The physicochemical properties of an intermediate temperature ionic liquid (ITIL), NaTFSA (10mol%)–CsTFSA (90mol%) (TFSA=bis(trifluoromethylsulfonyl)amide), were investigated as a new electrolyte for a sodium secondary battery operating at elevated temperatures (around 423K). The viscosity, ionic conductivity, density and electrochemical window of this ITIL measured at 423K were 42.5cP, 12.1mScm−1, 2.29gcm−3 and 4.9V, respectively. The electrodeposition of metallic sodium at the cathode limit was confirmed by X-ray diffraction. The charge–discharge properties of a NaCrO2 positive electrode in this ITIL at 423K were investigated. The discharge capacities at constant current rates of 10mAg−1 (after 10 cycles) and 100mAg−1 (after 50 cycles) were 83.0 and 66.4mAhg−1, respectively. Except for the initial few cycles, the coulombic efficiencies were higher than 99.5% during the charge–discharge tests, which indicates that the Na/NaTFSA–CsTFSA/NaCrO2 cell is promising as a new type of sodium secondary battery operating at intermediate temperatures.