New 'pomegranate-inspired' design for smaller batteries

An electrode designed like a pomegranate allows the use of silicon for a new generation of smaller, lighter rechargeable batteries for electric cars, cell phones and other devices.
The novel battery electrode features silicon nanoparticles clustered like pomegranate seeds in a tough carbon rind, said researchers at Stanford University and the US Department of Energy's Stanford Linear Accelerator Center (SLAC) National Accelerator Laboratory.
"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.

"Experiments showed our pomegranate-inspired anode operates at 97 per cent capacity even after 1,000 cycles of charging and discharging, which puts it well within the desired range for commercial operation," Cui said.
The anode, or negative electrode, is where energy is stored when a battery charges. Silicon anodes could store 10 times more charge than the graphite anodes in today's rechargeable lithium-ion batteries.
However, 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.

The new study builds on that work. 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.
Since each pomegranate cluster has just one-tenth the surface area of the individual particles inside it, a much smaller area is exposed to the electrolyte, thereby reducing the amount of gunk that forms to a manageable level.
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.
While these experiments show the technique works, Cui said, the team will have to solve two more problems to make it viable on a commercial scale: They need to simplify the process and find a cheaper source of silicon nanoparticles.
The study was published in the journal Nature Nanotechnology.