A closeup of the wood fibers used by the researchers in their sodium-ion battery
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.
