The physicochemical properties of molten alkali bis(trifluoromethylsulfonyl)amide, MTFSI (M=Li, K, Cs), mixture (x LiTFSI =0.20, x KTFSI =0.10, x CsTFSI =0.70) were studied to develop a new rechargeable lithium battery operating at intermediate temperature (100–180°C). The viscosity and ionic conductivity of this melt at 150°C are 87.2cP and 14.2mScm−1, respectively. The cyclic voltammetry revealed that the electrochemical window at 150°C is as wide as 5.0V, and that the electrochemical deposition/dissolution of lithium metal occurs at the cathode limit. A Li/MTFSI (M=Li, K, Cs)/LiFePO4 cell showed an excellent cycle performance at a constant current rate of C/10 at 150°C; 95% of the initial discharge capacity was maintained after 50 cycles. Except for the initial few cycles, the coulombic efficiencies were approximately 100% for all the cycles, indicating the stabilities of the molten MTFSI mixture and all the electrode materials.