Li-ion batteries may be ok for your smartphone, but when it comes to large-scale energy storage, the priorities suddenly shift from compactness and cycling performance (at which Li-ion batteries excel) to low cost and environmental feasibility (in which Li-ion batteries still have much room for improvement). A new "wood battery" could allow the emerging sodium-ion battery technology to fit the bill as a long-lasting, efficient and environmentally friendly battery for large-scale energy storage.

Scientists are speculating that sodium-ion batteries, currently in an early stage of development, could suit large-scale energy storage much better than Li-ion batteries, partially because sodium is cheap and plentiful and because sodium is environmentally benign. But for Na-ion batteries to become a viable energy-storage option there are still many obstacles to overcome, the greatest of which is the phenomenon known as sodiation.

With each charge/discharge cycle, the sodium ions cause the anode of the battery to swell by as much as 420 percent and then return to normal. This phenomenon, known as sodiation, can literally pulverize the anode after only 20 cycles, rendering the battery extremely short-lived. University of Maryland (UM) researchers Liangbing Hu and Teng Li found a way around this problem.

The stiff bases often used in existing batteries are too brittle to withstand the swelling and shrinking caused by sodiation, so the researchers turned to the much softer wood fibers. These have evolved to withstand these forces extremely well as they use capillary forces to transfer the sodium ions from the soil around them to the leaves of their tree. The resulting sodium-ion battery that uses wood fibers increases durability twenty-fold compared with previous designs.

The researchers coated a sliver of wood fibers with a thin layer (10 nm) of single-walled carbon nanotubes to make it electrically conductive, and then deposited a tin film over it. The soft wood fibers effectively neutralized the strong mechanical stresses of the sodiation process: Even after charging and discharging the battery hundreds of times, the wood ended up wrinkled but remarkably intact.

"Pushing sodium ions through tin anodes often weaken the tin’s connection to its base material," said Li, an associate professor of mechanical engineering. "But the wood fibers are soft enough to serve as a mechanical buffer, and thus can accommodate tin’s changes. This is the key to our long-lasting sodium-ion batteries."

"Wood fibers that make up a tree once held mineral-rich water, and so are ideal for storing liquid electrolytes, making them not only the base but an active part of the battery," added Hu.

In their testing, the researchers measured a stable cycling performance of 400 cycles with an initial capacity of 339 mAh/g, which is a marked improvement over previous designs.

Because sodium doesn't store energy quite as efficiently as lithium, there is little chance of this technology eventually finding its way to your next-generation gadget. However, because of their low cost and use of environmentally benign common materials, sodium-ion batteries could be used to store large amounts of energy from renewable energy sources, such as wind and solar.

A paper detailing the research was published on the journal Nano Letters.

Source: UMD