Investigation of Degradation Pathways in Fluoroethylene Carbonate Based Electrolytes via Chromatographic Techniques

For enhancing battery key performance indicators, like high voltage, safety, or lifetime, tailored electrolytes are crucial. Additives enable modifications of the solid electrolyte interphase (SEI) improving its electrochemical stability or mechanical strength. The formation and composition of the SEI still require further optimization. During operation, various side reactions of the electrolyte occur, resulting in continuous consumption of active material. The clarification of degradation mechanisms helps in understanding aging phenomena emerging during cycling. This article investigates different electrolytes formulations containing varying amounts fluoroethylene carbonate (FEC) for employment in high voltage applications. Long-term cycling revealed roll over effect occurring for the use of FEC and EC as cosolvents and highest capacity retention for FEC replacing EC. Subsequent post mortem electrolyte analysis by means of high-performance liquid chromatography coupled to quadrupole time of flight analyzer revealed the formation of FEC related degradation products. FEC undergoes subtraction of fluoride to form in situ VC which then continuously reacts with other electrolyte components. The identification of degradation products thus provides information about the degradation processes, confirmed for the first time by means of instrumental analytics.