The liquid metal battery (LMB) is attractive due to its simple construction, its circumvention of solid-state failure mechanisms and resultantly long lifetimes, and its particularly low levelized cost of energy. Here, we provide a study of a unique binary electrolyte, NaOH-NaI, in order to pursue a low-cost and low-temperature sodium-based liquid metal battery (LMB) for grid-scale electricity storage. Thermodynamic studies have confirmed a low eutectic melting temperature (220 • C) as well as provided data to complete the phase diagram of this system. X-ray diffraction has further supported the existence of a recently discovered compound, Na 7 (OH) 5 I 2 , as well as offered initial evidence toward a NaI-rich compound displaying Pm-3m symmetry. These phase equilibrium data have then been used to optimize parameters from a two-sublattice thermodynamic solution model to provide a starting point for study of higher order systems. Further, a detailed electrochemical study has identified the voltage window and related oxidation/reduction reactions and found greatly improved stability of the pure sodium electrode against the electrolyte. Finally, an Na|NaOH-NaI|Pb-Bi proof-of-concept cell was assembled. This cell achieved over 100 cycles and displayed leakage currents below 0.40 mA/cm 2 . These results highlight an exciting class of low-melting molten salt electrolytes that may enable low cost grid-scale storage. The pressing need for highly scalable and economically viable battery systems for grid-storage has prompted many researchers to pursue entirely new electrochemical approaches. One such technology that has recently grown from the university lab bench to the commercial production floor is the liquid metal battery (LMB) 1,2 -a system that takes advantage of a three liquid-layer design to store and deliver large quantities of energy at particularly low levelized costs.The LMB is designed with an electropositive liquid metal anode, A, separated from an electronegative liquid metal cathode, B, by a molten salt electrolyte. Upon discharge, the anode, A, oxidizes, transports across the A-itinerant electrolyte, and reduces at the cathode interface to form an A-B alloy. The reaction is driven by the partial free energy difference of A in the high activity negative electrode environment versus that of the alloyed metal A (in B) in the positive electrode.Because all three active battery components are liquid phase, the system is able to operate at high current densities with minimal overpotential losses. In addition, because the cell is restored to its virgin liquid state upon each recharge, the device is immune to solid-state failure mechanisms common in lithium-ion batteries 3 and, as a result, is expected to provide exceptionally long amortizable service lifetimes.In spite of the benefits of the LMB system, the high temperatures required to achieve a fully molten state present challenges when scaling the battery to production. Higher temperatures drive costs due to issues associated with device sealing, expensive wiri...