Lithium-ion (Li-ion) batteries have been the subject of intense research aimed at manufacturing them with nanomaterials that will let them better meet the demands of everything from laptops and mobile devices to all-electric vehicles (EVs).
A large portion of the research has been focused on developing nanomaterials for the anode of the Li-ion that will replace graphite. But there has definitely been a shift toward the cathode, as evidenced by research over the last few years.
Now a research team out of the University of Southern California (USC) has taken a novel approach to the improvement of Li-ion batteries with nanomaterials: tackling both the cathode and the anode simultaneously.
For the anode, the USC researchers developed an inexpensive method for producing porous silicon nanoparticles through ball milling and stain etching. For the cathode, they developed a method of coating sulfur powder with graphene oxide to improve performance.
Reports detailing the research projects were published in the journal Nano Letters, but in separate papers. The work on the silicon anode ("Large-Scale Fabrication, 3D Tomography, and Lithium-Ion Battery Application of Porous Silicon") was originally published online in November; details about the work on the cathode ("Solution Ionic Strength Engineering As a Generic Strategy to Coat Graphene Oxide (GO) on Various Functional Particles and Its Application in High-Performance Lithium–Sulfur (Li–S) Batteries") made the journal in December.
The silicon anode the research team developed demonstrated a stable capacity above 1100 milliamp-hours per gram (mAh/g) for 600 extended cycles, making it nearly three times as powerful and durable as a typical commercial anode. Meanwhile, the team's graphene oxide coating improved the sulfur cathode's capacity to 800 mAh/g for 1000 charge-discharge cycles, which is more than five times the capacity of commercial cathodes.
While the capacity numbers are good, the cycle numbers appear low—at least compared to the 6000 cycles reported for nanostructured silicon in anodes just two years ago. And early last year, researchers at Argonne National Laboratory were able to achieve a specific anode capacity of 1250 mAh/g and maintain it for 5000 charge-discharge cycles.
The USC researchers note that they also developed a silicon nanowire-based anode that had high capacity and was extremely durable, but the production method was prohibitively expensive. The researchers believe this new approach, which builds on their previous research, strikes an attractive balance between performance and cost-effective production.
"Our method of producing nanoporous silicon anodes is low-cost and scalable for mass production in industrial manufacturing, which makes silicon a promising anode material for the next generation of lithium-ion batteries," said Chongwu Zhou, an electrical engineering professor at USC's Viterbi School of Engineering, in a press release. "We believe it is the most promising approach to applying silicon anodes in lithium-ion batteries to improve capacity and performance."
Interestingly, the team’s use of graphene for the cathode hints at its effectiveness as an anode material. So-called decorated graphene, where nanoparticles are scattered around the surface of the one-atom-thick sheet, is relatively easy to produce cheaply and effectively. What's more, it not only has high storage capacity (or energy density), but also high power density. This is due to graphene’s inherent high conductivity.
Whether nanostructured silicon or graphene will win the day as an anode material remains to be seen; we can rest assured we will see higher capacity cathodes, which is the source of the battery’s energy.