The energy storage density of batteries has made remarkable strides in the last few decades, but people will always be happy with further improvements. The more charge you can stuff into a limited space, the longer cellphones will last and the farther electric cars will drive.

Right now, the anodes of lithium-ion batteries contain material that stores lithium in its structure. It would be more efficient to simply make the anode out of lithium metal itself, but early attempts to do so haven't worked out especially well, as the metal forms structures that rapidly degrade performance of the battery. Now, researchers have figured out how to put a carbon cap on top of the metal, keeping the lithium in its place and greatly enhancing the anode's stability.

The researchers behind the new paper, who are based at Stanford, nicely describe the problems with some of the previous work on lithium metal electrodes. To begin with, as charge moves in and out of the electrodes, they will necessarily grow and shrink with the changes in the amount of lithium present. This strains any electrolyte they're in contact with, frequently causing defects to appear at the electrode-electrolyte interface. Once these defects form, lithium metal will preferentially be added at these sites, causing extremely uneven growth.

Not only will this deform the battery further, but it can lead to the growth of lithium dendrites, which can then short out the battery. Having lithium deposits in just a few, concentrated areas also means that the heat involved with these chemical reactions is concentrated, increasing the risk of a thermal runaway.

The net result is that existing lithium anodes have led to batteries that see performance plunge by 100 charge-discharge cycles, and these batteries are prone to catastrophic failure.

One early attempt to keep the lithium under control involved forming a diamond-like coating of carbon over the lithium metal. This material, however, was prone to cracking as the electrode expanded and contracted. The new paper takes a variation on that, building a cap of amorphous carbon that is more flexible and robust than earlier coatings. (One measure of its robustness, called its Young's modulus, indicates it can withstand stresses up to 200 Gigapascals, the equivalent of two million atmospheres of pressure.)

To build the coat, the authors first packed down a layer of tiny polystyrene spheres that were roughly 500 nanometers across. A layer of amorphous carbon was deposited on top of the spheres, which ensured that the surface was lumpy. The polystyrene spheres were then ablated with heat, leaving the lumpy layer of carbon sitting on top of a copper backing. Lithium was then electrodeposited onto the copper, filling in the space below the carbon cap to form the electrode.

Even as the lithium metal grew during deposition, the carbon cap kept it from forming large dendritic spines. And no spines formed even after 150 charge/discharge cycles. As a result, the anode's performance remained excellent. While uncapped electrodes started to see performance drop after as few as 50 cycles, the carbon cap kept things completely stable out to at least 150 cycles, at which point the authors stopped testing.

The cycling Coulombic efficiency of the anode remained at 99 percent throughout these tests, meaning that the vast majority of the energy you put in to charge the battery comes back out during discharge.

All of which sounds pretty good, until the authors get around to noting that "the Coulombic efficiency needs to be improved to >99.9 percent for practical batteries." So, there's still a bit of work to do there. The other issue is that the authors don't test their batteries at very high charging rates, something that's rather important for both the phone and automobile use cases.

Still, the basic concept seems sound. Further study should provide a greater perspective on how rapidly these things can be safely charged and what the remaining problems are that prevent full cycling efficiency. If we can get lithium metal electrodes to work, then we can turn one of the major pieces of the battery into the material that carries charges around, which would save a fair bit of weight and space.

Nature Nanotechnology, 2014. DOI: 10.1038/NNANO.2014.152 (About DOIs).