Thursday, September 8, 2011
Lithium-ion batteries could hold up to 10 times as much energy per cell if silicon anodes were used instead of graphite ones. But manufacturers don't use silicon because such anodes degrade quickly as the battery is charged and discharged.
Researchers at the Georgia Institute of Technology and Clemson University think they might have found the ingredient that will make silicon anodes work—a common binding agent and food additive derived from algae and used in many household products. They say this material could not only make lithium-ion batteries more efficient, but also cleaner and cheaper to manufacture.
Lithium-ion batteries store energy by accumulating ions at the anode; during use, these ions migrate, via an electrolyte, to the cathode. The anodes are typically made by mixing an electroactive graphite powder with a polymer binder—typically polyvinylidene fluoride (PVDF)—dissolved in a solvent called NMP. The resulting slurry is spread on the metal foil used to collect electrical current, and dried.
If silicon particles are used as the basis of the electroactive powder, the battery's anode can hold more ions. But silicon particles swell as the battery is charged, increasing in volume up to four times their original size. This swelling causes cracks in the PVDF binder, damaging the anode. In research published today by Science, the Georgia Tech and Clemson scientists show that when alginate is used instead of PVDF, the anode can swell and the binder won't crack. This allows researchers to create a stable silicon anode that has, so far, been demonstrated to have eight times the capacity of the best graphite-based anodes.
The polymer alginate is made from brown algae, including the type which forms forests of giant kelp. It is already widely used as a gelling agent and a food additive. Initially, the researchers thought to replace PVDF with a combination of several different materials. Then, on theoretical grounds, they realized that a polymer with just the right kind of uniform structure could do all the things the binder was supposed to do, including providing good structural support while not chemically reacting with the electrolyte.
Gleb Yushin, one of the researchers and director of the Center for Nanostructured Materials for Energy Storage at Georgia Tech, says the team realized that some synthetic polymers, derived from plant cellulose, have structures that were close to what was needed, but weren't uniform enough. So the team began looking at aquatic plants. Says Yushin: "We thought that there might already be a polymer [we could use], because aquatic plants—especially those in seawater—are immersed in an electrolyte," and so would have evolved to prevent unwanted reactions. They came across alginate, which can be extracted by boiling kelp in soda water, and which has the uniform structure required.
Another advantage of alginate over PVDF is that, during anode manufacture, alginate can be dissolved in water, eliminating the need for NMP, potentially making for a cleaner manufacturing process. The researchers believe the binder could be integrated into existing anode manufacturing systems simply by swapping the PVDF and NMP supplies for alginate and water. The alginate could also be used to improve the performance of graphite-based anodes, allowing more charge and discharge cycles over the battery's lifetime.
The full potential of a silicon anode can't be exploited until researchers develop a matching cathode capable of handling the same amount of lithium ions. But even with existing cathodes, alginate-silicon anodes could increase the capacity of lithium-ion batteries by 30 to 40 percent, according to Yushin.