Researchers from Rice University, led by Lisa Biswali, have developed a cost-effective silicon anode (negative electrode) for rechargeable batteries that easily attains 600 charge-discharge cycles at 1,000 milliamp-hours per gram (mAh/g), which is only a third of its theoretical capacity, but almost thrice the current capacity of graphite anodes.
In search of a better replacement for graphite, scientists have been eyeing silicon which has a capacity of holding lithium ions ten times more than the commonly used graphite. The problem with silicon, though, is that when it becomes completely full of lithium (upon charging), it expands more than three times its initial volume (upon discharging). This large change in volume would easily degrade silicon anode with repetitive charge-discharge cycles.
So how do they address the problem? By increasing the surface area to volume ratio, the silicon will have more space for expansion. Biswali and her team came up with porous silicon, as shown in the SEM image above, which has shown promising properties. However, due to their delicate structure, these microporous silicon particles were difficult to handle for the manufacturers without damaging them.
A surprisingly simple solution to the problem is to eliminate the worry of damaging them by crashing them right away. This boosted further the lithium absorption capacity of the material as the crashed porous silicon offered more surface area. These crashed porous silicon particles are then mixed with a polymeric binder, polyacrylonitrile (PAN), as shown in the SEM micrograph below (porous silicon particles are encircled), for conductive and structural support.
The study’s lead author, Madhuri Thankur, said, “The surface area of our material is 46 square meters per gram. Crushed silicon is 0.71 square meters per gram. So our particles have more than 50 times the surface area, which gives us a larger surface area for lithiation, with plenty of void space to accommodate expansion.”
“The next step will be to test this porous silicon powder as an anode in a full battery. Our preliminary results with cobalt oxide as the cathode appear very promising, and there are new cathode materials that we’d like to investigate,” Biswali said.