Operando gas chromatography mass spectrometry for the continuous study of overcharge-induced electrolyte decomposition in lithium-ion batteries

Electrolyte decomposition, which occurs during operation of state-of-the-art Li-ion batteries (LIB), leads to the formation of a complex mixture of volatile chemical compounds. Here, a new method for operando gas chromatography mass spectrometry (GCMS) was developed to allow the time resolved investigation of gas mixtures evolving from a NMC811/graphite cell under normal and critical battery operation conditions. Decomposition of the carbonate-based electrolyte (1 M LiPF6/EC-EMC/2 % VC) led to the formation of up to 39 different volatile chemical compounds, which were classified into fluorinated hydrocarbons, hydrocarbons, carbon oxides, carbonyls, alcohols, ethers, fluoroalkyl silanes, carbonates, oxygen and water. Ethene was found as the most abundant hydrocarbon during cell formation. The onset potential corresponding to the evolution of the remaining gas species during charge was 4.6 V and coincided with a drop in potential related to dendrite formation or SEI decomposition. However, carbonyls and ethers showed the highest level of gas formation much later after the overcharging step. Fluorinated hydrocarbons were chromatographically separated to follow the decomposition of LiPF6. This work gives a comprehensive overview of electrolyte decomposition reactions and volatile chemical compounds formed in LIB after cell formation and while operating at an elevated potential.