Influence of precursor morphology and cathode processing on performance and cycle life of sodium-zinc chloride (Na-ZnCl2) battery cells

Abstract

Replacing nickel by cheap and abundant zinc may enable high-temperature sodium-nickel chloride (Na-NiCl2) batteries to become an economically viable and environmentally sustainable option for large-scale energy storage for stationary applications. However, changing the active cathode metal significantly affects the cathode microstructure, the electrochemical reaction mechanisms, the stability of cell components, and the specific cell energy. In this study, we investigate the influence of cathode microstructure on energy efficiency and cycle life of sodium-zinc chloride (Na-ZnCl2) cells operated at 300 °C. We correlate the dis-/charge cycling performance of Na-ZnCl2 cells with the ternary ZnCl2-NaCl-AlCl3 phase diagram, and identify mass transport through the secondary NaAlCl4 electrolyte as an important contribution to the cell resistance. These insights enable the design of tailored cathode microstructures, which we apply to cells with cathode granules and cathode pellets at an areal capacity of 50 mAh/cm2. With cathode pellets, we demonstrate >200 cycles at C/5 (10 mA/cm2), transferring a total capacity of 9 Ah/cm2 at >83% energy efficiency. We identify coarsening of zinc particles in the cathode microstructure as a major cause of performance degradation in terms of a reduction in energy efficiency. Our results set a basis to further enhance Na-ZnCl2 cells, e.g., by the use of suitable additives or structural elements to stabilize the cathode microstructure.