Cell design strategies for sodium-zinc chloride (Na-ZnCl2) batteries, and first demonstration of tubular cells with 38 Ah capacity

Abstract

At present, high-temperature sodium-nickel chloride (Na-NiCl2) batteries with nickel-based cathode are produced for specialty markets (e.g. backup power supply) at relatively low volumes. Replacing the nickel in the cathode by cheap and abundant zinc may enable this technology to provide sustainable large-scale energy storage for stationary applications at competitive cost. In this study, we derive design strategies from state-of-the-art Na-NiCl2 cells, and transfer them to Na-ZnCl2 cells. First, we evaluate the influence of cathode compartment filling level, sodium tetrachloroaluminate (NaAlCl4) secondary electrolyte volume, and active metal content on the pressure evolution during battery cell cycling. Based on this evaluation, we define a suitable Zn/NaCl cathode composition for integration into state-of-the-art high-temperature cells with tubular geometry. Second, we present cycling results of Na-ZnCl2 cells with 38 Ah capacity and 0.9 g/cm2 mass loading, corresponding to an areal capacity of 146 mAh/cm2 (100% state-of-charge, SOC). During operation at 280-300°C, tubular cells cycled to high SOC (97-100% SOC) exhibit voltage fading with each cycle. We show that this voltage fading is related to the coarsening and segregation of zinc metal at the cathode, while conductivity and crystalline phase content of the ceramic Na-β''-Al2O3 electrolyte are not significantly altered. In contrast, cell voltage and cathode structure remain stable when keeping the cell below 80% SOC. Under these cycling conditions, a cumulative capacity of >550 Ah (22 cycles, 2.1 Ah/cm2) was transferred at discharge current densities of 10 mA/cm2 (C/15). Our experiments demonstrate successful integration of zinc electrodes in state-of-the-art tubular cells at commercially relevant mass loadings.