The Empa and UNIGE researchers focused on a solid state sadium cell to provide higher performance with faster charging together with increased storage capacity and improved safety. Their battery uses a solid instead of a liquid electrolyte that enables the use of a metal anode by blocking the formation of dendrites, making it possible to store more energy while guaranteeing safety at a native voltage of 3V.
“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,” said Hans Hagemann, professor in the Physical Chemistry Department in UNIGE’s Faculty of Sciences. The researchers discovered that a boron-based substance, a closo-borane, enabled the sodium ions to circulate freely. As the closo-borane is an inorganic conductor, it removes the risk of the battery catching fire while recharging.
“The difficulty was establishing close contact between the battery’s three layers: the anode, consisting of solid metallic sodium; the cathode, a mixed sodium chromium oxide; and the electrolyte, the closo-borane,” said Léo Duchêne, a researcher at Empa’s Materials for Energy Conversion Laboratory and a PhD student in the Department of Physical Chemistry at UNIGE’s Faculty of Science. The researchers dissolved part of the battery electrolyte in a solvent before adding the sodium chromium oxide powder. Once the solvent had evaporated, they stacked the cathode powder composite with the electrolyte and anode, compressing the various layers to form the 3V battery, which has an energy density of 85 mAh/g at room temperature.
Researchers at Empa and UNIGE subsequently tested the battery. “The electro-chemical stability of the electrolyte we are using here can withstand three volts, whereas many solid electrolytes previously studied are damaged at the same voltage,” said Arndt Remhof, a researcher at Empa and leader of the project, which is supported by the Swiss National Science Foundation (SNSF) and the Swiss Competence Centre for Energy Research on Heat and Electricity Storage (SCCER-HaE). The scientists also tested the battery over 250 charge and discharge cycles, after which 85% of the energy capacity was still functional. “But it needs 1,200 cycles before the battery can be put on the market”, say the researchers. “In addition, we still have to test the battery at room temperature so we can confirm whether or not dendrites form, while increasing the voltage even more. Our experiments are still ongoing.”
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