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

Abstract

The electrochemical oxidation of LiPF 6 -based electrolytes is reported to generate POF 3 gas.In order to enable a quantitative analysis of the LiPF 6 decomposition reactions, we aimed to establish calibration factors for POF 3 and PF 5 in on-line electrochemical mass spectrometry (OEMS).Thermal decomposition of dry LiPF 6 is expected to yield PF 5 , but instead all PF 5 is detected as POF 3 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 PF 5 .However, we can still determine a cumulative calibration factor for POF 3 + PF 5 (referred to as "POF 3 "), which is then used to investigate the evolution of gases during the oxidation of an EC/LiPF 6 electrolyte on a carbon black electrode.Mechanistic experiments with protons or water added to EC/LiPF 6 electrolyte show that protons trigger the formation of PF 5 , while the kinetics for the hydrolysis of LiPF 6 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 PF 6 -to PF 5 and HF; the PF 5 is then detected as POF 3 in the OEMS.