Making batteries out of sand
Researchers at the University of California, Riverside, have come up with a magic ingredient that may improve the performance of lithium-ion batteries by a factor of three. It’s common beach sand.
“This is the holy grail — a low cost, non-toxic, environmentally friendly way to produce high performance lithium ion battery anodes,” said Zachary Favors, a graduate student working in the lab of professors Cengiz and Mihrimah Ozkan at UCR, where the research took place.
Beach-sand lithium-ion electric cars with a range of 300 miles instead of barely 100 – that’s a tall order. But Favors and the Ozkans seem to be on to something.
Favors said the idea came to him about six months ago when he was relaxing — where else? — on a beach in Southern California. He picked up a handful of sand and realized that it was mainly composed of quartz, which is silicon dioxide. Now, silicon has certainly played a large role in the California economy over the last several decades. Otherwise we wouldn’t have Silicon Valley. But that’s all been microchips and integrated circuits — “inscribing a computer on a grain of sand,” as you might call it. But batteries? To date no one had thought of applying silicon in battery technology before.
In his brief career as a graduate student, however, Favors became acutely aware of the challenges that face researchers in trying to devise more powerful and longer lasting batteries in the effort to make electrically driven vehicles a reasonable alternative to the gasoline-driven variety. So it got him thinking.
Favors’ research has focused on the anode, the negative terminal of the battery, which has traditionally been made from graphite. Researchers have tweaked and twisted the graphite in recent years, however, and haven’t been able to improve performance much. Now graphite, as you know, is made of carbon and carbon has the unique quality in that it has four available electrons, which is what makes it so versatile and the basis of all organic chemistry. But occupying the same position on the next line of the periodic table is silicon, which has the same properties. In fact, it is often said that if human beings hadn’t been made out of carbon, we might have been synthesized from silicon.
So Favors decided to see what would happen if he substituted silicon for carbon in a battery.
To do this, he first decided to find the beach sand that has the highest percentage of quartz. This turned out to be at the Cedar Creek Reservoir just east of Dallas, where he grew up! (Is there some karma at work here?) Sand in hand, he returned to the laboratory and began a very laborious process of trying to refine it into pure silicon.
The reason graphite works well as an anode, better than anthracite coal, for example, is because the molecules are loosely spaced, leaving plenty of room for electrons to gather. There’s been plenty of talk about grapheme and other carbon structures that would be an improvement but so far nothing has come of it. What Favors faced was the question of whether silicon could be shaped into such lattice-like structures that would also improve its ability to hold a charge.
Back in the lab, Favors took his Cedar Creek sand and ground it down to a very fine powder. Then he added salt and magnesium and heated the mixture. The salt absorbed the heat while the magnesium stripped the oxygen from the silicon dioxide, leaving a residue of pure silicon.
What Favors and the Ozkans discovered to their delight was that, when put through this procedure, the silicon assembled itself into a very porous, sponge-like formation that maximizes its ability to hold electrons. This “nano-silicon” has an energy density about three times that of the common graphite anodes, introducing the possibility of vast improvements in battery performance.
Silicon has been attempted to be used for battery material, but the problem has been that it degrades quickly and is hard to produce in large quantities. Another problem is that silicon tends to expand when taking a charge, often breaking in the process. But Favors and the Ozkans are hoping their approach can overcome these obstacles. The sponge-like quality of the lattice seems to reduce the swelling problem. And the basic elements — sand, salt and magnesium — are readily available, so costs should be manageable.
Favors, the Ozkans and their team of graduate students have published their findings in a paper entitled “Scalable Synthesis of Nano-Silicon from Beach Sand for Long Cycle Life Li-Ion Batteries” in Scientific Reports. They want to concentrate first on the lithium-ion batteries in cell phones, where substitution would be easy and the market is well established.
Still, it wouldn’t be surprising to find Elon Musk soon knocking on their door.
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