A novel non-flammable electrolyte containing methyl nonafluorobutyl ether for lithium secondary batteries

Use of nonflammable fluorinated ethers as electrolytes for lithium secondary batteries has been studied in terms of the flammability, ionic conductivity and cell charge–discharge performances including the rate capability, cycle life and abuse (nailing) test of a graphite/LiCoO2 cylindrical cell. By mixing appropriate amounts of methyl nonafluorobutyl ether (MFE) with cosolvents (e.g., ethyl methyl carbonate (EMC) and diethyl carbonate (DEC)), the mixed solution showed no flash point when evaluated by the Cleaveland open-cup flash point method (JIS 2265). The ionic conductivity was investigated for various electrolytes containing MFE with some lithium salts including LiN[SO2C2F5]2 (LiBETI) and LiN[SO2CF3]2 (LiTFSI). The solution properties of the electrolytes containing MFE were characterized in terms of the NMR chemical shifts and the diffusion coefficients by using the NMR pulse field gradient method. The graphite/LiCoO2 cylindrical cells assembled with 1 mol dm−3 LiBETI–MFE/EMC (80:20 vol %) discharged the designed capacity (1400 mAh) at a 0.1 C rate and sustained 80% of their initial capacity up to 50 cycles. No thermal runaway was detected and cell surface temperature increased very slowly in the nailing test which meant hardly any software and hardware protections were necessary.