Abstract
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.
Highlights
Lithium hexafluorophosphate (LiPF6) is commonly used as a salt in Li-ion battery electrolyte solutions due to its high ionic conductivity and passivating properties toward the aluminum cathode current collector.[1]
While dry LiPF6 is known to decompose to PF5 and HF, some POF3 was observed by TGAMS due to trace water impurities in the dry argon carrier gas
The thermal decomposition of wet LiPF6 in water-saturated argon carrier gas resulted in the formation of POF3, HF, and LiPO2F2
Summary
Lithium hexafluorophosphate (LiPF6) is commonly used as a salt in Li-ion battery electrolyte solutions due to its high ionic conductivity and passivating properties toward the aluminum cathode current collector.[1]. The hydrolysis of LiPF6 with trace water in Li-ion battery electrolyte solutions occurs already at room temperature, a complete conversion needs days to weeks.[17,18,19,20,21] The main product of LiPF6 Besides these two major decomposition pathways, LiPF6 affects the stability of the electrolyte solvents at high potentials. POF3 has been detected at high potentials in LiPF6-based electrolytes,[28,29,30,31,32,33] where it was ascribed to a reaction of LiPF6 or PF5 with water or other reactive oxygen-containing species formed during the oxidation of the electrolyte solvent. While this is plausible for experiments where water or reactive oxygen species are likely to be formed, i.e., in the presence of oxygen-releasing cathode materials,[28,29,34] the quantitative formation of POF3 during electrolyte oxidation on inert materials which do not release oxygen upon charge like high-voltage spinel or carbon black still lacks fundamental understanding.[31,32,33,34,35,36]
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