Most of the batteries we use, from our cell phones to our cars, rely on using lithium ions. As a result, their capacity is largely a product of how much lithium you can stuff into a given volume. Obviously, using a pure lithium electrode would provide the highest density possible. But there has been no way to control where the lithium ends up as a battery goes through charge/discharge cycles. The typical result is a set of lithium metal spines that short the whole system out.

As a result, a lot of effort has been put into finding other materials that can incorporate lithium into their structure. This lowers the total lithium content but keeps the battery from shorting out. However, a new paper suggests an intriguing alternative, describing a material that ensures lithium forms a smooth coating on its surface with no spines. What's this wonder material? A slightly modified version of asphalt.

Pavement from a chemistry perspective

Although the term "asphalt" is often used as a general term for blacktop pavement, it has a technical meaning as well: a viscous, semi-liquid hydrocarbon that's one of the components of the paving material. There are several different types of asphalt, but the team here worked with something called gilsonite, which is close enough to a solid to be mined. Like other hydrocarbons, it's a complex mix of molecules rather than a pure substance, and it contains things like nitrogen and sulfur due to its origin in biological material.

Using it for batteries is much less insane than it might seem given that many batteries already use various forms of carbon for electrodes. Many types of carbon conduct current well and form porous materials that can store a large volume of ions internally. But, as mentioned above, the team behind the new work (based at Rice University) wasn't looking to store lithium within the asphalt; instead, they were hoping to have the asphalt structure a sheet of lithium metal on its surface.

To prepare the asphalt, the researchers heated off most of the low-weight hydrocarbons and then treated the remaining solids with a strong basic solution (potassium hydroxide). When re-formed into a solid, this material formed a highly porous solid that's a bit like a sponge but much more porous—overall, its surface area was estimated at more than 3,000 square meters for each gram of material. The treatment with a strong base also chemically modified the hydrocarbons at the surface of the material, linking some of the carbon with oxygen. These oxygen atoms appear to help the surface interact with lithium metal.

To form an electrode, they needed something to improve its ability to conduct electricity. So, while the gilsonite was fragmented and suspended in a liquid, they mixed in some fragments of graphene ribbons. When this mix was used to form a porous solid, the ribbons were incorporated into it, making it evenly conductive. This turned out to be critical, as described below.

Once the asphalt was treated, the authors could add lithium to it simply by electroplating it. It formed an even coating on the surface. With that, the material was ready to be used as an electrode.

Testing 1, 2, 3...

Initial tests showed that it could run through repeated charge/discharge cycles without losing much capacity—at a charge transfer efficiency of over 95 percent, in fact. It also worked at a wide variety of charge/discharge rates, from taking 10 hours to discharge to taking as little as six minutes. The team tested the same material without the graphene mixed in, finding that spines of lithium metal formed on the surface, which could short the battery out. The researchers ascribe this to charge being poorly distributed when graphene was absent, causing lithium to build up in specific locations on the battery.

With everything looking promising, they built a battery with it. To make the other electrode, they relied on a somewhat similar approach. Sulfur and lithium have a strong affinity, meaning pure sulfur is able to store lots of lithium ions. But pure sulfur also engages in lots of additional chemical reactions, many of which damage an electrode or the battery as a whole. To avoid this, the researchers reacted sulfur with carbon, chemically linking it to the electrode material.

The results were rather impressive. When all of the electrode materials were considered, the battery's energy density was nearly 950 Watt-hours per kilogram. For comparison, the batteries in a Tesla are in the neighborhood of 250 Whr/kg.

As always, there's no way to know at this point whether this tech can be commercialized. Lots of promising looking results don't hold up under conditions needed for mass production. But this one has a major advantage in that gilsonite is cheap enough that we can afford to pave roads with it. And it's interesting for the different approach it took to stuffing as much lithium as possible into a given volume of battery.

ACS Nano, 2017. DOI: 10.1021/acsnano.7b05874 (About DOIs).