Increasing durability of lithium-ion batteries with silicon anodes

We often cover breakthroughs in energy storage technology since there is a growing need for usage of stored power with our gadgets and electric vehicles. A team of researchers from SLAC National Accelerator Laboratory, which is operated by Stanford University for the U.S. Dept. of Energy Office of Science, created a double-walled nanostructure which increases the number of cycles lithium-based batteries can handle before their properties start to degrade.

“This is a very exciting development toward our goal of creating smaller, lighter and longer-lasting batteries than are available today”, said Yi Cui, Stanford University materials scientist who led the team.

Lithium-ion batteries work by controlling the flow of lithium ions through a fluid electrolyte between its two terminals, called the anode and cathode. For more than a decade, various research groups tried to increase the amount of charge lithium-based batteries can store by replacing the graphite in one terminal with silicon. While silicon as the anode in these batteries enables up to four lithium ions bind to each of the atoms in a silicon anode instead just one for every six carbon atoms in currently used graphite anodes.

On the other hand, silicone anode also swells to as much as four times its initial volume and some of the electrolyte reacts with the silicon and inhibits further charging at that area. When lithium flows out of the anode during discharge, the anode shrinks back to its original size and the coating cracks, exposing fresh silicon to the electrolyte. Although the process enables 10 times more charge storage, it would be useless just after a couple of charging cycles.

In 2008, Cui founded a company, Amprius, which licensed rights to Stanford’s patents for his silicon nanowire anode technology. Over the past five years, Cui’s group has progressively improved the durability of silicon anodes by making them out of nanowires and then hollow silicon nanoparticles. The latest design consists of a double-walled silicon nanotube coated with a thin layer of silicon oxide – a very tough ceramic material.

This strong outer layer keeps the outside wall of the nanotube from expanding, so it stays intact. Instead, the silicon swells harmlessly into the hollow interior, which is also too small for electrolyte molecules to enter. After the first charging cycle, it operates for more than 6,000 cycles with 85 percent capacity remaining.

You probably noticed that even your rechargeable battery loses its maximum charge over time. Every time you fully deplete the power equal to one maximum charge, your battery has gone through one cycle. If you don’t use the entire charge at once since you do recharge your portable electronics, as most people do when they are charging more power hungry devices, the cycle can be calculated in percentages of amount of the battery being charged at that time. For example, a cycle is if you charge a battery 3 times after its charge dropped to two thirds of its maximum charge.

In comparison, currently used lithium-ion batteries are expected to retain up to 80% of their maximum charge for the first 300 cycles. That is why you notice your battery charge lasts less after 10 or 11 months if it is fully recharged each day.

While Cui aims to simplify the process for making the double-wall silicon nanotubes, other members of his group are developing new high-performance cathodes to combine with the new anode to form a battery with five times the performance of today’s lithium-ion technology.

For more information, read the article published in the journal Nature Nanotechnology: “Stable cycling of double-walled silicon nanotube battery anodes through solid–electrolyte interphase control”.