Impact of water on toxic hydrogen fluoride generation from the decomposition of LiPF6 in lithium-ion battery electrolytes

PresentationToxic hydrogen fluoride (HF) gas can be generated when LiPF6, a salt used in lithium-ion battery electrolytes, thermally decomposes and/or reacts with trace water. Simultaneous thermal analysis and mass spectrometry (STA-MS) was conducted on five different organic solvents containing LiPF6 to determine the temperatures at which HF is generated and the activation energies of the decomposition reaction. STA-MS allows the simultaneous direct observation of electrolyte thermal stability and hydrogen fluoride generation, something that is not possible with gas chromatography–mass spectrometry alone, thus it represents a more efficient and simple experimental approach. The five solvents tested in this study were anhydrous tetrahydrofurfuryl alcohol (THFA), 1,3- dioxolane (1,3-DL), diethyl carbonate (DEC), 1,2-dimethoxyethane (DME), and ethyl carbonate (EC). STA-MS analysis of the LiPF6 in these solvents revealed that HF generation occurred at different temperatures for each electrolyte. In the case of 1M LiPF6 in THFA, the addition of 1000 ppm of water reduced the thermal decomposition temperature compared to solid neat LiPF6. Except for EC, all of the other electrolyte systems exhibited a lower HF generation temperature and a lower reaction activation energy (Ea) when water was present. Additionally, from a risk assessment perspective, the results indicate that the HF generation starts from the SEI layer decomposition stage which occurs early in the thermal runaway mechanism of the lithium-ion battery. This research can be used to develop more thermally stable and safer electrolytes in the future, especially with respect to HF generation. In addition, this study highlights the need for research into measures to combat large-capacity lithium-ion battery fires, which may occur in electric vehicles and grid-scale energy storage systems