A team of researchers led by Professor Dennis Leung from the Department of Mechanical Engineering at the University of Hong Kong (HKU) have recently discovered a new possibility beyond lithium-ion-based batteries — a rechargeable aqueous battery with a magnesium metal anode.

While lithium-ion batteries are one of the most popular, they suffer from setbacks. These batteries are toxic and expensive, in addition to the global supply shortage of metals.

The recent innovation will most likely open an all-new direction for developing post-lithium-ion batteries.

“With a high theoretical capacity and negative electrochemical potential, magnesium is an attractive anode material,” said Professor Leung. “Magnesium is also non-toxic and earth-abundant,” he added.

Magnesium is typically considered hard to use in batteries due to its high reactivity. It is passivated when exposed to moisture, forming an impermeable oxidation film that blocks redox reactions.

While most researchers study magnesium batteries with non-aqueous organic electrolytes, they are often expensive, unstable, and poorly conductive.

 The Magnesium Alternative

The team’s findings bring attention to the overlooked rechargeable aqueous magnesium metal batteries. One of the key advantages of opting for magnesium is that it makes up over 2% of the earth’s crust and is more abundant than lithium by 1000 times.

Despite the challenge posed by magnesium’s sensitivity to moisture, Professor Leung maintains that aqueous electrolytes offer a safe and low-cost solution.

Contrary to traditional belief, rechargeability can be achieved in an aqueous magnesium battery system. The passivation film of magnesium can be regulated using an aqueous chloride-based “water-in-salt” electrolyte.

The team further found that the adsorption of chloride ions can protect the magnesium surface by partially dissolving oxides and exposing native metal for redox reactions. With limited free water, the chloride-based water-in-salt electrolyte successfully combats magnesium passivation.

“Using the novel water-in-salt electrolyte, the original passivation film can be converted into a conductive metallic oxide layer, providing ionic pathways for rechargeable battery operations,” said Ph.D. student Kee Wah Leong.

The resulting battery demonstrates rechargeability for over 700 stable cycles with a high discharge plateau of 2.4-2.0V. It exceeds the cell voltage of other multivalent-ion batteries, including zinc metal and aluminum metal batteries.

Though the voltage is not yet comparable to commercial lithium-ion batteries, its performance is said to have immense potential.

“The battery serves as a proof-of-concept and demonstrates for the first time the long-term cyclability of an aqueous Mg metal battery,” said Professor Leung.

Lithium rates have surged over seven times since May last year, creating challenges for the global electric vehicle (EV) battery supply chain. In a recent report, the International Energy Agency (IEA) said that the cobalt and nickel prices more than doubled from March 2021 to May 2022, stressing the supply chain as battery demand increased.

With the rising prices, the industry is already looking for cost-effective and readily available alternatives.

Earlier this year, researchers at Helmholtz Zentrum Berlin developed a pouch cell format with different electrolytes that can help increase the life of the lithium-sulfur battery.