More power to the magnesium battery

It is cheap, abundant and carries a hefty charge. Now the metal has been yoked to vanadium to better harness its capability

P Rajendran

Build a better battery and tech companies should beat a path to your door.

If an idea mooted by Sarabjit Banerjee of the Texas A&M University and his collaborators – to use cheaper and more abundant magnesium – are anything to go by, the paths to their doors may have to endure a great deal of wear and tear.

Lithium has been an energetic workhorse in batteries for some time, but researchers have been seeking other options. These include the far more abundant aluminum, sodium, potassium, calcium, or magnesium. They are looking for batteries that can carry more charge and can be recharged more often.

Batteries rely on charged species flowing from the positively charged side to the negative side. In lithium-based batteries, the positive side is made up of a lithium compound, the negative end of carbon. The two sides are separated by a “separator,” usually a porous polymer with a gel or liquid that will permit lithium ions to flow to the carbon side.

Lithium as an element has three negatively charged electrons (see figure), two in its inner shell, one in the outer one. That solitary outer electron can quickly be passed on to any another atom that can take in that electron. This allows lithium ions to serve as the currency for charge transfer.

Lithium makes up at best about 0.006 % of the Earth’s crust while magnesium accounts for about 2%. At current rates, as lithium becomes less accessible, mining may have to shift to sea water and undersea thermal vents – with attendant environmental and other consequences. Banerjee’s team also relies on vanadium, which while less abundant than magnesium, is still at least about five times more abundant than lithium – at 0.02%.

Magnesium is abundant and safer [in batteries],” Banerjee told Truly Curious. It also has a higher melting point.

“And there is a performance advantage,” he said. “It has two charges instead of one.”

On the risk of fire, lithium batteries, especially lithium cobalt batteries, also are particularly vulnerable despite safety checks in the form of self-closing polymer separators, electronic monitors, etc. This is because lithium tends to form fingers, or “dendrites,” that can slowly bridge the gap to the carbon, and once connected cause a short-circuit that may result in a fire. Magnesium does not form dendrites and so is deemed to be safer.

During charging, the magnesium ion, which, with two electrons gone, is not easy to get over to the anode, the negative side of the battery.

“That magnesium has a higher charge makes it hard to move through a lattice,” Banerjee pointed out. He said that if lithium ion, with one positive charge, was compared to a convict on a ball and chain, magnesium could be deemed a convict with two sets of them.

But if ever such a battery were possible, magnesium could provide almost four times the energy density of lithium. The main problem is not about letting magnesium ions through – they are small enough to fit – but to ensure the highly charged ions do not interact with the channel walls.

Banerjee and his team – including collaborators at the University of Illinois at Chicago, Pusan National University in Korea, and the Lawrence Berkeley National Laboratories – found that an oxide of vanadium (V₂O₅) could under a certain condition produce tunnels – in the form of nanowires – that would let magnesium ions through.

M Stanley Whittingham, a professor at Binghamton University, and a pioneer of modern rechargeable battery technology, had described a possible use for V₂O₅ but research on standard forms of V₂O₅ has not gone far.

The ideal condition that produced the nanowires was a “metastable” version of V₂O₅, with magnesium being sent into the nanowires either chemically or electrically.

The researchers have seen the magnesium-vanadium battery slowly improve its efficiency over multiple charging cycles, stabilizing at 90% at 85 cycles and retaining that figure after 100 cycles.

“We are still not fast enough to compete with Li charging,” Banerjee said. “There is still a lot of work to be done.” But more stable, efficient and cheaper batteries are worth the effort.

“Magnesium batteries are a massive holy grail for the community,” Banerjee said. “We’ve been looking for it for a long time.”

Asked what kept people working on it despite the many hurdles, he fell back to allegory:

“Al Capone was asked, ‘Why did you rob the bank.’ He answered, ‘Because that’s where the money is.’” *

The paper on their research appeared in Chem.

* The quote is actually attributed to infamous bank robber Willie Sutton though, in his autobiography, he denied saying it.

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