There is a demand for low-melting-point molten-salt electrolytes with high thermal and electrochemical stability for the development of high-performance sodium-ion batteries. Mixing sodium bis(fluorosulfonyl)amide (NaFSA) and sodium (fluorosulfonyl)(trifluoromethylsulfonyl)amide (NaFTA) results in a large depression in their melting points. In this study, the phase behavior and Na+ transport properties of binary mixtures of NaFSA and NaFTA were investigated. The mixture of NaFSA and NaFTA with a molar ratio of 8:2 has a melting temperature (T m) of 363 K, successfully achieving an ionic liquid consisting of single cationic (Na+) salts. This mixture easily forms a super-cooled liquid. The ionic conductivity (σ) of Na[(FSA)0.8(FTA)0.2] continuously varied from above T m to below T m, obeying the Vogel–Tamman–Fulcher equation, which coincides with its super-cooling nature. The ionic conductivity and apparent Na+ transference number (t Na+) under anion-blocking conditions at T m approached 10−3 S cm−1 and 0.92, respectively.