Understanding electrolyte decomposition of graphite/NCM811 cells at elevated operating voltage

Cells containing LiNi_xCo_yMn_1-x-yO₂ (NCM) materials with very high nickel content suffer from cathode material degradation upon cycling, especially when operating these cells at higher voltage. While the cathode particle degradation is a problem other issues contribute to electrochemical performance loss. Graphite/LiNi_0.8Co_0.1Mn_0.1O₂ cells have been cycled to different upper cutoff voltages and investigated with electrochemical impedance spectroscopy (EIS), gas analysis with online electrochemical mass spectroscopy (OEMS), nuclear magnetic resonance (NMR) spectroscopy, gas chromatography with MS (GCMS) and inductively coupled plasma MS (ICP-MS). Increased gas evolution was observed when the upper cutoff voltage was increased from 4.2 to 4.6 V. An increase in O₂ evolution upon cycling as well as an increase of CO₂ upon increasing the voltage, have been observed. More electrolyte decomposition products are observed on the anode for cells cycled to higher voltage. An increased amount of nickel has been detected on the graphite electrode in accordance with an increased reaction of the cathode surface with the electrolyte leading to transition metal dissolution. Finally, an increase in carbonate solvent transesterification products has been observed.