Lithium Salt Mixture with Fluorinated Ether Co-solvent Improves High Temperature Performance in Lithium-ion Batteries

Lithium-ion batteries are widely used as rechargeable batteries due to their relatively high energy density and excellent cycling efficiency. However, elevated temperatures (>50°C) cause decreased performance of the battery, including long-term cycle life and safety. Previously, a Li-ion battery with an electrolyte consisting of 1.0 M lithium hexafluorophosphate (LiPF6) in ethylene carbonate (EC):ethyl-methyl carbonate (EMC) (1:1 v/v) with a 2 wt. % vinylene carbonate (VC) additive had over 30% initial capacity retention after 40 cycles at 80 °C, outperforming other electrolyte compositions in the study. Lithium difluoro(oxalato)borate (LiDFOB) and lithium difluorophosphate (LiDFP) have previously been reported to improve the operating temperature range of lithium-ion batteries. Additionally, a fluorinated co-solvent, 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE), has been shown to improve oxidative stability. Electrolytes were screened using the baseline with addition of LiDFOB, LiDFP and TTE. A composition of 0.125 M LiDFOB, 0.5 M LiPF6 in 1:1:2 EC:EMC:TTE + 2 wt. % VC, 2wt. % LiDFP, was able to reach the 100 cycle limit from 4.2V to 2.5V at a C/5 rate in 2 out of 3 cells tested at 100°C and retain 77% of the initial capacity after the first 15 cycles, compared to only 50% after 15 cycles for the baseline electrolyte. Stepped temperature experiments at 40 °C, 60 °C, 80 °C and 100 °C show the TTE-based electrolyte was outperformed by the baseline up to 80 °C, but crucially, significantly improved battery performance at 100°C. Spiral wound three-electrode cells were constructed to better understand the independent processes occurring on the anode and the cathode. EIS and XPS analysis suggest low impedance film formation on the cathode results in improved capacity retention and prevents loss of lithium inventory. This work shows electrolyte design can lead to significant improvements in 100 °C cycling performance of lithium-ion batteries.