Clusters of silicon have energising applications

The team, from Stanford University and the US Department of Energy’s SLAC National Accelerator Laboratory, achieved their results by packing clusters of silicon nanoparticles in carbon to make the electrode.

‘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, reported in Nature Nanotechnology.

‘Experiments showed our…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.’

Silicon anodes could store 10 times more charge than the graphite anodes in today’s rechargeable lithium-ion batteries, but a significant challenge remains in that silicon swells and falls apart during battery charging. Similarly, it reacts with the battery’s electrolyte to form a coating that covers 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 parts and encasing the nanoparticles in so-called carbon ‘yolk shells’ that give them room to swell and shrink during charging.

The new study builds on that work. 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. The carbon holds the clusters together and provide a sturdy highway for electrical currents.

Since each cluster has 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 coating 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 anodes worked well when made in the thickness required for commercial battery performance.

Cui said that while these experiments show the technique works, the team will have to solve two more problems to make it commercially viable.

First, they need to simplify the process and then find a cheaper source of silicon nanoparticles. One possible source is rice husks that are unfit for human food, produced by the millions of tons and 20 percent silicon dioxide by weight.

According to Liu, they could be transformed into pure silicon nanoparticles relatively easily, as his team recently described in Scientific Reports.

‘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 in a statement.