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 degrees 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 vertical bar NaOH-NaI vertical bar 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. (C) The Author(s) 2015. Published by ECS. All rights reserved.