A new kind of electrodes for lithium ion batteries has been invented at Stanford University. They greatly improve the performance of a lithium ion battery and are made of silicon and conducting polymer hydrogel, a spongy material that resembles the one used in contact lenses.
“We’ve been trying to develop silicon-based electrodes for high-capacity lithium-ion batteries for several years,” said study co-author Yi Cui, an associate professor of materials science and engineering at Stanford. “Silicon has 10 times the charge storage capacity of carbon, the conventional material used in lithium-ion electrodes. The problem is that silicon expands and breaks.” Cui worked with Zhenan Bao, also a professor of chemical engineering at Stanford.
The reason why the researchers used a flexibile silicon-based material is that silicon can expand by 400 percent when combined with lithium. This is a problem of lithium ion batteries – repeated charging and discharging cracks normal carbon electrodes and the battery loses capacity. With this method, however, “each silicon nanoparticle is encapsulated within a conductive polymer surface coating and is connected to the hydrogel framework. That improves the battery’s overall stability.” Indeed, the new battery had retained its initial capacity after 5,000 charge/discharge cycles.
At first, they used a technique called “in situ synthesis polymerization” that coats the silicon nanoparticles with the conducting hydrogel. However, after observing that the porous hydrogel matrix is riddled with empty spaces that allow the silicon nanoparticles to expand when lithium is inserted, they tried mixing the silicon and hydrogen – the simplest way possible, which also proved to be the best.
Also, because the hydrogen contains water, which causes ignition in contact with lithium, they eliminated the water in the end. “You don’t want water inside a lithium-ion battery,” said Bao.
The researchers are optimistic that this technology will find its place on the market after a number of technical hurdles will be overcome.