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Molten-salt batteries, including sodium-nickel-chloride and sodium-sulfur batteries, are based on abundant raw materials. The former consist of a sodium-metal anode and a nickel-chloride cathode separated by a ceramic sodium-b’’-alumina electrolyte and is commercialized by our industry partner FZSonick. Upon discharge, sodium is transported to the cathode compartment and reacts to form sodium-chloride (table salt) and nickel. Sodium-nickel-chloride batteries are a very safe battery technology with no risk of thermal runaway or toxic gas evolution. In addition, these batteries unlike lithium-ion batteries tolerate environmental temperatures of -40°C to 60°C. This property makes these batteries a prime choice for delivering backup power for outdoor mobile telecom antennas. Our research aims at improving the power capability of these batteries to maintain competitiveness with lithium-ion battery technology for electromobility (delivery vehicles, shuttle buses. larger public buses, etc.) and stationary storage application (grid stabilization, etc.). Sodium-nickel-chloride batteries are also integrated and studied at the Empa Energy Hub
Fig. 1: a) Photograph of sodium nickel chloride cell with superimposed schematics. b) and c) Scanning electron microscopy images of sodium-beta’’-alumina electrolyte sintered at different conditions resulting in very different microstructure.
Our lab focuses on understanding the impact of microstructure and phase composition on the mechanical strength and on the ionic conductivity of the sodium-b’’-alumina electrolyte. These two properties are directly correlated with the power capability of the battery, but also with production cost, as thinner electrolytes results in improved power capability, but lower yield in production. Although known since the 1970s, processing of sodium-b’’-alumina into a dense material with high ion conductivity is challenging, in particular due to significant sodium loss during sintering. Reports in literature relating microstructure to ionic transport properties are lacking and reported values for the ionic conductivity vary significantly. We developed a detailed understanding, how microstructure and phase composition affect ionic conductivity and mechanical strength. Our results are also relevant for related ceramic electrolytes for all-solid-state lithium-ion batteries.
Swiss Federal Office of Energy, InnoSuisse, FZSonick.
 E. Cuervo-Reyes, E. Roedern, Y. Yun, C. Battaglia, submitted
 M.-C. Bay et al., in preparation
 M. Heinz et al., in preparation