Influence of bis(2,2,2-trifluoroethyl) carbonate flame retarding co-solvent on interfacial chemistry in carbonate ester lithium-ion battery electrolytes†

The development of flame-retarding battery electrolytes may be achieved by the inclusion of non-flammable solvents in existing conventional electrolyte formulations. Here the use of one such promising solvent, bis(2,2,2-trifluoroethyl) carbonate (TFEC), mixed with conventional lithium-ion battery solvents ethylene carbonate and ethyl methyl carbonate, achieves comparable or superior electrochemical performance to a state-of-the-art benchmark (up to 90% capacity retention between 5th and 200th cycle, compared with 76% for the benchmark). Further electrochemical analysis indicates comparable cell resistance and rate capability, though a TFEC content beyond 90 vol% leads to increased resistance and rapid capacity fading. This was found to be caused by lithium trapping in the graphite electrodes and formation of a thinner solid electrolyte interphase with a distinct chemistry, as determined by X-ray photoelectron spectroscopy. TFEC's low Li⁺-solvating ability likely significantly influences these electrolytes' physico-chemical and electrochemical behaviour.