The dream of long-lasting batteries is a little closer now that scientists have made progress on a big problem: a battery material that holds a lot of energy but breaks down quickly. They’ve found a way to use little “pulleys” to keep the material, silicon, intact.
Manufacturers are interested in making a battery component called the anode with silicon instead of graphite. Silicon can hold up to five times more energy — but it also expands up to 400 percent and then cracks, which means the battery life is terrible. In an article published today in the journal Science, scientists engineered pulleys out of molecules. These pulleys hold the silicon in place so it doesn’t expand. In tests, the batteries retained 98 percent effectiveness after hundreds of cycles, and lasted just as long as a battery with a graphite anode. This could be a key development for things like electric vehicles, which need batteries that can store a lot of energy for a long time.
Let’s take a step back and look at how batteries work. Batteries have electrical conductors on opposite sides. One electrode, called the cathode, is made of lithium and holds positively charged electrons. The opposite electrode, called the anode, holds negatively charged ions. During charging, lithium ions move from the cathode to the anode. When the battery is in use, the lithium moves in the opposite direction.
The anode is the part that’s usually made out of graphite. Earlier research into silicon anodes experimented with a binder, or a polymer that (you guessed it) tries to hold the silicon together. But these were stiff and couldn’t move with the silicon or hold the particles firmly, says study co-author Jang Wook Choi, an engineer at the Korea Advanced Institute of Science and Technology (KAIST).
In today’s study, Choi’s team created a binder that is both elastic enough to accommodate the expansion, and strong enough to keep the silicon from expanding so much that it scatters. The pulley, called polyrotaxane, consists of rings on a “thread” that can slide up and down. Because the rings can move, they can follow the silicon as it expands but use force to keep it from expanding too much, explains co-author Ali Coskun, also at KAIST. They hold the silicon particles so they don’t disintegrate while changing.
Coskun did his post-doctoral fellowship with Northwestern University’s Fraser Stoddart, who received the Nobel Prize in Chemistry in 2016 for his work on molecular machines and the bonds that make these pulleys possible. “People think that this sort of research is just looking at fancy molecules, but this is as real as it gets,” says Coskun. “This research can make an impact with real-life applications.”
One of the limitations is that the design is rather complicated, so it may be expensive to commercialize, says Soojin Park, an engineer at Korea’s Ulsan National Institute of Science & Technology who was not involved with the study. Nevertheless, Coskun and Choi have already partnered with “a major Korean battery maker” to try to bring this to market. “Electrical vehicles can be a major application, but this can be helpful in all applications that need more energy,” says Choi. “Just imagine a cellphone charge lasting three days instead of one.”