Impact of Aluminum Trifluoride AlF₃ Impurities in Electrolyte on NMC₈₁₁/Graphite Lithium-Ion Battery Performance

The impact of aluminum fluoride (AlF3) as electrolyte impurity in lithium-ion battery cells has been studied. Deteriorated capacity retention of NMC811/Graphite full cells is observed especially when they are operated at high temperature. We found an acceleration of electrolyte degradation during battery cell operation in presence of AlF3 impurities. The NMC811 cathode and Graphite anode electrodes were characterized by cross-section scanning electron microscopy focused-ion beam, Raman spectroscopy, and X-ray photoelectron spectroscopy methods. Cells with AlF₃, especially when cycled at elevated temperatures, form polymers on the cathode and anode surfaces, likely due to electrolyte degradation. Corresponding pathways for the reaction forming the SEI and CEI layers are postulated. In particular, we proposed a formation path for polyethylene and polyethylene glycol polymers on the anode and cathode surfaces due to the presence of aluminum alkoxide as a catalyst activated by the AlF3 impurities.