This research is bearing fruit.
Batteries designed like pomegranates – with charge-holding pieces of silicon bunched together like seeds inside a hard rind – could one day hold 10 times as much charge as traditional rechargeable lithium-ion batteries, scientists claim.
"While a couple of challenges remain, this design brings us closer to using silicon … in smaller, lighter and more powerful batteries for products like cellphones, tablets and electric cars," said Yi Cui, an associate professor at Stanford University and the Department of Energy’s SLAC National Accelerator Laboratory, who led the research. "Experiments showed our pomegranate-inspired [design] 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."
The key is both in the materials and the design: silicon holds far more of a charge than the graphite that’s used in today’s rechargeable lithium-ion batteries, but the material is especially brittle. Graphite holds its form, whereas silicon swells and then falls apart during charging.
That’s where nanoscience comes in.
Previously, researchers had found promise using silicon nanowires and nanoparticles – pieces of silicon so tiny that they were too small to break. They grouped the wires and particles together, and then encased them in carbon “yolk shells” so they’d have room to grow and shrink during charging.
This latest study then built on that research. Stanford and SLAC scientists cribbed a process from the paint, cosmetics and oil industries to gather and then hold the yolk shells together in larger clusters – providing “a sturdy highway for electrical currents,” according to a statement on the findings distributed by Stanford.
The clusters are too small to be seen individually, but grouped together they form a fine black powder, which can be poured onto a piece of foil to form an anode, or the part of a battery that holds a charge. When scaled-up in lab tests to the thickness of a commercial battery, the anodes “worked well,” the statement said.
Some challenges remain: Silicon nanoparticles remain expensive, but could potentially be harvested from rice husks. The process itself also remains complicated and costly – at least for now.
"To me it's very exciting to see how much progress we've made in the last seven or eight years," Cui said.
The findings were first published in the journal Nature Nanotechnology on Sunday.