Effects of carbon-chain length of trifluoroacetate co-solvents for lithium-ion battery electrolytes using at low temperature

Trifluoroacetate is suitable as a co-solvent of rechargeable lithium-ion battery electrolyte for low temperature use. In this work, the following four trifluoroacetate solvents have been studied: (1) methyl trifluoroacetate (MTFA), (2) ethyl trifluoroacetate (ETFA), (3) n-butyl trifluoroacetate (NBTFA), (4) n-hexyl trifluoroacetate (NHTFA). Our efforts focus on the effect of carbon-chain length in trifluorinated acetateʹs molecular structure. These solvents have been incorporated into multi-component carbonate-based electrolytes and evaluated in lithium–graphite cells. FTIR spectrum has been used to analyze the dissociation of LiPF6 in a single co-solvent. The migration abilities of solvated Li+ have been characterized by ionic conductivities and viscosities. It could be concluded that the trifluoroacetate with long carbon-chain has a weak ability to dissociate LiPF6 salt into free ions, and simultaneously decreases mobility of solvated Li+ in the modified electrolyte. The charge–discharge test has shown a larger capacity shrink of lithium de-intercalation at low temperature and higher polarization potential on graphite electrode. As a low-temperature co-solvent, the carbon-chain length of alcohol group in trifluoroacetate structure should be selected as short as possible.