Discussions about batteries often revolve around energy density. What we want is a battery that stores a whole lot of energy in a very tiny volume, preferably in a manner that doesn’t involve explosions or fire. At the cutting edge of research, what we get are batteries that are a mix of amazing and amazingly bad.
Modern batteries are, quite frankly, a miracle compared to ye olde lead acid battery. Yet they still contain less energy per unit mass than the equivalent mass of wood. Essentially, we simply don’t pack enough atoms into a small enough volume to compete with hydrocarbons. But, now it seems that graphene—it’s always graphene—might help pack lithium in.
The invisible metal
Although there are many ways to make a lithium-ion battery, the chemistry boils down to the following: lithium is stored in some form at one electrode. The lithium is released as an ion, where it travels to another electrode and reacts. At the same time, the electrons that complete the reaction travel out into the world via one electrode, do some work, and end up at the other electrode, where they complete the reaction.
The key here is that the lithium is usually stored as a light and low-density lithium carbide. Finding materials that increase the density of lithium is one way to increase battery capacity.
Here is where battery research often runs into problems. Lithium is a very light element. Carbon, the other main constituent of a battery, is also a very light element. When viewed through an electron microscope, they look almost identical. That makes it very difficult to examine how lithium builds up at an electrode and makes it hard to see the variations in structures that it forms as it is stored (or how those structures come apart as it is removed).
It is worse than that, though. Electron microscopes usually use quite energetic electrons to create an image. The electrons have more than enough energy to knock carbon and lithium atoms out of the structure being examined. By the time you have created your image, you have destroyed the structure you imaged. Not ideal.
Enter a group of scientists with a transmission electron microscope that has been designed to work with low-energy electrons. The microscope still has sufficient resolution to see single atoms, so structures can be determined. By examining how much energy the electrons lose as they go through the sample, the researchers can also figure out the sample contents. Finally, the time it takes to gather the image is short enough (about one second) that the researchers can observe the build up and decay of structures as the battery is used.
A lithium sandwich
Since transmission electron microscopy requires that electrons pass through the sample, the carbon-lithium layer had to be very thin. The researchers chose to use a ribbon of a graphene double-layer (graphene is a single layer of graphene with the carbon atoms arranged in a honeycomb pattern). A blob of electrolyte-containing lithium ions was placed at one end of the graphene ribbon.
A series of electrodes were placed along the ribbon to measure and set voltages. The voltages were used to drive lithium into the ribbon and allow it to leave again. When lithium accumulates in the ribbon, the resistance drops, allowing a second set of electrodes to detect the presence of lithium.
The researchers don’t say it, but I think they were quite surprised by what happened. The lithium moves quite rapidly in the gap between the two graphene ribbons. On the scale of their graph, lithium appears between the electrodes instantly. From the movie, it looks like it takes about 14s to travel 50 micrometers, which I think is shockingly fast.
The amount of lithium is also pretty surprising. By examining the structure and elemental composition, the researchers found that the lithium was not forming a lithium carbide, as expected. Instead, it was forming multiple layers of crystalline lithium with only the outermost layer binding to the carbon. But the metallic lithium was not in its usual form. Instead, the lithium forms a high-density state that is normally found at low temperature or very high pressure.
Don’t get overexcited
This is quite interesting, and it may even prove useful. But not yet. For on thing, the high-density lithium only forms between two sheets of very nearly perfect graphene, not the sort of graphene that you can buy from a manufacturer. Indeed, near the edges of imperfections, the energy imparted by the electrons in the electron microscope was enough to boil off the lithium metal.
Even if we could get large amounts of high-quality, double-layer graphene sheets, there is no certainty that the lithium will diffuse as deeply as required during a charging cycle. It is pretty easy to imagine the first lithium ion building up in a clump that blocks the rest of the lithium from moving into the sandwich.
It is also not certain that the graphene survives the process for very long. This is one of the main problems with batteries involving metallic lithium: the electrodes destroy themselves over multiple cycles. We’ve no idea if graphene will last any longer than current electrode designs.
That said, the researchers are not presenting this as a battery-ready technology. Rather, it is an excellent example of how an experimental necessity has led to an interesting new set of observations that we will probably learn a lot from. And, if we are lucky, it will eventually help make batteries better.
