Magnesium battery uses cathode of fool’s gold

Technology News | November 18, 2019

By Nick Flaherty

Materials & processes Batteries / Power Supplies

Researchers in Switzerland have developed a low cost magnesium battery for large energy storage systems in the electricity grid.

Maksym Kovalenko, Marc Walter and their colleagues at the Laboratory for Thin Films and Photovoltaics, part of ETH Zurich, combined a magnesium anode with an electrolyte made of magnesium and sodium ions. Nanocrystals made of crystalline iron sulphide, iron pyrites, known as fool’s gold, as the cathode.

The sodium ions from the electrolyte migrate to the cathode during discharging. When the battery is recharged, the pyrite re-releases the sodium ions. This sodium-magnesium hybrid battery already works in the lab and has several advantages: The magnesium as the anode is far safer than highly flammable lithium. And the test battery in the lab already withstood 40 charging and discharging cycles without compromising its performance.

The protype cell has an average discharge voltage of 1.1 V and a cathodic capacity of 189 mAh/g at a current of 200 mA/g. This gave the Mg/FeS2 hybrid battery cell energy densities of up to 210 Wh/kg, comparable to commercial Li-ion batteries and approximately twice as high as state-of-the-art magnesium-ion batteries based on Mo6S8 cathodes.

The biggest advantage of the magnesium battery is that the elements are easily affordable and in plentiful supply: Iron sulfide nanocrystals, for instance, can be produced by grinding dry metallic iron with sulfur in conventional ball-mills. Iron, magnesium, sodium, and sulfur are amongst the commonest elements. One kilogram of magnesium costs at most four Swiss francs, which makes it 15 times cheaper than lithium.

There are also savings in the cell construction. Lithium ion batteries require relatively expensive copper foil to collect and conduct away the electricity. For the fool’s gold battery, however, aluminium foil is perfectly sufficient.

Magnesium battery for ESS

The researchers primarily see potential in the low cost battery technology in large network storage batteries to store terrawatts of energy. The fool’s gold battery is not suitable for electric cars as the output is too low.

“The battery’s full potential has not been exhausted yet,” said Kovalenko, who teaches as a professor at ETH Zurich’s Department of Chemistry and Applied Biosciences alongside his research at Empa. “If we refine the electrolytes, we’re bound to be able to increase the electric voltage of the sodium-magnesium hybrid cell even further and to extend its cycling life.

Further significant gains in the energy density are expected from the development of Na/Mg electrolytes with a broader electrochemical stability window

“We also look for investors willing to support research into such post-Li-ion technologies and bring them to the market,” said Kovalenko.

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