Design guidelines for next-generation sodium-nickel-chloride batteries

Sodium-nickel-chloride batteries have a proven track record for backup power applications, but also show great potential for large-scale stationary electricity storage currently dominated by lithium-ion batteries. While lithium-ion cells rely on critical cobalt and lithium, sodium-nickel-chloride batteries are based on abundant, non-critical sodium chloride, nickel, and alumina, and are thus ideally suited for large-scale deployment. However, to be competitive with lithium-ion batteries, the charge and discharge rate capabilities of sodium-nickel-chloride batteries need to be improved and cell production cost needs to be reduced. In this PhD thesis, rate limiting processes in state-of-the-art sodium-metal-chloride cells are assessed and measures are proposed to improve their performance and reduce cost. On the negative electrode, plating and stripping of liquid sodium metal from a ceramic Na-ß"-alumina electrolyte is investigated at 250 °C in a home-built, specifically-designed high-temperature electrochemical cell. Operating the negative electrode above the melting temperature of sodium eliminates mass transport limitations at the sodium/Na-ß"-alumina interface and enables stable cycling at unprecedentedly high current densities above 1000 mA/cm2 without sodium metal dendrite formation. On the positive electrode, the chlorination and de-chlorination conversion reactions of nickel and iron in sodium tetrachloroaluminate electrolyte are investigated at 300 °C employing planar model electrodes. Analysis of cyclic voltammetry data reveals a diffusion limitation during oxidation with a diffusion coefficient of 2.3·10-3 cm2/s for the nickel electrode and an upper bound of 1.2·10-5 cm2/s for the iron electrode. Post-mortem analysis of the electrodes shows that chlorination of the nickel electrode proceeds via uniform oxidation of nickel and the formation of NiCl2 platelets on the surface of the electrode. In contrast, the chlorination of the iron electrodes proceeds via stress-induced cracking, resulting in non-uniform iron oxidation and the pulverization of the iron electrode. For enhanced rate capability, it is thus important to improve metal-ion diffusion in the tetrachloroaluminate electrolyte, preferably by enhancing metal-ion mobility. A complementary approach to improve the rate capability of sodium-nickel-chloride batteries is to modify the microstructure of the positive electrode. By moving from planar model electrodes to high-capacity porous electrodes, it is shown how microstructure design can improve sodium ion transport across the porous positive electrode. By these measures, the nominal discharge current density could be increased from 70 to 150 mA/cm2 demonstrating the potential for further improvements in rate capability.To reduce production cost of sodium-nickel-chloride batteries, the replacement of tubular Na-ß"-alumina electrolytes by planar Na-ß"-alumina electrolyte discs is investigated. Despite their potential for lower cost, the use of planar discs results in severely higher mechanical stress on the electrolyte. A practical solution to mitigate the gas pressure difference is to seal the cell below atmospheric gas pressure allowing implementation of planar electrolyte discs.Insights into the transport and conversion processes at the electrodes presented in this PhD thesis will contribute to enable competitive high-power sodium-nickel-chloride batteries for stationary electricity storage.