Unlike traditional lithium-ion batteries, magnesium batteries are considered to be safe, as they are not flammable or subject to exploding. They are also less costly, as magnesium is a material in abundant supply.
However, up until now, magnesium battery designs have demonstrated a limited ability to store energy. Progress has stagnated in this area, say the University of Houston researchers, due to the need for a better battery cathode, and more efficient electrolytes – the medium through which the ionic charge flows between cathode and anode.
Specifically, the researchers say, the battery chemistry issues involve “the sluggish scission [breakage of a chemical bond] of magnesium-chloride bond and slow diffusion of divalent magnesium cations [positively charged ions] in [battery] cathodes.” Now, the researchers report, they have found a new way to design the cathode that “upends the conventional wisdom” that the magnesium-chloride bond must be broken before introducing magnesium into the host, resulting in a drastic increase in the battery’s storage capacity.
“We are combining a nanostructured cathode and a new understanding of the magnesium electrolyte,” says Yan Yao, associate professor of electrical and computer engineering at the University of Houston and lead author on the paper. “That’s new.”
“Magnesium ion is known to be hard to insert into a host,” adds Hyun Deog Yoo, postdoctoral fellow and first author on a paper on the research. “First of all, it is very difficult to break magnesium-chloride bonds. More than that, magnesium ions produced in that way move extremely slowly in the host. That altogether lowers the battery’s efficiency.”
The new battery stores energy by inserting magnesium monochloride into a host, such as titanium disulfide, in which the gaps have been expanded to accept magnesium monochloride molecules. By retaining the magnesium-chloride bond, Yao says, the cathode demonstrated much faster diffusion than traditional magnesium versions.
According to the researchers, the new battery has a storage capacity of 400 mAh/g, compared with 100 mAh/g for earlier magnesium batteries. Commercial lithium-ion batteries have a cathode capacity of about 200 mAh/g. Voltage of the new battery is said to be about one volt, compared to the three or four volts provided by lithium batteries.
“We hope this is a general strategy,” says Yoo. “Inserting various polyatomic ions in higher voltage hosts, we eventually aim to create higher-energy batteries at a lower price, especially for electric vehicles.”
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