Rice University researchers have developed yet another li-ion (lithium-ion) silicon battery anode, but this time it is a crushed, porous silicon anode. The researchers “have refined silicon-based lithium-ion technology by literally crushing their previous work to make a high-capacity, long-lived and low-cost anode material with serious commercial potential for rechargeable lithium batteries,” a Rice University press release notes. The research was led by Sibani Lisa Biswal and Madhuri Thakur.
The new li-ion silicon battery anode lasts much longer than previous designs. It has sustained operation through 600 charge–discharge cycles at 1,000 milliamp hours per gram (this means it was charged 600 times). “This is a significant improvement over the 350 mAh/g capacity of current graphite anodes,” the press release notes, putting it “squarely in the realm of next-generation battery technology competing to lower the cost and extend the range of electric vehicles.”
Li-ion and Lead-acid Battery Life
Notably, while typical lithium-ion batteries can be cycled 1,200 times, people don’t actually get that many cycles out of them. Due to normal usage of most devices, people do not usually get to cycle them that many times, only a few hundred times. This is due to the fact that lithium-ion batteries self-degrade and die before people can get the 1,200 cycles out of them. Therefore, 600 cycles is not as bad as it sounds. (For comparison, lead-acid batteries can usually be cycled 500-800 times.)
Additionally, another one of the researchers’ anodes “continues to cycle at a C/5 rate (five-hour charge and five-hour discharge) and is expected to remain at 1,000 mAh/g for more than 700 cycles.”
Another important point to note is that the application of batteries greatly affects their lifespan. Very heavy usage of lithium-ion batteries can shorten their lifespan to even less than the time it takes them to self-degrade. This is why some lead-acid batteries can last 5 years (when used for battery backup applications such as UPSes), while some lithium-ion batteries last only 3 years, despite the fact that lithium-ion batteries have a longer cycle life.
Silicon Anode–based Batteries
Of the last two silicon anode-based batteries I’ve seen: the earliest one used a solid, rigid silicon anode which cracked promptly; and the other a silicon nanowire anode which lasted longer but was still unreliable. One reason scientists keep pursuing (and successfully improving) silicon-based li-ion batteries is because they have tremendous potential. They can achieve an energy density of 1,000 Wh per kg, as opposed to the 95-180 Wh per kg that typical batteries achieve. Lithium polymer batteries are on the high (and expensive) end of the battery market and they are the ones that achieve 180 Wh/kg.
This means that these batteries are extremely lightweight, 10 times lighter than mainstream lithium-ion batteries. Specifically, these Rice batteries don’t store that much, but still much more than average (1,000 mAh per gram as opposed to 350 mAh per gram). This enables electric vehicles to travel at least several hundred miles (close to 1,000 miles) per charge, putting the even Tesla Roadster and Model S range and performance to shame.
Why the Delay in Using Silicon Anode–based Batteries
So, if silicon-anode batteries are so great, why the delay in using them? Of course, “there’s a problem,” the Rice release notes. “Silicon more than triples its volume when completely lithiated. When repeated, this swelling and shrinking causes silicon to quickly break down.”
So, here’s more from Rice University on how researchers have tried to address this so far, and how Thakur and Biswal are doing things differently: Many researchers have been working on strategies to make silicon more suitable for battery use. Scientists at Rice and elsewhere have created nanostructured silicon with a high surface-to-volume ratio, which allows the silicon to accommodate a larger volume expansion. Biswal, lead author Thakur and co-author Michael Wong, a professor of chemical and biomolecular engineering and of chemistry, tried the opposite approach; they etched pores into silicon wafers to give the material room to expand. By earlier this year, they had advanced to making sponge-like silicon films that showed even more promise.
But even those films presented a problem for manufacturers, Thakur said. “They’re not easy to handle and would be difficult to scale up.” But by crushing the sponges into porous grains, the material gains far more surface area to soak up lithium ions.
Biswal held up two vials, one holding 50 milligrams of crushed silicon, the other 50 milligrams of porous silicon powder. The difference between them was obvious. “The surface area of our material is 46 square meters per gram,” she said. “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 porous silicon powder is mixed with a binder, pyrolyzed polyacrylonitrile (PAN), which offers conductive and structural support.
“As a powder, they can be used in large-scale roll-to-roll processing by industry,” Thakur said. “The material is very simple to synthesize, cost-effective and gives high energy capacity over a large number of cycles.” “This work shows just how important and useful it is to be able to control the internal pores and the external size of the silicon particles,” Wong said.
“The next step will be to test this porous silicon powder as an anode in a full battery,” Biswal said. “Our preliminary results with cobalt oxide as the cathode appear very promising, and there are new cathode materials that we’d like to investigate.”