Could sodium and magnesium replace lithium in batteries?

Scientists at the Swiss Federal Laboratories for Materials Science and Technology (Empa) are on a quest to find new materials which can be used in rechargeable batteries and eventually provide alternatives to lithium batteries.

Lithium-based batteries have several drawbacks, including the limited availability of the raw material itself and the numerous safety issues, which are primarily associated with the use of a flammable liquid compound. Arndt Remhof and his team at Empa recently demonstrated the potential of sodium and magnesium in the development of alternative technologies based exclusively on solid elements.

The researchers set out to develop solid-state battery cells using a solid compound (as opposed to cells which are based on a liquid electrolyte), the design of which poses a significant technical problem. Ions — whether they are lithium, sodium or magnesium — must be allowed to move through a solid medium. By moving from one pole to the other inside the battery, ions (positive charge) facilitate the displacement of electrons (negative charge) and thus the discharge of an electrical current through an external circuit.

To facilitate the displacement of ions, the researchers developed solid electrolytes with crystalline structure. By substituting lithium with sodium or magnesium, Remhof’s team had to completely overhaul their crystalline architecture and use new components and manufacturing processes.

The team has now developed a solid electrolyte that facilitates good mobility of sodium ions at 20°C. This last point is crucial: ions require a source of heat in order to move, and inducing a reaction at room temperature poses a technical challenge. The electrolyte is also non-flammable and is chemically stable up to 300°C, which addresses the various safety concerns associated with lithium-ion batteries. Furthermore, unlike lithium, there are huge reserves of sodium.

“Availability is our key argument,” said Léo Duchêne, first author on the study, which has now been published in the journal Chemical Communications. “However, it stores less energy than the equivalent mass of lithium and thus could prove to be a good solution if the size of the battery isn’t a factor for its application.”

The same team has also developed a solid magnesium-based electrolyte, the results of which have been published in the journal Scientific Reports. It is much more difficult to set this element in motion; however, magnesium is lightweight and abundant, with no risk of it exploding. More importantly, a magnesium ion has two positive charges, whereas lithium only has one. This means that it stores almost twice as much energy in the same volume.

Some experimental electrolytes have already been used to stimulate magnesium ions to move, but at temperatures in excess of 400°C. The electrolytes used by the Swiss scientists have already recorded similar conductivities at 70°C.

“This is pioneering research and a proof of concept,” said Elsa Roedern, who led the experiments. “We are still a long way from having a complete and functional prototype, but we have taken the first important step towards achieving our goal.”