|TEM images of an individual DWSiNT before (a) and after (b) lithiation, respectively. They show the tube wall expanded towards the inside, while the outside diameter of the tube remains constant. Wu et al., Supplementary Material. Click to enlarge.|
Although silicon has a large charge storage capacity, making it an attractive anode material for advanced Li-ion batteries, the pulverization it experiences during cycling and an unstable solid–electrolyte interphase has limited the cycle life of silicon anodes to the order of hundreds of cycles.
A team of researchers led by Dr. Yi Cui of Stanford University and SLAC National Accelerator Laboratory has now shown that anodes consisting of an active silicon nanotube surrounded by an ion-permeable silicon oxide shell can cycle more than 6,000 times in half cells while retaining more than 85% of their initial capacity. A paper on the double-walled silicon nanotubes (DWSiNT) was published in the journal Nature Nanotechnology.
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. This latest design consists of a double-walled silicon nanotube coated with a thin layer of silicon oxide—a very tough ceramic material.
The outer surface of the silicon nanotube is prevented from expansion by the oxide shell, and the expanding inner surface is not exposed to the electrolyte, resulting in a stable solid–electrolyte interphase. Batteries containing these double-walled silicon nanotube anodes exhibit charge capacities approximately eight times larger than conventional carbon anodes and charging rates of up to 20C (a rate of 1C corresponds to complete charge or discharge in one hour).—Wu et al.
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.
Cui said future research is aimed at simplifying the process for making the double-wall silicon nanotubes. Others in 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.
In 2008, Cui founded a company, Amprius, which licensed rights to Stanford’s patents for his silicon nanowire anode technology. Its near-term goal is to produce a battery with double the energy density of today’s lithium-ion batteries.
Hui Wu, Gerentt Chan, Jang Wook Choi, Ill Ryu, Yan Yao, Matthew T. McDowell, Seok Woo Lee, Ariel Jackson, Yuan Yang, Liangbing Hu & Yi Cui (2012) Stable cycling of double-walled silicon nanotube battery anodes through solid–electrolyte interphase control. Nature Nanotechnology 7, 310–315 doi: 10.1038/nnano.2012.35