In the search for high-performance materials for fabricating lithium-based batteries, silicon has emerged as a strong contender for anodes because it has ten times the capacity for lithium uptake compared to graphitic carbon, leading to more powerful batteries. However, this performance comes at a price — the silicon changes volume during lithium uptake, which makes it degrade as it is charged and discharged.

To address this problem, Jaephil Cho at the Ulsan National Institute of Science and Technology in Korea and colleagues1 have developed a high-performance anode material based on silicon nanotubes. Their anode can be cycled multiple times, and displays ten times the capacity of graphitic carbon even after 200 charge–discharge cycles.

Fig. 1: Transmission electron microscopy image of the carbon-coated silicon nanotube anode material.

Using silicon nanotubes (Fig. 1), the researchers increased the surface area of the anode that the electrolyte can cover in the battery, which means that lithium uptake was quicker and more effective. In particular, the tubular structure allowed the inner surface of the tubes to be accessed directly. They also added a thin carbon coating to the material, which stabilized the interface with the electrolyte in the batteries to allow the anode to be cycled more times.

The lifetime of batteries with the silicon nanotube anodes was comparable with that of batteries with fully optimized commercially available graphitic carbon anodes. Even after 200 charge-discharge cycles, the performance was still 89% that of the first cycle.

Talking about the motivation for the work, Cho says that when the volume changes in standard silicon anode materials on cycling, “the mechanical strain on the brittle silicon material is so great that silicon anodes tend to crack after they are charged and discharged only a few times. So my group has been developing nanostructured silicon nanotubes to better withstand these stresses.”

He also has high hopes that the new anodes could make a big difference for real-world applications. “In a hybrid car, the battery lasts only 30 minutes using current technology,” says Cho. “If the new silicon anode can be matched to a cathode with comparable storage capacity, the resulting battery should be able to run a car for three to four hours without recharging.”