A pioneering investigative study by researchers from the Swiss Federal Laboratories for Materials Science and Technology, Empa,
demonstrates the potential of magnesium and sodium in the development of alternative technologies (including, importantly, rechargeable batteries that may someday replace lithium-ion technology) that are based exclusively on solid elements.
The scientific team, headed by Empa group leader, Arndt Remhof Ph.D., has developed solid-state battery cells using a solid compound (as opposed to cells based on a liquid electrolyte), a design solution which posed its own significant technical problem. Positively-charged ions--whether lithium, sodium or magnesium--must be able to move through a solid medium, i.e. from one pole to the other inside a battery. The ions facilitate the displacement of negatively-charged electrons, and thus the discharge of an electrical current through an external circuit.
Dr. Remhof and his collaborators developed solid electrolytes with crystalline structure, substituting magnesium or sodium for lithium.
The solid electrolyte enables good mobility of sodium ions at 20 degrees-- important because a source of heat is required for ions to move; triggering a reaction at room temperature is technically challenging. The electrolyte is non-flammable with chemical stability up to 300 degrees, a characteristic that addresses the various safety issues associated with lithium-based batteries, including the recurrence of exploding mobile phones, which are powered by that type of battery.
And unlike lithium, sodium is cheap and plentiful.
"Availability is our key argument," says Léo Duchêne of Empa and first author of the research paper. "However, it (sodium) 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 research group has also created a magnesium-based solid electrolyte. Although making this element move is difficult, it is an attractive material because it is readily available in large quantity, is lightweight, and poses no risk of explosion. Regarding magnesium’s application in power cells, the most important point is the presence of two positive charges in a magnesium ion, as opposed to one positive charge in a lithium ion--that means magnesium can store nearly twice the energy in the same volume.
Some experimental electrolytes have been employed to induce the movement of magnesium ions, but at temperatures above 400 degrees. The electrolytes developed by the Swiss researchers have already shown similar conductivities at 70 degrees.
The research project (the Novel Ionic Conductors project) assembled researchers from Empa, the University of Geneva, the Paul Scherrer Institute and the Henryk Niewodniczanski Institute for Nuclear Physics (Poland). It has been funded by the Swiss National Science Foundation since 2015 as part of the Sinergia programme, which supports collaborative and interdisciplinary research. "What we have managed to achieve in less than two years is quite extraordinary!" says Dr. Remhof.
The Swiss Federal Laboratories for Materials Science and Technology is an interdisciplinary Swiss research institute for applied materials sciences and technology, founded in 1880. As part of the Swiss Federal Institutes of Technology Domain, it is an institution of the Swiss federation. (Empa is the German acronym for Eidgenössische Materialprüfungs- und Forschungsanstalt). Empa describes its vision as “Materials and Technologies for a Sustainable Future.”
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