Despite their popular use today, lithium-ion batteries are low on being toxic and expensive, and there is a global shortage of metal. But for decades, researchers have been trying to find alternatives that are more environmentally friendly, safer and cheaper. The research team of Professor Dennis Leung from the Department of Mechanical Engineering at the University of Hong Kong (HKU) has discovered a new possibility: an aqueous battery with a magnesium metal anode, advances in post-lithium-ion-batteries.
The innovation opens a new direction for the development of post-lithium-ion batteries. The team’s findings (advances in post-lithium-ion-batteries) which were published in ACS Energy Letters, in an article titled “Reversibility of a high-voltage, Cl-assisted, aquatic Mg metal battery enabled by a water-in-salt electrolyte,” are attracting attention to the neglected rechargeable aqueous battery, Magnesium (Mg) metal batteries.
“With high theoretical capacitance and negative electrochemical potential, magnesium is an attractive anode material,” Professor Leung said. “Magnesium is also non-toxic and abundant in the world.” Mg makes up more than 2% of the Earth’s crust, which is 1000 times more than lithium. For a long time, Mg metals were considered difficult to use in batteries because of their high reactivity. Magnesium passes through when exposed to water, forming an oxidation film that inhibits redox reactions.
Many researchers are studying magnesium batteries with non-liquid electrolytes, but they are often expensive, unstable, and inefficient. Professor Leung emphasizes that aqueous electrolytes provide a good and cheap solution, despite the challenge of magnesium being sensitive to water. “It will make a promising candidate for cheap and durable batteries if we can unlock the potential of Mg batteries with water.”
And that’s what his team has discovered. They discovered that contrary to traditional belief, the capacity can be recovered in a liquid magnesium battery. Mg passivation films can be prepared using a chloride-based “water-in-salt” aqueous electrolyte. A “water in salt” electrolyte is a saturated mixture where the concentration of solute exceeds that of the solvent.
“There is little availability of free water and water-to-salt electrolyte prevents dehydration and addresses the main cause of excess,” Dr. Wending Pan, a postdoctoral fellow in the Department of Mechanical Engineering explained the study of water and saline electrolytes. The team also found that diffusion of chloride ions can protect the surface of Mg by dissolving oxides in one part and exposing the base metal to redox reactions. In excess of free water, the electrolyte based on water chloride and salt effectively combats the excess of magnesium.
“Using a novel water-in-salt electrolyte, the original passivation film can be transformed into a metal oxide layer, providing an ion pathway for battery performance,” said PhD student Kee. Wah Leong said, who studied the surface of magnesium anode, in detail.
The resulting battery exhibits good charging capacity for more than 700 stable cycles with a high discharge of 2.4 to 2.0 V, which exceeds the cell voltage of other multivalent ion batteries, including Zn and Al metal batteries metal. Although the voltage is not yet as high as that of commercial lithium-ion batteries, its performance can be improved with further development.
“The battery serves as a proof of concept and demonstrates for the first time the long-term cyclability of an aqueous metal Mg battery,” said Professor Leung.
