While lithium-ion batteries remain popular despite drawbacks of being toxic and expensive as a result of a global supply shortage of metal. Thus, for decades, researchers have been working to identify and develop alternatives which are more environmental-friendly, safer, and of lower cost.
A team of researchers led by Professor Dennis Leung from the Department of Mechanical Engineering at the University of Hong Kong (HKU) has discovered a new possibility – a rechargeable aqueous battery with a magnesium metal anode. The innovation opens a new direction for the development of post-lithium-ion batteries.
The team’s findings, which were published in ACS Energy Letters, in an article titled “Reversibility of a high-voltage, Cl-assisted, aqueous Mg metal battery enabled by a water-in-salt electrolyte,” bring attention to the overlooked rechargeable aqueous Magnesium (Mg) metal batteries.
Professor Leung stated that with a high theoretical capacity and negative electrochemical potential, magnesium is an attractive anode material. He noted that magnesium is also non-toxic and earth abundant.
Over 2% of the earth’s crust is comprised of magnesium (Mg), which is more abundant than lithium by one thousand times. For several years, Mg metals were considered difficult to work with in batteries as a result of their high reactivity. Mg is passivated when exposed to moisture, forming an impermeable oxidation film that blocks redox reactions. Most researchers study Mg batteries with non-aqueous organic electrolytes, but they are often costly, unstable, and poorly conductive.
Professor Leung maintains that aqueous electrolytes do offer a safe and low-cost solution, despite the challenge posed by magnesium’s sensitivity to moisture. He said that it would make a promising candidate for low-cost and sustainable batteries if we can unlock the potential of aqueous Mg batteries.
And that is what his team has discovered. They found that contrary to traditional belief, rechargeability can be achieved in an aqueous Mg battery system. The Mg passivation film can be regulated using an aqueous chloride-based “water-in-salt” electrolyte.
A “water-in-salt” electrolyte is a supersaturated mixture where the mass of solute outweighs that of the solvent. Dr Wending Pan, a postdoctoral fellow from the Department of Mechanical Engineering who specializes in the study of water-in-salt electrolytes said that the limited availability of free water in the water-in-salt electrolyte restricts water decomposition and addresses the main cause of passivation.
The team also found that the adsorption of chloride ions can protect the Mg 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.
By utilizing 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, according to PhD student Kee Wah Leong, who studied the surface of the magnesium anode in detail.
The resulting battery demonstrates excellent rechargeability for over 700 stable cycles with a high discharge plateau of 2.4-2.0 V, which exceeds the cell voltage of other multivalent-ion batteries, including Zn metal and Al metal batteries. Although the voltage is not yet comparable to commercial lithium-ion batteries, its performance could be boosted by further development.
It was noted that the battery serves as a proof-of-concept and demonstrates for the first time the long-term cyclability of an aqueous Mg metal battery.
Recent research found that while current battery technologies such as lead acid and lithium-ion batteries will dominate the battery market over the next decade, advanced and post lithium-ion batteries – given the right conditions – are expected to potentially take a considerable dent of about 10% of whole battery market by 2026.
This can be attributed to some of them are already in use in niche segments that will present high growth in the following 10 years. Advanced and post-lithium-ion battery technologies are projected to reach a market value of US$14 billion in 2026.
