Cation Mixtures of Alkali Metal (Fluorosulfonyl)(trifluoromethylsulfonyl)Amide as Electrolytes for Lithium Secondary Battery

Cation mixtures of alkali metal (fluorosulfonyl)(trifluoromethylsulfonyl)amides showed reduced melting points and improved transport properties from the single lithium salt (Li[fTfN]) by the addition of other alkali metal cations. In particular, Li 0.4 Cs 0.6 [fTfN] has a significantly reduced melting point (from 100°C to 33°C), decreased viscosity (from 17 Pa·s to 330 mPa·s at 110°C) and enhanced specific conductivity (from 0.20 mS·cm −1 to 2.9 mS·cm −1 at 110°C). This melt is an ionic liquid at room temperature range although it contains no organic cation. Its electrochemical stability is 5.1 V from deposition of lithium metal to decomposition of fTfN anion. The charge-discharge of a Li 0.4 Cs 0.6 [fTfN] in Li ∥ LiCoO 2 and Li ∥ LiFePO 4 cell was successful at 45∼110°C. In particular, rate performance and cycle stability with LiFePO 4 were superior to an organic electrolyte at the same temperature due to its high thermal stability.