Organofluorophosphates As Electrochemical Aging Products in Lithium Ion Battery Electrolytes

Abstract

The lithium ion battery (LIB) electrolyte is a delicate system susceptible to a manifold of influences both from the outside and inside. Thermal stress and overcharging of the LIB can result in unwanted side reactions such as rapid electrolyte degradation and the so-called thermal runaway, respectively. Here, the degradation of the conducting salt, namely lithium hexafluorophosphate (LiPF6 )poses many risks to the LIB and the environment. Hydrolysis of LiPF6 leads to the formation of hydrofluoric acid (HF) and POF3 which can react with the organic compounds within the electrolyte to form highly toxic organofluorophosphates (OFPs). The formation of OFPs in LIBs due to thermal stress has been covered in literature. In this study the electrochemically induced formation of OFPs is discussed. Electrolytes containing fluoroethylene carbonate (FEC) as a film-forming additive indicated the presence of OFPs forming at operation voltages of 4.8 V in self-assembled LIB coin cells. Cycling of the coin cells with cut-off voltages of 4.8 V gave rise to 15 non-acidic and two acidic OFPs after 10 charge/discharge steps. The quantity of FEC had an impact on the amount of OFPs formed which gives reason to incorporate FEC in the formation pathways proposed in this study. The formation pathway of OFPs through EC-polymerization proposed in literature is evaluated and an alternative mechanism with FEC as the carbonyl-carbon donor is presented. Structure elucidation and separation of the formed OFPs is achieved by utilization of a) hydrophilic interaction liquid chromatography (HILIC) for acidic OFPs and b) reversed-phase (RP) chromatography for non-acidic OFPs hyphenated to a high-resolution ion trap time-of-flight mass spectrometer (IT-TOF-MS). Based on the findings in this study and due to the highly toxic nature of OFPs further investigation of the formation of OFPs in FEC-containing electrolytes is necessary. Further, the impact of the operation voltages needs to be assessed to ensure for safe and efficient LIBs.