Because the battery uses a solid electrolyte, it allows a metal anode to be used. “But we still had to find a suitable solid ionic conductor that, as well as being non-toxic, was chemically and thermally stable, and that would allow the sodium to move easily between the anode and the cathode,” explained UNIGE Professor Hans Hagemann. The researchers discovered that closo-borane enabled the sodium ions to circulate freely. And, since closo-borane is an inorganic conductor, it removes the risk of the battery catching fire while recharging.
According to EMPA researcher Leo Duchene, the difficulty was establishing close contact between the battery’s three layers – a metallic sodium the node; a mixed sodium chromium oxide cathode; and the closo-borane electrolyte. The solution was to dissolve part of the battery electrolyte in a solvent before adding sodium chromium oxide powder. Once the solvent evaporated, the cathode powder composite was stacked with the electrolyte and anode, with the various layers compressed to form the battery.
Said to withstand 3V, the battery was tested for more than 250 charge and discharge cycles, after which it retained 85% of its original energy capacity. However, the research team says further work needs to be done in order to commercialise the battery.