Identification of solid electrolyte interphase formed on graphite electrode cycled in trifluoroethyl aliphatic carboxylate-based electrolytes for low-temperature lithium-ion batteries

Trifluoroethyl aliphatic carboxylates with different length of carbon-chain in acyl groups have been introduced into carbonate-based electrolyte as co-solvents to improve the low-temperature performance of lithium-ion batteries, both in capacity retention and lowering polarization of graphite electrode. To identify the further influence of trifluoroethyl aliphatic carboxylates on graphite electrode, the components and properties of the surface film on graphite electrode cycled in different electrolytes are investigated using Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and electrochemical measurements. The IR and XPS results show that the chemical species of the solid electrolyte interphase (SEI) on graphite electrode strongly depend on the selection of co-solvent. For instance, among those species, the content of RCOOLi increases with an increasing number of carbon atoms in RCOOCH2CF3 molecule, wherein R was an alkyl with 1, 3, or 5 carbon atoms. We suggest that the thickness and components of the SEI film play a crucial role on the enhanced low-temperature performance of the lithium-ion batteries.