The project spanned several years and involved scientists from three universities and three US laboratories, working both experimentally and theoretically. The key is expanding a titanium disulphide cathode to allow magnesium chloride molecules in whole, rather than having to use energy to break the molecular bonds. This is a four step process.
“We are combining a nanostructured cathode and a new understanding of the magnesium electrolyte,” said Yan Yao, associate professor of electrical and computer engineering at the University of Houston. “That’s new.”
This allows batteries with a capacity of 400 mAh/g, up from 100 mAh/g for earlier magnesium batteries and twice that of commercial lithium ion batteries with a cathode capacity of 200 mAh/g. However, the voltage of the new battery remains low at about 1V, compared to 3 to 4 V for lithium batteries.
“Magnesium ion is known to be hard to insert into a host,” said postdoctoral fellow Hyun Deog YooYoo. “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.”
Increasing the gaps in the cathode from 0.57 nm to 1.8 nm allows the magnesium chloride to be inserted, says Yao. “Combined theoretical modeling, spectroscopic analysis, and electrochemical study reveal fast diffusion kinetics of magnesium monochloride cations without scission of magnesium chloride bond,” he said. “The large capacity accompanies excellent rate and cycling performances even at room temperature, opening up possibilities for a variety of effective intercalation hosts for multivalent-ion batteries.”
“We hope this is a general strategy,” Yoo said. “Inserting various polyatomic ions in higher voltage hosts, we eventually aim to create higher-energy batteries at a lower price, especially for electric vehicles.”
Various teams are working on magnesium battery technology as part of the US energy programme.