This illustration shows the composition of the new solid sodium battery. Image: Empa.
This illustration shows the composition of the new solid sodium battery. Image: Empa.

To meet the expectations of today’s consumers, batteries need to be lighter, more powerful and longer lasting. Currently, lithium-ion batteries are the most important battery technology, but they are expensive and contain a flammable liquid, which can represent a safety hazard. In order to satisfy the growing demand for better batteries, for use in electric cars and renewable energy storage, researchers from Empa, the Swiss Federal Laboratories for Materials Science and Technology and the University of Geneva (UNIGE), all in Switzerland, have now devised a new battery prototype known as ‘all-solid-state’.

This battery has the potential to store more energy while maintaining high safety and reliability levels. Furthermore, the battery is based on sodium, a cheap alternative to lithium. This novel battery prototype is described in a paper in Energy and Environmental Science.

All batteries comprise an anode (the negative pole), a cathode (the positive pole) and an electrolyte. When a lithium-ion battery charges, the lithium ions leave the cathode and move to the anode. To prevent the formation of lithium dendrites – a kind of microscopic stalagmite that can induce short circuits in the battery that may cause it to catch fire – the anode in commercial batteries is made from graphite rather than metallic lithium, even though this ultra-light metal would increase the amount of energy that can be stored.

When setting out to produce an enhanced battery, with faster charging, increased storage capacity and improved safety, the Empa and UNIGE researchers decided to use sodium rather than lithium and a solid electrolyte, instead of the conventional liquid one. By physically blocking the formation of dendrites, a solid electrolyte should allow them to utilize a metal anode, 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,” says Hans Hagemann, professor in the Physical Chemistry Department in UNIGE’s Faculty of Sciences. The researchers discovered that a boron-based substance known as a closo-borane allowed the sodium ions to circulate freely. Furthermore, since 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,” explains 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’ solution was to dissolve 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 together with the electrolyte and anode, compressing the various layers to form the battery.

The researchers then tested this new battery prototype. “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,” says 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 its energy capacity was still functional. “But it needs 1200 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.”

This story is adapted from material from the University of Geneva, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.