Nanowire batteries to get seven-minute charge

Silicon nanowires could form the basis for very fast charging batteries with three times the capacity of today’s best lithium-ion products.

In work described at the International Electron Device Meeting in San Francisco this week, Professor Yi Cui of Stanford University said an extension to work published at the beginning of the year could yield a lithium-ion battery that can recharge in a matter of minutes rather than hours and which is potentially safer than today’s fire-prone designs.

Cui’s initial work focused on using silicon nanowires to capture lithium ions as the battery charges. The wires expand as the lithium moves into the silicon, turning it into an amorphous form. Potentially, the stored charge is ten times that of the powdered graphite currently used in lithium-ion batteries.

In work to be published in an upcoming issue of the journal Nano Letters, Cui’s team built nanowires in two steps. The first produces a thin crystalline core. The second forms an amorphous silicon cladding which captures the bulk of the lithium. This structure improves the stability of the nanowire.

“You can charge a battery extremely fast this way: it can take just seven minutes to recharge,” Cui claimed. “The single crystal core remains intact and it retains stability.

The maximum capacity is lower than the theoretical maximum possible with silicon: around 800mAh/g versus more than 4000. “But that is already three times more than powdered graphite,” Cui explained.

The material choice for the cathode is more involved, said Cui – the silicon can only be used in the anode. However, forming existing materials into nanowires seems to show better results than the powders used today. “Replacing the carbon in the anode alone almost doubles the capacity of the capacity. The increase in capacity is around 80 per cent.”

Cui said the technology is manufacturable. “We don’t have to use high-purity silicon. It does not have to be solar-cell grade. So there is plenty of silicon available to make batteries. And, in terms of safety, we believe silicon can be safer than carbon.”

The next step, Cui told Engineering & Technology, is to establish how the process can be scaled up to industrial levels and to work on safety tests. The nanowires grow at a rate of around 10µm/minute, which is fast enough to be economic in production, he claimed.

For larger batteries, the Stanford team is looking at variants that use water as a solvent rather than the flammable organic materials used in smaller, conventional lithium-ion batteries.

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