Dr. Yi Cui, Assistant Professor of Materials Science and Engineering at Stanford University and a partner in the start-up, Amprius, is working on a new battery technology that could last for 6,000 charge and discharge cycles without depleting capacity below 85 percent. The goal is to create smaller, lighter and longer-lasting batteries than are currently available, Cui said.
Cui, who has worked with Lithium-ion battery technology in the past, said he has now abandoned lithium ions and replaced them with either sodium or potassium ions for the new, longer lasting technology. For example, a typical laptop battery can go for about 1,000 charge/discharge cycles before its starts to be able to hold 80 percent of the original charge. The battery technology significantly outperforms current power products.
Lithium-ion batteries are widely used to power everything from electric vehicles to portable electronics because they can store a relatively large amount of energy in a relatively lightweight package. The battery works by controlling the flow of lithium ions through a fluid electrolyte between its two terminals, called the anode and cathode.
The new double-walled silicon nanotube anode is produced via a four-step process [see diagram]: Polymer nanofibers (green) are made, then heated (with, and then without, air) until they are reduced to carbon (black). Silicon (light blue) is coated over the outside of the carbon fibers. Finally, heating in air drives off the carbon and creates the tube as well as the clamping oxide layer (red).
“The promise—and peril—of using silicon as the anode in these batteries comes from the way the lithium ions bond with the anode during the charging cycle. Up to four lithium ions bind to each of the atoms in a silicon anode—compared to just one for every six carbon atoms in today’s graphite anode—which allows it to store much more charge,” said an explanation of the technology from Stanford’s SLAC National Accelerator Laboratory.
Over the past five years, Cui’s group is said to have improved the durability of silicon anodes by making them out of nanowires and then hollow silicon nanoparticles. His latest design consists of a double-walled silicon nanotube coated with a thin layer of silicon oxide, a very tough ceramic material, the university said.
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 rem
The IEEE reported there is a weight penalty with the battery technology, which means that it will not be powering any laptops or electric vehicles in the near future. However, the engineering group said it might be the perfect fit for large-scale energy storage on the electrical grid.
“At a rate of several cycles per day, this electrode would have a good 30 years of useful life on the electrical grid,” said Colin Wessells, a graduate student in materials science and engineering who is the lead author of a paper describing the research, published last week in Nature Communications.
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 short-term goal is to produce a battery with double the energy density of today’s lithium-ion batteries.