Sodium solid-state batteries with improved performance, safety developed

Geneva: Scientists have devised a new sodium-based battery that has the potential to store more energy while maintaining high safety and reliability levels. Researchers from Empa, the Swiss Federal Laboratories for Materials Science and Technology and the University of Geneva (UNIGE) in Switzerland focused on the advantages of a “solid” battery: faster charging together with increased storage capacity and improved safety.

The battery prototype known as all-solid-state 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. “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,” said Hans Hagemann, from the UNIGE.

In the study published in the journal Energy and Environmental Science, the researchers discovered that a boron-based substance, a closo-borane, enabled the sodium ions to circulate freely. Since the closo-borane is an inorganic conductor, it removes the risk of the battery catching fire while recharging. It is a material, in other words, with numerous promising properties. “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 Leo Duchene, from the UNIGE.

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 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, leader of the project.

The scientists also tested the battery over 250 charge and discharge cycles, after which 85 per cent of the energy capacity was still functional. “It needs 1,200 cycles before the battery can be put on the market. 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,” the researchers added.