A Fluorinated Ether Co-solvent Enables High Operating Temperature Li-ion Batteries

Li-ion batteries are commonly used as electrochemical energy storage systems due to their high energy density. However, few Li-ion batteries can reliably function at elevated temperatures, which is necessary for space and defense applications. In this study, Li-ion electrolytes were prepared and investigated for use at 100 °C. The previously developed baseline electrolyte, 1.0 M lithium hexafluorophosphate (LiPF 6 ) in 1:1 ethylene carbonate (EC):ethyl-methyl carbonate (v/v) with 2 wt% vinylene carbonate (VC), was altered to observe the effects of the lithium salts lithium difluoro(oxalato)borate (LiDFOB) and lithium difluorophosphate (LiDFP), and the fluorinated co-solvent 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE). The resulting formulations showed significantly improved capacity retention at 100 °C in multiple cell configurations. X-ray photoelectron spectroscopy characterization of the electrodes following cycling at high temperatures with the improved electrolyte revealed the cathode-electrolyte-interface to be boron-rich, while the graphite anodes were found to have little boron but were more fluorine rich compared to the baseline anodes. Raman spectroscopy determined notable changes in solvation structure upon addition of the TTE diluent. Overall, the use of various Li salts as well as the TTE co-solvent improved specific capacity retention at 100 °C in Li-ion cells.