The key is a stretchy, highly conductive polymer binder that can be used to hold together silicon, tin, and other materials that can store a lot of energy but that are ordinarily unstable. Researchers at the Lawrence Berkeley National Laboratory painstakingly engineered this new polymer binder and used it to make a silicon anode for a rechargeable lithium-ion battery with a storage capacity 30 percent greater than those on the market today. It’s also more stable over time than previously developed electrodes.
When a lithium-ion battery is charged, lithium ions are taken up by one of the electrodes, called the anode. The more lithium the anode can hold, the more energy the battery can store. Silicon is one of the most promising anode materials: it can store 10 times more lithium than graphite, which is used to make the anodes in the lithium-ion batteries on the market today. "Graphite soaks up lithium like a sponge, holding its shape, but silicon is more like a balloon," says Gao Liu, a researcher at the Berkeley Lab’s Environmental Energy Technologies Division.
However, because the silicon anodes swell and shrink, changing in volume by three or four times as they’re charged and discharged, the capacity of the battery fades over time. "After a few rounds of charge and discharge, pretty soon the silicon particles are not in touch with each other," which means the anode can’t conduct electricity, says Liu.
One approach to the problem is to structure these anodes in a totally different way, for example growing shaggy arrays of silicon nanowires that can bend, swell, and move around as lithium enters and exits. This approach is being commercialized by Amprius, a startup in Palo Alto, California. But growing nanowires requires new processes that aren’t normally used in battery manufacturing.
Today’s anodes are made by painting a solvent-based slurry of graphite particles held together with a binder, a simple process that keeps costs low. The Berkeley researchers believe the key to making new battery materials like silicon work is to stick with this manufacturing process. That meant coming up with a rubbery binder that would stick to silicon particles, remain highly conductive in the harsh environment of the anode, and stretch and contract as the anode swells and deflates.
Most work on advanced batteries has focused on the active materials, but "we have pushed these materials to the limit," says Yury Gogotsi, professor of materials science and engineering at Drexel University. "Now what’s limiting us are the binders."
Reading through papers on silicon battery binders, Liu noticed that researchers were making "fatal mistakes"—choosing polymers that lose their conductivity in the kinds of conditions found in an anode, for example. He worked with theoretical chemists to come up with a list of polymers with the right electrical properties for the job. Once they found one, they altered it to make it much stickier. Once they developed and characterized this new material, they were able to make silicon anodes using conventional processes, and test them in batteries.
The Berkeley group’s anodes have been tested in over 650 charging cycles. They maintain a storage capacity of 1,400 milliamp hours per gram—much greater than the 300 or so stored by conventional anodes. Full batteries incorporating the anodes store about 30 percent more total energy than a commercial lithium-ion battery. Typically, battery capacity increases by about 5 percent a year, Liu notes. He says they’ve tested the binder in other battery anodes, including those made of tin, that have similar potential and problems, and that it should work for any such materials.
The storage capacity of these batteries is nearly as good as those made from pure silicon nanowires with no binders, says Yi Cui, professor of materials science and engineering at Stanford and one of the founders of Amprius. That’s impressive, he says, considering that the binder doesn’t store any lithium.
Liu’s group is now collaborating with researchers at 3M on the anode research. 3M is scaling up production of silicon-based battery materials designed to not expand quite so much during charging, says Kevin Eberman, who is developing battery materials products at 3M Electronics in St. Paul, Minnesota. But to make them work, a good binder is key. The company is providing the Berkeley group with materials to test. Liu says the Berkeley group has patented the binders, and is in talks with a few companies about ways to commercialize them.