UMD Makes a Wood Battery for Pinocchio
In a remarkable display of creativity, the University of Maryland has portrayed a prototype nanobattery created by researchers as being made of wood, sending the blogosphere into a tizzy.
"A sliver of wood coated with tin could make a tiny, long-lasting, efficient and environmentally friendly battery," the university claims. A video strengthens that impression.
However, the truth is a little different. It seems the researchers experimented with using wood fibers for the anode of a sodium-ion battery, rather than making a battery out of wood.
"We use wood fiber as a replacement of the traditional stiff substrate to release the stress and as an ion reservoir for the tin electrode," Hongli Zhu, the lead researcher, told TechNewsWorld.
What Are Little Sodium-Ion Batteries Made Of?
Sodium-ion batteries are reusable batteries that use sodium ions, no surprise there, to store energy. They have aroused interest because sodium is much more plentiful than lithium and sodium-ion batteries will cost less than the lithium-ion ones being used in consumer electronic products and hybrid cars today.
Normal sodium cells have a storage capacity of 400 W-hr kg, which is relatively high. However, they can't maintain a strong charge after being repeatedly charged and discharged -- most sodium-ion batteries retain about 50 percent of their original capacity after 50 cycles. Researchers are looking at different materials for the anodes and cathodes to solve this problem.
One approach tried earlier this year, also at the University of Maryland, used samples of nickel-coated genetically modified tobacco mosaic virus as the substrate for the electrodes.
The Mechanics of Sodium-Ion Batteries
Sodium-ion batteries store energy in chemical bonds in their cathodes. When they are being charged, positively charged sodium ions migrate toward the anode, a process called "sodiation," while charge-balancing electrons go from the cathode through the external circuit containing the charger and into the anode. These processes are reversed during discharge.
Tin oxide is used for the anodes because it has a high energy density. However, it expands and contracts severely during sodiation and desodiation, Zhu said, with the volume increasing by 420 percent. That reduces access to the power stored.
Further, tin particles tend to aggregate into large particles and then break up, and that affects cycling stability, other researchers have found.
Getting to the Wood
Zhu's team took microfibers from yellow pines to serve as a substrate and deposited a thin film of tin on them through electrochemical deposition. "Very small" amounts of wood fiber were used, with a 1 cm square sample weighing less than 3 mg, Zhu said.
"The tin has to be a continuous film, or you won't have electrical conductivity," Harold Kung, professor of chemical and biological engineering at Northwestern University, told TechNewsWorld.
The expansion and contraction of tin in the anode during sodiation and desodiation weakens its connection to its substrate, but the wood fibers are resilient so they serve as a buffer and reduce the effect of this stress. Further, the wood fibers are porous, and the tunnels through their structure let ions travel through their inner and outer surfaces.
The result was that the experimental battery had a stable cycling performance of 400 cycles with an initial capacity of 339 mAh/g. That's a significantly longer life than other nanostructures using tin anodes, the team said.
Such nanobatteries are "extremely suitable for low-cost, large-scale energy storage" of the type required by an electricity grid, noted Zhu.
"I have never seen anything like this before, but I like the sustainability aspect of it, especially for grid-type storage," said Jim McGregor, principal analyst at Tirias Research.
It's not likely that such batteries will show up in consumer electronics products, because sodium doesn't store energy as efficiently as lithium.
The research was supported by the University of Maryland and the United States National Science Foundation.