Chemical technologist Harry Pratt synthesizes a copper-based ionic liquid. Photo: Randy Montoya |
Sandia National Laboratories
researchers have developed a new family of liquid salt electrolytes, known as
MetILs, that could lead to batteries able to cost-effectively store three times
more energy than today’s batteries.
The research,
published in Dalton Transactions, might
lead to devices that can help economically and reliably incorporate large-scale
intermittent renewable energy sources, like solar and wind, into the nation’s
electric grid.
The grid was designed
for steady power sources, making fluctuating electricity from intermittent
renewable energy difficult to accommodate. Better energy storage techniques
help even out the flow of such fluctuating sources, and Sandia researchers are
studying new ways to develop a more flexible, cost-effective, and reliable
electric grid with improved energy storage.
“The U.S. and the
world need significant breakthroughs in battery technology for renewable energy
sources to replace today’s carbon-based energy systems,” said Anthony Medina,
director of Sandia’s Energetic Components Realization program. “MetILs are a
new, promising battery chemistry that might provide the next generation of
stationary storage battery technology, replacing lead-acid and lithium-ion
batteries and providing significantly higher energy storage density for these
applications.”
For the past 20 years,
lithium-ion batteries have been at the forefront of energy storage research.
Their compact, lightweight design is well suited for cell phones, laptop
computers, and personal electronics, but lithium-ion batteries are expensive
and degradation issues limit their use in stationary, high-capacity application
on the nation’s electric grid.
Sandia researcher and
inorganic chemist Travis Anderson is leading a team developing the next
generation of flow batteries. A flow battery pumps a solution of free-floating
charged metal ions, dissolved in an electrolyte—substance with free-floating ions
that conducts electricity—from an external tank through an electrochemical cell
to convert chemical energy into electricity. Flow batteries are rapidly charged
and discharged by changing the charge state of the electrolyte, and the
electroactive material can be easily re-used many times. Anderson said flow batteries can sustain more
than 14,000 cycles in the laboratory, equivalent to more than 20 years of
energy storage, which would be unusual in a lithium-ion battery.
However, flow battery
grid storage systems are roughly the size of a house and can cost more than
equivalent lithium-ion batteries. The goal of researchers is to make flow
batteries smaller and cheaper, while increasing the amount of energy stored for
a given volume, or energy density.
Flow batteries have
been fielded in the U.S., Japan, and Australia. A number of systems—up
to 25 MW—are in the process of being demonstrated under the American Recovery
and Reinvestment Act (ARRA) administered by DOE’s Energy Storage Systems
Research program. Zinc bromine and vanadium redox systems are among the top
contenders. But the materials involved are moderately toxic, and vanadium is
subject to major price fluctuations. In addition, the aqueous solution limits
the amount of material that can be dissolved and how much energy can be stored,
and outside temperature can hurt performance.
Sandia researchers have discovered a new family of liquid salt electrolytes that could lead to batteries with three times greater energy density than other available storage technologies. The MetILs are, from left to right: copper-based compound, cobalt-based compound, manganese-based compound, iron-based compound, nickel-based compound, and vanadium-based compound. Image: Randy Montoya |
Sandia is pioneering
research on flow batteries that avoid these problems by not using water. Anderson assembled a
multidisciplinary team of experts from the Labs, including electrochemist David
Ingersoll, organic chemist Chad Staiger and chemical technologists Harry Pratt
and Jonathan Leonard. What they’ve designed is a new family of electrochemically
reversible, metal-based ionic liquids, or MetILs, which are based on
inexpensive, non-toxic materials that are readily available within the U.S.,
such as iron, copper, and manganese.
“Instead of
dissolving the salt into a solvent, our salt is a solvent,” Anderson said. “We’re able to get a much
higher concentration of the active metal because we’re not limited by
saturation. It’s actually in the formula. So we can cost-effectively triple our
energy density, which drastically reduces the necessary size of the battery,
just by the nature of the material.”
The electrochemical
efficiency, or ability to reverse charge, in MetILs is far greater than
anything else published to date. The team has prepared nearly 200 combinations
of cations, anions and ligands, and of those, five outperform the
electrochemical efficiency of ferrocene, which has long been considered the
gold standard.
A common problem when
mixing positively and negatively charged species is that these species will
start aggregating together, eventually causing the solution to turn gummy and
clog the battery membrane and electrode surfaces. The team addressed that
challenge by developing asymmetric cations, or positively charged ions, that
resemble a soccer ball. In this analogy, the black pentagons represent
negatively charged areas and the white hexagons represent positively charged
regions. Such an arrangement lowers the melting point by preventing the ionic
liquid constituents from bonding and becoming a solid, while the partial charge
still allows electrons to flow freely through the cell to generate a current.
The team is funded by
the Department of Energy’s Office of Electricity Delivery and Energy Reliability.
Imre Gyuk, energy storage systems program manager for that office, has been a
champion of Sandia’s efforts and provided the necessary funding.
“The MetILs approach
represents an ingenious, out-of-the-box solution to the cathode/electrolyte
paradigm. Because it is based on readily available, inexpensive precursors, it
may well lead to innovative, cost-effective storage systems with major impacts
on the entire U.S.
grid,” said Gyuk.
The findings apply to
new flow battery cathode materials. The next step for the Sandia team is to
find similar materials for flow battery anodes, and researchers are encouraged
by their progress.
“There are three
things you’re juggling at the same time, and they aren’t always related:
viscosity, electrical conductivity and the fundamental electrochemical
efficiency,” Anderson
said. “The excitement of having all three things go right at the same time,
it’s like finding the treasure, but without the map. We’re creating that map,
and we’re very excited by the possibilities.”
