25 °C, about 4% of oxidized electrolyte molecules leads to the degradation of the material, probably due to the dissolution of surface transition metal. By lowering the operating temperature, the electrolyte degradation kinetics decreased, leading proportionally to better cycling stability. Perspectives of this work are also drawn.