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.