An all-fluorinated electrolyte with 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetra-fluoropropylether as co-solvent for lithium-ion cells with advanced electrochemical and safety properties at high voltage

To enable high-performance lithium-ion cells at high voltage, this work proposes a series of all-fluorinated electrolytes with fluoroethylene carbonate (FEC), bis (2,2,2-trifluoroethyl) carbonate (TFEC), and 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropylether (TTE) as co-solvents. It is found that a moderate increase in lithium salt concentration within the FEC-TFEC-TTE solvent enhances the long-term cycling performance of cells at high voltage, while excessive lithium salt cannot further improve the cells' performance but leads to high cost and viscosity and harms the cells' high-temperature property. Thus, the blend named FTT1.5, i.e., 1.5 M lithium hexafluorophosphate (LiPF6) in FEC, TFEC and TTE with a ratio of 2:6:2, is selected, which outperforms not only the commercial electrolyte (EE) but another all-fluorinated electrolyte, i.e., 1.5 M LiPF6 in FEC, methyl (2,2,2trifluoroethyl) carbonate (FEMC), and TTE with a ratio of 2:6:2, implying that TFEC cooperates better with FEC and TTE than FEMC. This mainly results from the low binding energy between TFEC and Li+ and the enhanced solvation structure that enables fast Li+ desolvation, reduces lithium plating, and suppresses parasitic reactions at high voltage. The strengthened combination between FEC and TTE with Li+ attributes to the improvement of the anode stability and the generation of reliable and thin passivation layers rich in lithium fluoride. As a result, side reactions at high voltage are successfully restrained, to further obtain high-voltage cells with promising electrochemical performance. Besides this, both the all-fluorinated electrolyte and the system utilizing this electrolyte demonstrate more advanced safety properties when subjected to typical abusing scenarios, with no thermal runaway.