The development of renewable energy has become a high priority for environmental sustainability and energy independence. Some renewable energies (e.g., solar and wind), however, are intermittent, and require integration with energy storage systems to provide predictable and dispatchable supply of electricity. Redox flow batteries (RFBs) are excellent candidates for such grid-scale energy storage of electricity. However, the state-of-the-art RFBs have low volumetric and gravimetric energy densities (20 – 33 Wh/L and 15 – 25 Wh/kg) and their costs are high for many applications. To address these issues, we propose a novel concept of hybrid flow batteries consisting of a molten Na-Cs anode and an aqueous NaI catholyte separated by a NaSICON membrane. A number of carbonaceous electrodes are studied using cyclic voltammetry (CV) for their potentials as the positive electrode of the aqueous NaI catholyte. The charge transfer impedance, interfacial impedance and NaSICON membrane impedance of the Na-Cs ║ NaI hybrid flow battery are analyzed using electrochemical impedance spectroscopy (EIS). The performance of the Na-Cs ║ NaI hybrid flow battery is evaluated through galvanostatic charge/discharge cycles. This study demonstrates, for the first time, the feasibility of the Na-Cs ║ NaI hybrid flow battery and shows that the Na-Cs ║ NaI hybrid flow battery has the potential to achieve the following properties simultaneously: (i) an aqueous NaI catholyte with good cycle stability, (ii) a durable and low impedance NaSICON membrane for a large number of cycles, (iii) stable interfaces at both anode/membrane and cathode/membrane interfaces, (iv) a molten Na-Cs anode capable of repeated Na plating and stripping, and (v) a flow battery with high Coulombic efficiency, high voltaic efficiency, and high energy efficiency.