For the last couple of years the big news for lithium-ion (Li-ion) batteries has been the replacement of graphite in the anodes with silicon.
This move from graphite to silicon has been eagerly pursued in order to address one of the fundamental operations of Li-ion batteries: the movement of lithium ions from the cathode to the anode and their storage there. The more lithium ions that the anode can store, the longer the battery will stay charged. The charge can be increased by a factor of ten by replacing the graphite with silicon.
But there is a drawback with these silicon electrodes. They swell to three times their original size when charged and the charging and discharging of the battery soon renders the silicon useless as an electrode in a battery. For this reason, researchers have been working with various nanostructured silicon materials that take advantage of the superior storage capacity of silicon but are not as susceptible to the deterioration caused by charge-discharge cycles.
Now researchers at Pacific Northwest National Laboratory (PNNL) are taking a closer look at why these nanostructured silicon electrodes perform better than the graphite variety. "The electrodes expand as they get charged, and that shortens the lifespan of the battery," lead researcher Chongmin Wang at the Department of Energy's Pacific Northwest National Laboratory, was quoted as saying. "We want to learn how to improve their lifespan, because silicon-carbon nanofiber electrodes have great potential for rechargeable batteries."
The research, which was published in the journal Nano Letters, first tested how much lithium the electrodes could hold and how long they would last by putting the electrodes—which were made from carbon nanofibers wrapped by a thin layer of silicon—into a half-cell.
The researchers were initially impressed by the results, which showed that material maintained a very good capacity of about 1000 milliAmp-hours per gram of material after 100 charge-discharge cycles, five to 10 times the capacity of conventional electrodes in lithium ion batteries. However, they remained skeptical that the material would withstand continued charge-discharge cycles.
What they discovered while looking at the electrode through a transmission electron microscope confirmed some previous research and revealed some new phenomena. The research observed, as previous work has shown, that charging the battery does cause lithium ions to flow into the silicon. But what this research discovered was that the addition of the carbon to the silicon speeds up this process considerably—as much as 60 times faster than silicon alone.
It also turns out—just as expected—that the silicon nanocomposite electrode does swell to three times its size just as its silicon cousin, but it does so evenly avoiding the imperfections that are caused by the uneven expansion of silicon alone.
Another interesting discovery from this research builds on previous work that discovered that once the ratio of lithium to silicon reached 15 to 4 in these electrodes the material crystallizes. The PNNL research discovered that the crystallization happens all at once rather than gradually—it ‘snaps’ into its crystallized form—a phenomenon known as congruent phase transition.
The key determination though was to find out how this charging and discharging affected the electrode. The results were not encouraging. It turns out the charge-discharge process leaves the surface of the electrode looking like a potholed stretch of road.
"We can see the electrode's surface go from smooth to rough as we charge and discharge it. We think as it cycles, small defects occur, and the defects accumulate," said Wang.
While this may not sound like good news, there is the positive side in that the thin silicon reacts better than thicker silicon and is more durable. “"In the current design, because the silicon is so thin, you don't get bigger cavities, just like little gas bubbles in shallow water come up to the surface. If the water is deep, the bubbles come together and form bigger bubbles," says Wang.
The researchers will next be looking into optimizing the bonding between the carbon nanofibers and the silicon to improve both the performance and the lifetime of the electrodes.