Magnesium-ion batteries could prove that two electrons are better than one

Magnesium-ion batteries could prove that two electrons are better than one

A lithium ion battery essentially works by using lithium (ions?!) to ferry electrons back and forth between a positive and a negative electrode. These are basically electrical freighters that load up with electrons on the negative side of the battery and, when given the proper signal, sail on over to the positive side to offload their cargo — this is discharging a battery, and when the electrons leave the lithium carriers at the other side, they can flow down their gradient and create a current to power our device. The now-empty lithium freighters are bound to the positive electrode, but they can be reloaded by running the current backwards. The now electron-laden molecule naturally dissociates and sails back to the first, storage electrode. Once all available ions are thus stored in a high energy state, he battery is recharged.

The freighter analogy works well because, just as cargo comes in units of shipping containers, electricity comes in units of electrons. A lithium “ion” is, for the purposes of batteries, going to be able to hold only a single electron; Li¹⁺ is the ion we typically use in lithium ion batteries. But magnesium is most readily usable in the form of Mg²⁺ ions, meaning that it could ferry a maximum of two electrons per freighter-trip across the battery. Theoretically, that allows a huge increase in the density of energy storage — double, actually, if magnesium ions are present in the same density as lithium ions.

The very basics of lithium ion function.

But as we all know, theory doesn’t always manifest perfectly in the real world. Foremost among the confounding problems is that fact that when you have twice as many negatively charged electrons per atom, you have twice the negative charge per atom — and a stronger negative charge means a stronger attraction to positive ions. So, while magnesium ions do carry twice the electrical load, they are also more sluggish in doing so because the physical process of diffusing across the electrolyte-filled gap between the two electrodes is slowed by literal hangers-on.

This computer model shows how the orange magnesium ion is coordinated by only four nearby ions in the electrolyte.

Recent research shows that this might not be as large a problem as scientists had feared, purporting to prove that a magnesium ion is only fettered by its four closest neighbor ions — the space gets too crowded for anything more than that. This means that there’s a fairly low upper limit on the amount of interference magnesium ions might experience when they’re doing their job in the battery, which means we might not have to do much to overcome this downside at all.
Future research will almost certainly be into tailoring the perfect electrolyte to getting both full battery function and easy ionic movement. Kristin Persson at Lawrence Berkeley laboratories has tested thousands of different electrolyte-electrode combinations in hopes of finding one that will allow us to exploit the magnesium ion for all it’s worth. Supercomputers run fundamental physical simulations, looking at everything from charge density to atomic geometry to see how Mg²⁺ ions can be made productive members of battery society.

The dual carbon battery could make magnesium irrelevant — if it goes anywhere.

Magnesium also has a lot of other advantages, not the least of which being that it isn’t lithium, and is much cheaper to acquire and use. Toyota has invested in the technology, and Elon Musk has stated openly that Tesla and its battery Gigafactory are ready for magnesium should it become the standard; since so few other aspects of the battery design would be affected by the lithium-magnesium transition, the factory could be retooled to pump out double-dense Tesla Mg-ion batteries very easily.
Given the enormous possible benefits of switching from lithium to magnesium, it seems like a foregone conclusion that it will happen eventually — unless something entirely better comes along first. Japan’s dual carbon batteries, lithium-air contraptions, and even resurrected hydrogen fuel cells could all very plausibly rise to dominate the industry. It’s all about which technology actually makes it to market — and no matter what, we’ve definitely got at least a few more years of lithium.