If you’re using a large battery for a specialized purpose—say grid-scale storage or an electric vehicle—then it’s possible to tweak the battery chemistry, provide a little bit of excess capacity, and carefully manage its charging and discharging so that it enjoys a long life span. But for consumer electronics, the batteries are smaller, the need for light weight dictates the chemistry, and the demand for quick charging can be higher. So most batteries in our gadgets start to see serious degradation after just a couple of years of use.
A big contributor to that is an internal fragmentation of the electrode materials. This leaves some of the electrode material disconnected from the battery’s charge handling system, essentially stranding the material inside the battery and trapping some of the lithium uselessly. Now, researchers have found that, for at least one battery chemistry, it’s possible to partially reverse some of this decay, boosting the remaining capacity of the battery by up to 30 percent.
The only problem is that not many batteries use the specific chemistry tested here. But it does show how understanding what’s going on inside batteries can provide us with ways to extend their lifespan.
That other thing that silicon is good for
The chemistry in question involves the use of a silicon anode. Lithium can be stored within solid silicon particles at a much higher density than most other anode materials. So, a lithium-silicon battery has the potential to provide an enormous boost to capacity, which would definitely be valuable.
Unfortunately, stuffing all that lithium into silicon particles does expand their volume considerably, and the particles tend to fragment, leading to a rapid decay in capacity. Several battery companies, like StoreDot and Amprius, have figured out workarounds; others simply limit the silicon mixed in with the standard graphite electrode material, giving it plenty of space to expand. However, pure lithium-silicon batteries are relatively rare and are already built around limiting the problems caused by silicon’s expansion.
The new work, then, is based on a hypothetical: What if we just threw silicon particles in, let them fragment, and then fixed them afterward?
As mentioned, the reason fragmentation is a problem is that it leads to small chunks of silicon that have essentially dropped off the grid—they’re no longer in contact with the system that shuttles charges into and out of the electrode. In many cases, these particles are also partly filled with lithium, which takes it out of circulation, cutting the battery’s capacity even if there’s sufficient electrode material around.
The researchers involved here, all based at Stanford University, decided there was a way to nudge these fragments back into contact with the electrical system and demonstrated it could restore a lot of capacity to a badly degraded battery.
Bringing things together
The idea behind the new work was that it could be possible to attract the fragments of silicon to an electrode, or at least some other material connected to the charge-handling network. On their own, the fragments in the anode shouldn’t have a net charge; when the lithium gives up an electron there, it should go back into solution. But the lithium is unlikely to be evenly distributed across the fragment, making them a polar material—net neutral, but with regions of higher and lower electron densities. And polar materials will move in an uneven electric field.
And, because of the uneven, chaotic structure of an electrode down at the nano scale, any voltage applied to it will create an uneven electric field. Depending on its local structure, that may attract or repel some of the particles. But because these are mostly within the electrode’s structure, most of the fragments of silicon are likely to bump into some other part of electrode in short order. And that could potentially re-establish a connection to the electrode’s current handling system.
To demonstrate that what should happen in theory actually does happen in an electrode, the researchers started by taking a used electrode and brushing some of its surface off into a solution. They then passed a voltage through the solution and confirmed the small bits of material from the battery started moving toward one of the electrodes that they used to apply a voltage to the solution. So, things worked as expected.
They then started testing this in used batteries with a pure silicon anode and confirmed that this restored some capacity to them. Testing various voltages suggested the higher the voltage you applied, the more capacity you restored. The researchers settled on four volts, as it was the highest they could go without risking degrading the battery’s electrolyte. They also tested how long to apply the voltage, finding that the capacity restoration hit a plateau at about five minutes.
So, there you have it: four volts for five minutes is enough to get disconnected fragments of anode to reintegrate into the electrical system of a battery.
Big restoration
How well did this work? Some of the numbers are striking. After about 20 charge/discharge cycles, it was possible to increase the electrode’s capacity by over 30 percent. The increased capacity served as a new baseline for the battery’s decay, suggesting that the re-established connections are effectively like new. If they did over 200 cycles before applying the pulse, the recovery was shockingly good: the remaining capacity of the battery more than doubled, going up by 140 percent. This worked on the isolated electrode and when it was incorporated into a battery with a lithium-iron-phosphate cathode.
All of that is great, but we’re talking about a battery with a performance that decayed to well under half its initial capacity after just 200 cycles. In other words, not a battery that’s likely to be especially useful for a gadget that may be charged multiple times in a week.
The question is whether the same approach might restore some capacity in the sorts of lithium-silicon batteries that degrade more slowly. The paper describing this work doesn’t touch on this issue, but there is a hint: The senior author is Yi Cui, who was the founder of Amprius, one of the companies that’s building lithium-silicon batteries.
It’s also possible that this approach may work with some non-silicon electrode materials. It probably won’t work with the main alternative to silicon, graphite, given that graphite is composed of lots of large sheets of graphene that don’t fragment in the same way as silicon. However, there may be other electrode materials under consideration that are prone to fragmentation and could benefit from something similar.
