Molten Salt Electrolyte for Na-ZnCl2 All Liquid Battery for Grid Storage

Zeolite Battery Research Africa (ZEBRA) batteries (Na-NiCl2 solid electrolyte batteries, SEBs) have commercial applications in, e.g., energy storage due to their low costs and recyclability, long lifetime and high safety. In commercial ZEBRA batteries, Ni electrode and beta’’-alumina solid electrolyte (BASE) have a > 70% share of the cell material costs. Na-ZnCl2 all liquid batteries (ALBs), which replace Ni with abundant and low-cost Zn and BASE electrolyte with molten salt electrolyte, could have lower costs, longer lifetime and higher safety, are therefore promising as grid storage. However, compared to SEBs, ALBs are in an early development stage, particularly on their molten salt electrolytes, which have significant effect on the battery performance. The physical and chemical properties of the salt electrolyte like melting temperatures and solubilities of electrode materials (i.e., Na and Zn metal) are vital for the molten salt electrolyte selection, battery cell design and optimization. In this work, the binary and ternary phase diagrams of salt mixtures containing NaCl, CaCl2, BaCl2, SrCl2, KCl, obtained via FactSage simulation and DSC measurements and the solubilities of electrode materials are presented and used for the selection of the molten salt electrolyte. There are multiple criteria for the selection of the molten salt electrolyte, including high electromotive force (EMF) for suitable electrochemical properties, low melting temperature for large charge/discharge range, low solubilities of electrode materials for low self-discharge, low material costs and high material abundance for easy scale-up. According to these criteria, the NaCl-CaCl2-BaCl2 and NaCl-SrCl2-KCl salt mixtures are selected as the two most promising ALB molten salt electrolytes, which are suggested to be tested in the ALB demonstrators under development.