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Engineering a battery fast enough to make recharging like refueling

Layers of phosphorene sheets form black carbon.
Enlarge / Layers of phosphorene sheets form black carbon.

Right now, electric vehicles are limited by the range that their batteries allow. That’s because recharging the vehicles, even under ideal situations, can’t be done as quickly as refueling an internal combustion vehicle. So far, most of the effort on extending the range has been focused on increasing a battery’s capacity. But it could be just as effective to create a battery that can charge much more quickly, making a recharge as fast and simple as filling your tank.

There are no shortage of ideas about how this might be arranged, but a paper published earlier this week in Science suggests an unusual way that it might be accomplished: using a material called black phosphorus, which forms atom-thick sheets with lithium-sized channels in it. On its own, black phosphorus isn’t a great material for batteries, but a Chinese-US team has figured out how to manipulate it so it works much better. Even if black phosphorus doesn’t end up working out as a battery material, the paper provides some insight into the logic and process of developing batteries.

Paint it black

So, what is black phosphorus? The easiest way to understand it is by comparisons to graphite, a material that’s already in use as an electrode for lithium-ion batteries. Graphite is a form of carbon that’s just a large collection of graphene sheets layered on top of each other. Graphene, in turn, is a sheet formed by an enormous molecule formed by carbon atoms bonded to each other, with the carbons arranged in a hexagonal pattern. In the same way, black phosphorus is composed of many layered sheets of an atom-thick material called phosphorene.

But there are key differences between the materials. To begin with, phosphorus is a larger atom with more electrons than carbon, and so it can interact with more lithium atoms, an essential feature for battery electrodes. The other key difference is that the bonds formed by carbon atoms ensure that graphene is essentially flat, no thicker than the atoms of carbon it’s formed from. The phosphorene sheets, as you can see above, are most clearly not flat. Neighboring atoms are bound at angles that give the sheet a series of ridges or channels.

It’s that feature that drew the researchers’ interest, since those angles form an avenue to get lithium ions into and out of the material quickly. And, because each phosphorus atom can interact with multiple lithium ions, we only know of two materials with higher theoretical electrode capacities—one of them being lithium metal itself. Finally, black phosphorus conducts electricity well, an important feature for a battery electrode.

So, why isn’t everyone already using black phosphorus? Well, mostly because it doesn’t work. Like other electrode materials, the black phosphorus expands as lithium ions get packed in, increasing the risk of a structural failure during charge/discharge cycles. And at the edges of sheets, chemical bonds can form between the different layers, sealing off some of the channels. To