The Origin of Rapid Capacity Loss in 1,1,1-Trifluoroethyl Methyl Carbonate - Based Lithium-Ion Battery Electrolytes

1,1,1-trifluoroethyl methyl carbonate (FEMC) is a popular non-flammable solvent for lithium-ion battery electrolytes, although its high irreversible capacity means it can only be used with film-forming additives like fluoroethylene carbonate (FEC). This work studies the origin of the high irreversible capacity of FEMC-containing cells. Scanning electron microscopy and Raman spectroscopy of graphite anodes after charging and discharging in an FEMC electrolyte show evidence of significant physical and chemical graphite degradation, likely caused by solvent co-intercalation, which is probably responsible for a large portion of the capacity loss. X-ray photoelectron spectroscopy analysis of the anodes shows very low graphite signals, a sign of graphite degradation, formation of a thick solid electrolyte interphase (SEI), or both. When a small amount of FEC is added to FEMC, co-intercalation does not occur. FEC reduction occurs at a higher potential versus Li/Li+ than FEMC co-intercalation. It also forms a significantly different and thinner SEI containing more carbon, less fluorine, and no apparent FEMC decomposition products. Non-flammable lithium-ion battery electrolytes based on 1,1,1-trifluoroethyl methyl carbonate (FEMC) perform poorly as a result of solvent co-intercalation into graphite and formation of a blocking solid electrolyte interphase (SEI). The addition of a small amount of the film-forming electrolyte additive fluoroethylene carbonate (FEC) effectively passivates the graphite surface, protecting its structure, and leading to high performance. image