Electrolyte Solutions for Rechargeable Li-Ion Batteries Based on Fluorinated Solvents

Electrolyte solutions based on fluorinated solvents were studied in high-voltage Li-ion cells using lithium as the anode and Li1.2Mn0.56Co0.08Ni0.16O2 as the cathode. Excellent performance was achieved by replacing the conventional alkyl carbonate solvents in the electrolyte solutions by fluorinated cosolvents. Replacement of EC by DEC and by their fluorinated counterparts FEC, 2FEC, and fluorinated ether (F-EPE) considerably improved the cycling behavior of the cells charged up to 4.8 V. The improvement achieved is attributed to formation of a stable and protective surface films on the cathode particles due to unique surface reactions that are enabled by the nature of the fluorinated solvent molecules. The surface films formed on the lithiated transition metal oxide cathodes isolate the active mass, which is highly reactive toward the electrophilic alkyl carbonates, from continuous detrimental reactions with solution species. The positive effect of fluorinated electrolyte solutions on the performance of high-voltage cathodes was exploited in experiments with full graphite-Li1.2Mn0.56Co0.08Ni0.16O2 cells. Excellent cycling performance was recorded (1000 cycles) with solutions containing 2FEC, 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (F-EPE) and 1% TMSP, tris(trimethylsilyl)-phosphate (TMSP), which also provided very good results with Li1.2Mn0.56Co0.08Ni0.16O2 cells in shorter experiments. The extraordinary electrochemical stability of this electrolyte solution makes it a suitable candidate for other high-voltage cathode materials.