Researchers at Stanford University and the Department of Energy's SLAC National Accelerator Laboratory have developed an electrode with silicon nanoparticles clustered like seeds in a tough carbon rind for a new generation of lithium-ion batteries.
Experiments showed the 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, according to Yi Cui, an associate professor at Stanford and SLAC who led the research.
Silicon anodes could store 10 times more charge than the graphite anodes in today's rechargeable lithium-ion batteries, but they also have major drawbacks: The brittle silicon swells and falls apart during battery charging, and it reacts with the battery's electrolyte to form gunk 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.
Researchers 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 sturdy highway for electrical currents.
Each pomegranate cluster has just one-tenth the surface area of the individual particles inside it, exposing a much smaller area to the electrolyte, thereby reducing the amount of gunk that forms to a manageable level.
While these experiments show the technique works, Cui said, the team will have to simplify the process and find a cheaper source of silicon nanoparticles to make it viable on a commercial scale. One possible source is rice husks. They are unfit for human consumption, produced by the millions of tons and 20 percent silicon dioxide by weight. According to research team member Nian Liu, they could be transformed into pure silicon nanoparticles relatively easily.
"To me it's very exciting to see how much progress we've made in the last seven or eight years and how we have solved the problems one by one," Cui said.
Details of the research are in the latest issue of Nature Nanotechnology.
The research was funded by the DOE Office of Energy Efficiency and Renewable Energy through the Batteries for Advanced Transportation Technologies program.
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