Let’s all take a moment and give a shout-out to lithium—more specifically to lithium ions, most specifically to lithium-ion batteries. Without them, you’d practically be cut off. It’s lithium ion batteries that power your laptop, your smartphone, and your tablet, not to mention your car if you drive a hybrid. For all they do, lithium-ion batteries are simple things, consisting of a cathode and an anode immersed in a lithium solution. They’re also frustrating things—rechargeable, which is nice, except that it sometimes feels recharging is all they do. Part of that is due to the anodes, which are typically made of graphite, but scientists have been pondering making them out of silicon, which could boost the batteries’ lifetime by 30 to 50%. Now a Korean team of researchers has published a paper in the Proceedings of the National Academy of Sciences suggesting an unusual source for that silicon: rice.
Rice husks, the outer shell that’s removed when rice is processed and packaged, are around 20% silica by weight. (Silica is the same substance that makes up sand—a silicon atom bonded to two oxygen atoms.) Rice is a staple crop for a third of the world’s population, with more than a hundred million tons of husks piling up each year. So far, there hasn’t been much call for them, so they tend to be recycled into things like animal feed.
The reason the team thinks this silica, rather than the silica that you can scoop up on the beach, is a good source for battery anodes, is that it comes prearranged in the kind of nano-scale structure that engineers try to build for such purposes. The big problem with silicon as a battery component is that when the battery charges, the silicon expands more than 300%, and then shrinks back down as the charge goes down. After repeated cycles of swelling and shrinking like a carnival balloon, the silicon, quite understandably, starts to fall apart. But using silicon that’s patterned at the nanoscale seems to help with that problem.
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Silicon with this durability and flexibility can be expensive to produce, but rice silica, whose whole purpose is to form a protective shell around the kernel that insects and pathogens can’t penetrate, has those properties naturally. When Jang Wook Choi at Korea Advanced Institute of Science and Technology, senior author of the paper, saw microscope images of it in a colleague’s lab, he lit up: “I became very interested because that structure might be ideal for silicon anodes,” he recalls. “I know that chemistry, I know that structure will be good.”
Good, that is, is if the structure would stick around once the silica was converted to silicon by means of chemical reactions. Through trial and error, the team perfected a method—involving a carefully calibrated dip in acid and a couple heat treatments—for removing the oxygen atoms from rice silica without upsetting the prized structure. Then they spread the purified silicon, now a yellowish powder, on a sheet of metal foil and set up a simplified version of a battery. They found the silicon could withstand at least 200 charge cycles while maintaining 100% of its capacity—not yet up to the standards required for lithium-ion batteries, but promising.
It’s not the first time researchers have looked at rice to find silicon. In fact, on the other side of the world, the lab of Yi Cui at Stanford University lit upon the same idea. Those researchers published a paper in late May with similar findings. “It’s such an amazing source of silicon,” Yi says. “If you can convert silicon dioxide into silicon, that provides a lot of silicon nanostructure at low cost.”
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You won’t be finding rice husks in your cell phone battery in the near future, if ever—there are still many kinks to be worked out, some quite significant. Jeff Dahn, a battery scientist at Dalhousie University in Nova Scotia, Canada, points out one major issue: when lab mock-ups of silicon-anode batteries using rice husks are charged, they tend to degrade the lithium solution faster than would be acceptable in a commercial battery. So an improvement in one component of the battery leads to a breakdown in another.
Yi Cui acknowledges the challenges but he is confident that improvements are forthcoming. “The purpose of these papers was to find a low-cost silicon source. Certainly the battery data could be better if we spend more time on it.” Meanwhile, the benefits of silicon anodes continue to beckon. Despite the swelling and shrinking, and despite the problems of degrading lithium, the prospect of more energy-dense batteries still attracts researchers. Lots of minds are likely to produce lots of solutions—whether they’re rice or something else.
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