Gel-coated nanowires yield batteries with ‘indefinite’ cycle life
The basic structures being investigated are charge-storage mechanisms based around nanowires; the ‘wires’ in question are deposited structures (wires only the sense they are long, thin conductors). In the work reported, the wires are gold, and they are coated or sheathed in manganese dioxide (MnO2). The device is essentially a capacitor, with the charge-storing capacitance being formed between interleaved arrays of the wires, all suspended in an electrolyte medium.
The use of nanowires is known to offer higher charge storage than films of identical materials. Thousands of times thinner than a human hair, they’re highly conductive and feature a large surface area for the storage and transfer of electrons. The gold wires (laid down on glass) are 240 nm across and 35 nm thick; various shell thicknesses between 143 and 300 nm were evaluated. Prior work has found that these filaments are extremely fragile and don’t hold up well to repeated discharging and recharging, or cycling. In a typical lithium-ion battery, they expand and grow brittle, which leads to cracking.
UCI researchers have solved this problem by coating a gold nanowire in a manganese dioxide shell and encasing the assembly in an electrolyte made of a Plexiglas-like gel: a gel of poly-methyl-methacrylate and LiClO4 (lithium perchlorate, source of the lithium ions to form a lithium ion battery). The combination is reliable and resistant to failure.
The study leader, UCI doctoral candidate Mya Le Thai, cycled the testing electrode up to 200,000 times over three months without detecting any loss of capacity or power and without fracturing any nanowires. The findings were published today in the American Chemical Society’s Energy Letters.
Hard work combined with serendipity paid off in this case, according to senior author Reginald Penner. “Mya was playing around, and she coated this whole thing with a very thin gel layer and started to cycle it,” said Penner, chair of UCI’s chemistry department. “She discovered that just by using this gel, she could cycle it hundreds of thousands of times without losing any capacity.”
“That was crazy,” he added, “because these things typically die in dramatic fashion after 5,000 or 6,000 or 7,000 cycles at most.” The structure used by the UCI workers intentionally employed nanowires of high aspect ratio (very long relative to their width) in order to reveal or amplify the effects of any failures: none were seen.
The researchers think the gel plasticizes the metal oxide in the battery and gives it flexibility, preventing cracking.
“The coated electrode holds its shape much better, making it a more reliable option,” Thai said. “This research proves that a nanowire-based battery electrode can have a long lifetime and that we can make these kinds of batteries a reality.”
The study was conducted in coordination with the Nanostructures for Electrical Energy Storage Energy Frontier Research Center at the University of Maryland, with funding from the Basic Energy Sciences division of the U.S. Department of Energy.
University of California, Irvine: www.uci.edu