Lithium-ion (Li-ion) batteries are ubiquitous but flawed, especially for electric vehicle applications. The problem has been poor charge life. But researchers have shown that if you replace the graphite on the anodes with silicon, the charge can be increased by a factor of ten. Problem is that after a few charge-discharge cycles the silicon cracks and becomes inoperable from the expansion and contraction of the material.
One solution: nanostructured silicon anodes that improve on the traditional silicon variety in this area of charge-discharge cycles. While there’s been improvement from silicon’s poor performance, the nanostructured variety still doesn’t measure up to plain old graphite in this regard.
That's a shame, because silicon anodes just crush graphite when it comes charge life. So, if there were a way to get silicon to work, it would offer some considerable benefits to Li-ion batteries, and nanotechnology has been the prayer. At least one commercial interest believes that the nano-based solution has already been developed for enabling silicon anodes to survive a large number of charge-discharge cycles.
Now research, led by Yi Cui of Stanford, who holds the distinction of having the most cited paper at ACS journal Nano Letters over the past 10 years, has come up with a nanostructured silicon capable of 6,000 cycles while maintaining 85% of its capacity.
That sounds good…for lithium-ion batteries, but Cui demonstrated just last year that potassium or sodium ions in place of the lithium variety can create batteries capable of 40,000 cycles while maintaining 83% of its charge. The issue there was that they had the cathode sorted but hadn’t yet developed the anode.
Then there's the issue of applications. The sodium- or potassium-based ion battery was targeted for large-scale energy storage on the electrical grid. Again, the Li-ion battery Cui and his colleagues has developed is being targeted for electric vehicle applications.
Cui himself started a company, Amprius, in which he was going to use his silicon nanowire anode technology with the aim of doubling the energy density of Li-ion batteries. I hope the company succeeds, but even if you achieve a doubling of energy density in the Li-ion battery you still only get to 400Wh/kg for powering vehicles. According to Stephen Chu, Li-ion batteries will need to get to 1000Wh/kg to really be competitive with fossil fuel-powered vehicles. Why can’t powering laptops be good enough for these batteries?
Dexter Johnson is a contributing editor at IEEE Spectrum, with a focus on nanotechnology.