Scientists and engineers have used a material from brown algae to improve the performance of battery electrodes. Image: Wikipedia Commons |
By looking to Mother Nature for solutions, researchers have identified a
promising new binder material for lithium-ion battery electrodes that could not
only boost energy storage, but also eliminate the use of toxic compounds now
used in manufacturing the components.
Known as alginate, the material is extracted from common, fast-growing brown
algae. In tests so far, it has helped boost energy storage and output for both
graphite-based electrodes used in existing batteries and silicon-based
electrodes being developed for future generations of batteries.
The research, the result of collaboration between scientists and engineers
at the Georgia Institute of Technology and Clemson University,
was reported in Science Express. The
project was supported by the two universities, as well as by a Honda Initiation
Grant and a grant from NASA.
“Making less expensive batteries that can store more energy and last
longer with the help of alginate could provide a large and long-lasting impact
on the community,” says Gleb Yushin, an assistant professor in Georgia
Tech’s School of Materials Science and Engineering.
“These batteries could contribute to building a more energy-efficient
economy with extended-range electric cars, as well as cell phones and notebook
computers that run longer on battery power—all with environmentally friendly
manufacturing technologies.”
Working with Igor Luzinov at Clemson
University, the
scientists looked at ways to improve binder materials in batteries. The binder
is a critical component that suspends the silicon or graphite particles that
actively interact with the electrolyte that provides battery power.
“We specifically looked at materials that had evolved in natural
systems, such as aquatic plants which grow in salt water with a high
concentration of ions,” says Luzinov, a professor in Clemson’s School of Materials Science and Engineering.
“Since electrodes in batteries are immersed in a liquid electrolyte, we
felt that aquatic plants—in particular, plants growing in such an aggressive
environment as salt water—would be excellent candidates for natural
binders.”
Finding just the right material is an important step toward improving the
performance of lithium-ion batteries, which are essential to a broad range of
applications, from cars to cell phones. The popular and lightweight batteries
work by transferring lithium ions between two electrodes—a cathode and an anode—through
a liquid electrolyte. The more efficiently the lithium ions can enter the two
electrodes during charge and discharge cycles, the larger the battery’s
capacity will be.
Existing lithium-ion batteries rely on anodes made from graphite, a form of
carbon. Silicon-based anodes theoretically offer as much as a ten-fold capacity
improvement over graphite anodes, but silicon-based anodes have so far not been
stable enough for practical use.
Among the challenges for binder materials are that anodes to be used in
future batteries must allow for the expansion and contraction of the silicon
nanoparticles, and that existing electrodes use a polyvinylidene fluoride
binder manufactured using a toxic solvent.
Alginates—low-cost materials that are already used in foods, pharmaceutical
products, paper, and other applications—are attractive because of their
uniformly distributed carboxylic groups. Other materials, such as carboxymethyl
cellulose, can be processed to include the carboxylic groups, but that adds to
their cost and does not provide the natural uniform distribution of alginates.
The alginate is extracted from the seaweed through a simple soda
(Na2CO3)-based process that generates a uniform material. The anodes can then
be produced through an environmentally friendly process that uses a water-based
slurry to suspend the silicon or graphite nanoparticles. The new alginate
electrodes are compatible with existing production techniques and can be integrated
into existing battery designs, Yushin says.
Use of the alginate may help address one of the most difficult problems
limiting the use of high-energy silicon anodes. When batteries begin operating,
decomposition of the lithium-ion electrolyte forms a solid electrolyte
interface (SEI) on the surface of the anode. The SEI must be stable, allow
lithium ions to pass through it, yet restrict the flow of fresh electrolyte.
With graphite particles, whose volume does not change, the SEI remains
stable. However, because the volume of silicon nanoparticles changes during
operation of the battery, cracks can form and allow additional electrolyte
decomposition until the pores that allow ion flow become clogged, causing
battery failure. Alginate not only binds silicon nanoparticles to each other
and to the metal foil of the anode, but they also coat the silicon
nanoparticles themselves and provide a strong support for the SEI, preventing
degradation.
Thus far, the researchers have demonstrated that the alginate can produce
battery anodes with reversible capacity eight times greater than that of
today’s best graphite electrodes. The anode also demonstrates a coulombic efficiency
approaching 100% and has been operated through more than 1,000 charge-discharge
cycles without failure.
