Quantification of PF5 and POF3 from Side Reactions of LiPF6 in Li-Ion Batteries

The electrochemical oxidation of LiPF6-based electrolytes is reported to generate POF3 gas. In order to enable a quantitative analysis of the LiPF6 decomposition reactions, we aimed to establish calibration factors for POF3 and PF5 in on-line electrochemical mass spectrometry (OEMS). Thermal decomposition of dry LiPF6 is expected to yield PF5, but instead all PF5 is detected as POF3 in our OEMS setup, rendering a differentiation of the two gases impossible and presenting an artefact which likely occurs with most on-line mass spectrometry systems due to the high reactivity of PF5. However, we can still determine a cumulative calibration factor for POF3 + PF5 (referred to as "POF3"), which is then used to investigate the evolution of gases during the oxidation of an EC/LiPF6 electrolyte on a carbon black electrode. Mechanistic experiments with protons or water added to EC/LiPF6 electrolyte show that protons trigger the formation of PF5, while the kinetics for the hydrolysis of LiPF6 with water at room temperature are too slow to be detectable. These findings let us conclude that the oxidation of EC generates highly acidic species, which cause the decomposition of PF6− to PF5 and HF; the PF5 is then detected as POF3 in the OEMS.