Organofluorophosphates as Oxidative Degradation Products in High-Voltage Lithium Ion Batteries with NMC or LNMO Cathodes

Abstract

Organofluorophosphates (OFPs) have been reported to pose substantial health hazards due to their structural similarities to pesticides and nerve agents. Formation of OFPs in lithium ion batteries (LIBs) due to hydrolysis of the conducting salt lithium hexafluorophosphate (LiPF6) and the reaction with the organic carbonate solvents that make up the electrolyte has been discussed in literature. The oxidative formation of OFPs in electrolytes containing fluoroethylene carbonate (FEC) and vinylene carbonate (VC) as film-forming additives is presented in this study. Further the impact of potentially reactive positive electrode surfaces is investigated with the layered metal oxide NCM622 which is ascribed to release reactive oxygen species at high voltages and the spinel type LNMO as a typical high-voltage material. Cycling of the self-assembled LIB coin cells (CR2032) at cut-off voltages of 4.8 V gave rise to a number of degradation products including potentially highly toxic OFPs. Here, the presence of the film-forming additive had a massive impact on the amount of OFPs formed during electrochemical cycling experiments, which raises further concerns for the utilization of film-forming additives for high voltage applications. The formation pathway of OFPs through EC-polymerization proposed in literature is evaluated and an alternative mechanism with FEC/VC as the carbonyl carbon-donor is presented. Structure elucidation and separation of the formed OFPs is achieved by utilization of reversed-phase (RP) chromatography hyphenated to a high-resolution ion trap time-of-flight mass spectrometer (IT-TOF-MS). The findings presented in this study support further investigation of the formation of OFPs in film-forming additive-containing electrolytes, quantitative approaches and toxicological assessments due to the highly toxic nature of OFPs.