Pomegranate inspires silicon anode breakthrough for lithium-ion batteries
By designing an electrode that looks like a pomegranate but has silicon nanoparticles clustered like seeds in a tough carbon rind the researchers believe they have overcome several of the remaining obstacles to using a silicon anode for a new generation of lithium-ion batteries.
"While a couple of challenges remain, this design brings us closer to using silicon anodes in smaller, lighter and more powerful batteries for products like cell phones, tablets and electric cars,” said Yi Cui, an associate professor at Stanford and SLAC who led the research which was in Nature Nanotechnology.
“Experiments showed our pomegranate-inspired anode operates at 97 percent capacity even after 1,000 cycles of charging and discharging, which puts it well within the desired range for commercial operation,” explained Yi Cui.
Silicon anodes should be able to store 10 times more charge than the graphite anodes used by today’s rechargeable lithium-ion batteries, but they also have major drawbacks. The brittle silicon has a tendency to swell and fall apart during battery charging, and also reacts with the battery’s electrolyte to form a scum that coats the anode and degrades its performance.
Over the past eight years, Cui’s team has tackled the breakage problem by using silicon nanowires or nanoparticles that are too small to break into even smaller bits and encasing the nanoparticles in carbon ‘yolk shells’ that give them room to swell and shrink during charging.
Top: Silicon nanoparticles are encased in carbon ‘yolk shells’ and clustered like seeds in a pomegranate. Each cluster has a carbon rind that holds it together, conducts electricity and minimizes reactions with the battery’s electrolyte that can degrade performance. Bottom: Silicon nanoparticles swell during battery charging to completely fill their yolk shells; no space is wasted, and the shells stay intact. (Nian Liu, Zhenda Lu and Yi Cui/Stanford)
Graduate student Nian Liu and postdoctoral researcher Zhenda Lu used a microemulsion technique common in the oil, paint and cosmetic industries to gather silicon yolk shells into clusters, and coated each cluster with a second, thicker layer of carbon. These carbon rinds hold the pomegranate clusters together and provide a highway for electrical currents.
The novel battery electrode features silicon nanoparticles clustered like pomegranate seeds in a tough carbon rind. (Illustration by Greg Stewart/SLAC)
Each pomegranate cluster has one-tenth the surface area of the individual particles inside it which means that a much smaller area is exposed to the electrolyte and reduces the amount of the coating that reduced performance.
Although the clusters are too small to see individually, together they form a fine black powder that can be used to coat a piece of foil and form an anode. Lab tests showed that pomegranate anodes worked well when made in the thickness required for commercial battery performance.
There are still two more problems the scientists have to solve to make the pomegranate battery a viable on a commercial scale solution.
Firstly the process needs to be refined and simplifed. A cheaper source of silicon nanoparticles also needs to be found. Again seeking inspiration from food the scientists are looking into whether rice husks which are unfit for human consumption but are produced by the millions of tons could be the answer because they comprise 20 percent silicon dioxide by weight. The researchers believe the rice husks could be transformed into pure silicon nanoparticles relatively easily.
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