Despite their ubiquitous usage today, lithium-ion batteries have the downsides of being poisonous and costly, with the additional complexity of a worldwide supply deficit of the metal. For decades, academics have sought to find alternatives that are more ecologically friendly, safer, and of lower cost.
A team of researchers headed by Professor Dennis Leung from the Department of Mechanical Engineering at the University of Hong Kong (HKU) has identified a new possibility—a rechargeable aqueous battery with a magnesium metal anode. The breakthrough provides a new route for the development of post-lithium-ion batteries.
The team’s results, which were published in ACS Energy Letters in a paper titled “Reversibility of a high-voltage, Cl-regulated, aqueous Mg metal battery facilitated by a water-in-salt electrolyte,” concentrate attention on the ignored rechargeable aqueous magnesium (Mg) metal batteries.
“With a large theoretical capacity and negative electrochemical potential, magnesium is a desirable anode material,” stated Professor Leung. “Magnesium is also non-toxic and earth-abundant.”
Mg makes up over 2% of the Earth’s crust and is 1,000 times more plentiful than lithium. Mg metals were long thought difficult to utilise in batteries because of their high reactivity. Mg is passivated when exposed to moisture, generating an impenetrable oxidation coating that prevents redox processes. Most researchers examine Mg batteries using non-aqueous organic electrolytes, but they are generally pricey, unstable, and poorly conductive.
Professor Leung argues that aqueous electrolytes do provide a safe and low-cost alternative, despite the issue presented by magnesium’s sensitivity to moisture. “It would make a good contender for low-cost and sustainable batteries if we can uncover the potential of aqueous Mg batteries.”
And that is what his team has uncovered. They discovered that contrary to common thought, rechargeability may be accomplished in an aqueous Mg battery system. The Mg passivation film may be controlled using an aqueous chloride-based “water-in-salt” electrolyte.
A “water-in-salt” electrolyte is a supersaturated combination where the mass of the solute dominates that of the solvent. “The limited availability of free water in the water-in-salt electrolyte restricts water decomposition and addresses the main cause of passivation,” explained Dr Wending Pan, a postdoctoral fellow from the Department of Mechanical Engineering who specialises in the study of water-in-salt electrolytes.
The scientists also showed that the adsorption of chloride ions may preserve the Mg surface by partly dissolving oxides and exposing native metal for redox reactions. With limited free water, the chloride-based water-in-salt electrolyte effectively combats magnesium passivation.
“Using the innovative water-in-salt electrolyte, the original passivation film may be transformed into a conductive metal oxide layer, offering ionic pathways for rechargeable battery operations,” stated PhD student Kee Wah Leong, who researched the surface of the magnesium anode in detail.
The resultant battery displays outstanding rechargeability for more than 700 stable cycles with a high discharge plateau of 2.4–2.0 V, which surpasses the cell voltage of existing multivalent-ion batteries, including Zn metal and Al metal batteries. Although the voltage is not currently equivalent to commercial lithium-ion batteries, its performance might be enhanced by additional research.
“The battery serves as a proof-of-concept and illustrates for the first time the long-term cyclability of an aqueous Mg metal battery,” stated Professor Leung.
