New Pomegranate-like Silicon Electrode is Future of Lithium-Ion Batteries

Inspired by the structure of a pomegranate, a team of international experts has created a new battery electrode that solves the existing drawbacks of lithium-ion batteries.

Researchers at Stanford University and the Department of Energy's SLAC National Accelerator Laboratory have created a novel battery electrode that has silicon nanoparticles clustered like pomegranate seeds in a conductive carbon layer. The structure can help utilize silicon effectively and efficiently in new generation lithium-ion batteries. 

The anode (negative electrode) stores energy when the battery is charged. Silicon anodes have the capacity of storing over 10 times more energy when compared to graphite anodes in rechargeable lithium ion batteries.

But these batteries have certain setbacks that include structural degradation and instability. The brittle silicon gets enlarged and ruptures during charging and reacts with the electrolyte of the battery to form gunk that deposit on the anode.

The research team led by Yi Cui, an associate professor at Stanford and SLAC, overcame the challenges by using silicon nanowires and particles that are extremely small, even to break into little pieces. The researchers then covered the nanoparticles in carbon 'yolk shells' that offer sufficient space to swell and shrink during the charging process.

"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 Cui. "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."

In order to get the silicon yolk shells into clusters, graduate student Nian Liu and postdoctoral researcher Zhenda Lu used a technique called microemulsion, commonly found in oil, paint and cosmetics.

After clubbing the silicon yolk shells in clusters, the experts further layered each cluster with another thick coating of carbon. These carbon rinds hold the pomegranate-like structures together and offer a clear pathway for current to flow.

Also, only a small section of the area is exposed to the electrolyte. About just one-tenth of the surface area of each pomegranate cluster is inside. This lowers the formation of gunk.

The clusters being extremely minute are too difficult to mark individually. They look like fine black powder that can be applied to a piece of foil and later converted into an anode.

Tests conducted in a laboratory revealed that the pomegranate anodes functioned exceptionally well when they were designed with the right thickness needed in a commercial battery.

In order to make the new design viable on a commercial scale they need to work on two more challenges that include simplification of the process and cheaper source of silicon nanoparticles. They have targeted 'rice husk' unfit for human food that could be easily converted into pure silicon nanoparticles.

"To me it's very exciting to see how much progress we've made in the last seven or eight years," Cui said, "and how we have solved the problems one by one."

According to the inventors, this novel design allows smaller, lighter rechargeable batteries for electric cars, cell phones as well as other devices.

The study was documented in the Journal Nature Nanotechnology.