Solid new battery technology could be safer and more reliable

Swiss researchers have developed a new battery prototype that could potentially store more energy, while maintaining high levels of safety and reliability.

The technology, known as ‘all-solid state,’ is based on sodium, a cheap alternative to the lithium that sits inside most modern batteries. When a battery charges, ions leave the cathode and move to the anode. Most lithium-ion batteries use a graphite anode to prevent the formation of lithium dendrites – branching growths that can cause short circuits and fires.

But, using a metallic anode would increase the amount of energy that could be stored, so that’s exactly what researchers from Empa, Switzerland’s federal laboratory for materials science and technology, and the University of Geneva have done.

Their battery uses a solid electrolyte instead of a liquid, which allows them to use a metal anode without risking the formation of dendrites.

“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 Hans Hagemann, professor of physical chemistry at Geneva.

They settled on a close-borane, a boron-based conductor that enabled to sodium ions to circulate freely within the battery. Because it’s an inorganic conductor, it removes the risk of the battery catching fire while recharging, according to the researchers.

“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 researcher Leo Duchane.

To solve this problem, they dissolved part of the electrolyte in a solvent, before adding sodium chromium oxide powder. After the solvent had evaporated, they stacked the cathde powder composite with the electrolyte and anode and compressed the layers to form the battery.

When testing the battery, they found that could withstand three volts – a level above solid electrolytes which have been previously studied, according to Arndt Remhof, the project leader. They also tested it over 250 charge and discharge cycles, and found that 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."