Graphene handles the issues that come with an electrode’s lithium moving elsewhere.
Lithium ion batteries, as the name implies, work by shuffling lithium atoms between a battery’s two electrodes. So, increasing a battery’s capacity is largely about finding ways to put more lithium into those electrodes. These efforts, however, have run into significant problems.
If lithium is a large fraction of your electrode material, then moving it out can cause the electrode to shrink. Moving it back in can lead to lithium deposits in the wrong places, shorting out the battery.
Now, a research team from Stanford has figured out how to wrap lots of lithium in graphene. The resulting structure holds a place open for lithium when it leaves, allowing it to flow back to where it started.
Tests of the resulting material, which they call a lithium-graphene foil, show it could enable batteries with close to twice the energy density of existing lithium batteries.
Lithium behaving badly
One obvious solution to increasing the amount of lithium in an electrode is simply to use lithium metal itself. But that’s not the easiest thing to do. Lithium metal is less reactive than the other members of its column of the periodic table (I’m looking at you, sodium and potassium), but it still reacts with air, water, and many electrolyte materials.
In addition, when lithium leaves the electrode and returns, there’s no way to control where it re-forms metal. After a few charge/discharge cycles, the lithium electrode starts to form sharp spikes that can ultimately grow large enough to short out the battery.
To have better control over how lithium behaves at the electrode, the Stanford group has looked into the use of some lithium-rich alloys. Lithium, for example, forms a complex with silicon where there are typically over four lithium atoms for each atom of silicon. When the lithium leaves the electrode, the silicon stays behind, providing a structure to incorporate the lithium when it returns on the other half of the charge/discharge cycle.
While this solves the problems with lithium metal, it creates a new one: volume changes. The silicon left behind when the lithium runs to the other electrode simply doesn’t take up as much volume as it does when the same electrode is filled with the lithium-silicon mix.
As a result, the electrode expands and contracts dramatically during a charge-discharge cycle, putting the battery under physical stress. (Mind you, a lithium metal electrode disappears entirely, possibly causing an even larger mechanical stress.)
And that would seem to leave us stuck. Limiting the expansion/contraction of the electrode material would seem to require limiting the amount of lithium that moves into and out of it. Which would, of course, mean limiting the energy density of the battery.
Between the sheets
In the new work, the researchers take their earlier lithium-silicon work and combine it with graphene. Graphene is a single-atom-thick sheet of carbon atoms linked together, and it has a number of properties that make it good for batteries. It conducts electricity well, making it easy to shift charges to and from the lithium when the battery charges and discharges. It’s also extremely thin, which means that packing a lot of graphene molecules into the electrode doesn’t take up much space. And critically for this work, graphene is mechanically tough.
To make their electrode material, the team made nanoparticles of the lithium-silicon material. These were then mixed in with graphene sheets in an eight-to-one ratio. A small amount of a plastic precursor was added, and the whole mixture was spread across a plastic block. Once spread, the polymer precursor created a thin film of polymer on top of the graphene-nanoparticle mix. This could be peeled off, and then the graphene-nanoparticle mix could be peeled off the block as a sheet.
The resulting material, which they call a foil, contains large clusters of the nanoparticles typically surrounded by three to five layers of graphene. Depending on how thick you make the foil, there can be several layers of nanoparticle clusters, each separated by graphene.
The graphene sheets make the material pretty robust, as you can fold and unfold it and then still use it as a battery electrode. They also help keep the air from reacting with the lithium inside. Even after two weeks of being exposed to the air, the foil retained about 95 percent of its capacity as an electrode. Lower the fraction of graphene used in the starting mix and air becomes a problem, with the electrode losing nearly half of its capacity in the same two weeks.
And it worked pretty well as an electrode. When the lithium left, the nanoparticles did shrink, but the graphene sheets held the structure together and kept it from shrinking. And it retained 98 percent of its original capacity even after 400 charge-discharge cycles. Perhaps most importantly, when paired with a vanadium oxide cathode, the energy density was just over 500 Watt-hours per kilogram. Current lithium-ion batteries top out at about half that.
Normally, work like this can take a while to get out of an academic lab and have a company start looking into it. In this case, however, the head of the research group Yi Cui already has a startup company with batteries on the market. So, this could take somewhat less time for a thorough commercial evaluation. The biggest sticking point may be the cost of the graphene. A quick search suggests that graphene is still thousands of dollars per kilogram, although it has come down, and lots of people are looking for ways to make it even less expensive.
If they succeed, then the rest of the components of this electrode are pretty cheap. And the process for making it seems pretty simple.