Regular readers may recall that the silicon electrode for lithium ion batteries is the current leading candidate for a big capacity increase. A study led by Chongmin Wang at the Department of Energy’s Pacific Northwest National Laboratory (PNNL) has been published online in the journal Nano Letters examining a new type of silicon-carbon nanocomposite electrode.
To refresh, silicon’s advantage is a high capacity for energy storage comes from taking on a much larger charge than today’s electrodes. Silicon’s disadvantage is that it swells up when charged, expanding up to 3 times its discharged size. The expansion and contraction over charge and discharge cycles quickly destroys the silicon structure that makes an electrode.
The PNNL study examines a new type of silicon-carbon nanocomposite electrode revealing details of how they function and how repeated use could wear them down. The study also provides clues to why this material performs better than silicon alone.
Right off the silicon-carbon electrode equipped battery has an electrical capacity five times higher than conventional lithium battery electrodes. There’s a strong motive in place for more research because silicon-carbon nanocomposite electrodes could lead to longer-lasting, cheaper rechargeable batteries for electric vehicles. A five-fold increase changes the economics for consumers in a major way.
Wang explains the PNNL role, “The electrodes expand as they get charged, and that shortens the lifespan of the battery. We want to learn how to improve their lifespan, because silicon-carbon nanofiber electrodes have great potential for rechargeable batteries.”
Wang led a multi-institution effort to test the nano-sized electrodes consisting of carbon nanofibers coated with silicon. The carbon’s high conductivity, which lets electricity flow, nicely complements silicon’s high capacity, which stores it. Researchers at Department of Energy’s Oak Ridge National Laboratory and Applied Sciences Inc. and General Motors Global R&D Center created the silicon-carbon nanofibers and forwarded the electrodes to the team at PNNL to probe their behavior while functioning.
Then the PNNL team tested how much lithium the electrodes could hold and how long they lasted using a small testing battery called a half-cell. At 100 charge-discharge cycles, the electrodes still maintained a very good capacity of about 1000 milliAmp-hours per gram of material. That’s 5 to 10 times the capacity of conventional electrodes in lithium ion batteries. So far so good.
The team knew the expansion and contraction of the silicon could be a problem for the battery’s longevity, since stretching tends to wear things out. (Like kinking a wire quickly back and forth till it separates – note smaller wires take longer to break than large ones – so stay with this.)
To see how well the electrodes weather the repeated stretching, Wang popped a specially designed, tiny battery into a transmission electron microscope, which can view objects nanometers wide. They zoomed in on the tiny battery’s electrode allowing the team to study the electrode in use, and they took images and video while the tiny battery was being charged and discharged. There are images and videos on the study’s Supporting Information page – so click here!
Previous work has shown charging causes lithium ions to flow into the silicon. The PLLN team’s study showed lithium ions flowed into the silicon layer along the length of the carbon nanofiber at a rate of about 130 nanometers per second. This is about 60 times faster than silicon alone, suggesting that the underlying carbon improves silicon’s charging speed. There’s a bonus worth more examination.
The team expected the silicon layer to swell up about 300 percent as the lithium entered. But the combination of the carbon support and the silicon’s unstructured quality allowed it to swell evenly. That compares favorably to silicon alone, which swells unevenly, causing imperfections.
Beyond the swelling up, lithium is known to cause other changes to the silicon. The combination of lithium and silicon initially form an unstructured, glassy layer. Then, when the lithium to silicon ratio hits 15 to 4, the glassy layer quickly crystallizes, as seen in previous researcher’s work.
The team examined the crystallization process in the microscope to better understand it. In the microscope video, they could see the crystallization advance as the lithium filled in the silicon and reached the 15:4 ratio.
Here’s the breakthrough – the team found that this crystallization is different from the classic way that many substances crystallize, which builds from a starting point. Rather, the lithium and silicon layer snapped into a crystal all at once when the ratio hit precisely 15 to 4. Computational analyses of this crystallization verified its snappy nature, a type of crystallization known as congruent phase transition.
And the crystallization isn’t permanent. Upon discharging, the team found that the crystal layer became glassy again, as the concentration of lithium dropped on its way out of the silicon.
That’s the key to the huge capacity.
On the longevity front the team charged and discharged the tiny battery 4 times. Comparing the same region of the electrode between the first and fourth charging, the team saw the surface become rough, similar to a road with potholes.
Wang has an explanation about the changes being likely due to lithium ions leaving a bit of damage in their wake upon discharging, “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.”
The fact that the silicon layer is very thin makes it more durable than thicker silicon. In thick silicon, the holes that lithium ions leave behind can come together to form large cavities. “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,” is Wang’s metaphor.
For the future work the team expects to explore the thickness of the silicon layer and how well it bonds with the underlying carbon to optimize the performance and lifetime of the electrodes.
Lots of possibility here. Great work and a great start. And a big thanks to the American Chemical Society for making the images and videos available. Seeing an electrode in action is quite a treat as well as highly instructional.
So far this is the best solution seen to getting the silicon advantage into the lithium ion battery chemistry. Silicon has tempted and bedeviled for years. The potential using the preliminary numbers are tremendous bait – a 5 to 10 fold increase in capacity could double range for 40% of the mass. Plus those flow rates suggest very quick charges and discharges offering simpler engineering, better performance and fast recharge. This is a technology to watch very carefully.