This paper first derives a model to describe a class of Na–metal–halide secondary batteries, using molten sodium as the anode, NASICON as the sodium-ion-conducting separator, and copper-iodide chemistry in an aqueous electrolyte for the cathode. The model is based upon solving transient conservation equations using a Nernst–Planck–Poisson (NPP) formulation. The broad objective is to develop a predictive model that can assist the design and development of large-scale grid-storage batteries. However, the model-predicted results and discussion are focused on a laboratory-scale battery. Several examples are discussed, considering the effects of current density and catholyte molar concentrations on battery performance.