Chemical aspects of the degradation of lithium-ion batteries based on layered oxide LiNi0.6Mn0.2Co0.2O2 and graphite

The change in the composition of the electrolyte after life cycle testing (cycling) of lithium-ion batteries (LIBs) was studied. The cell with a nominal capacity of 22 A h was composed of a cathode based on nickel-rich layered lithium oxide LiNi0.6Mn0.2Co0.2O2 (NMC622) and an anode based on graphite. NMR and high-resolution mass spectrometry demonstrated the continuous decomposition of dimethyl carbonate and ethyl methyl carbonate, related to the disruption of the formation of protective surface layers on the graphite electrode. The degradation of the LIB is related to the formation of polyethylene oxide oligomers of various compositions as a result of the decomposition of the electrolyte components and the precipitation of the salt MeOCO2Li, which is poorly soluble in carbonate solvents, on the separator. A water content of more than 20 ppm in the electrolyte leads to the hydrolysis of the salt LiPF6 with the formation of HPO2F2 and HF. The presence of HF facilitates the dissolution of the components of the surface film at the graphite/electrolyte interface with the regeneration of H2O and the formation of a "fresh" surface on the graphite, which participates in the electrochemical decomposition of the carbonate solvents. Organophosphate C2H5O4P is formed upon the interaction of the electrolyte components with HF.