Tech Briefs Magazine - July 2021 - 44

Power & Energy
Electrolyte Boost Improves Performance of Aqueous
Dual-Ion Batteries
New cell chemistry utilizes less costly and more abundant materials than lithium-ion batteries.
Pacific Northwest National Laboratory, Richland, Washington
W
idespread adoption of renewable
energy in the power grid requires
the right kind of battery - one that is
safe, sustainable, powerful, long-lasting,
and made from materials that are plentiful
and ethically sourced. Researchers
have formulated a new type of cell chemistry
for dual-ion batteries (DIB) called
graphite||zinc metal aqueous dual-ion
battery, which uses a zinc anode and a
natural graphite cathode in an aqueous,
or water-in-bisalt, electrolyte.
The use of aqueous electrolytes is not
new, nor is the use of graphite. Lithiumion
(Li-ion) batteries use graph ite as the
anode component and non-aqueous DIBs
use graphite as both the anode and the
cathode. What's new is combining the two
in a new chemistry. To do that, the team
gave the aqueous electrolyte an extra
boost by using a highly concentrated
water-in-bisalt solution. The solution
widens the electrochemical stability window
of the electrolyte and enables graph -
ite as a cathode material in a practical
aqueous system. This helps stabilize the
electrolyte at high voltages, allowing the
graphite to electrochemically oxidize
before the aqueous electrolyte.
The battery showed promising performance
during testing. At approximately
2.3 to 2.5 volts, it achieved one of the
highest operating potentials of any aqueous
battery. But the new cell chemistry
doesn't only improve battery perfor mance
- it's also better for the environment.
Cathodes made of highly abundant car -
bon-based materials, like natural graph -
ite, are less costly and more sustainable
than environmentally harmful, scarce,
and expensive metals, like nickel and
cobalt, that are regularly used in Li-ion
batteries. Using an aqueous electrolyte
also makes DIBs safer as they are nonflammable
compared to commercial Liion
batteries, which use non-aqueous
electrolytes exclusively.
In DIBs, both the positive cathode
and negative electrode can be made of
low-cost carbon-based materials like
graphite. This makes DIBs a particularly
promising solution to support the widespread
adoption of renewable energy
sources like wind and solar for the
power grid. Until now, the use of graph -
44
Cov
In DIBs, both cations and anions are active and move in parallel from the electrolyte to the anode
and cathode, respectively, in an accordion-like fashion. (Graphic by Cortland Johnson, adapted
from images by Ismael Rodríguez Pérez/Pacific Northwest National Laboratory)
ite as a cathode has been limited by the
narrow electrochemical stability of wa -
ter, which caps out at 1.23 volts. The
electrochemical stability window is the
potential range between which the electrolyte
is neither oxidized nor re duced
(decomposed) and an important measuring
stick for the efficiency of an electrolyte
in contact with an electrode.
Graphite would require a much wider
stability window.
Each battery cell has three main parts:
a positive electrode called a cathode, a
negative electrode called an anode, and
an electrolyte. In Li-ion batteries, power
is generated when the Li-ions (positively
charged ions or cations) flow from the
cathode to the anode and back again in
a rocking chair motion through the electrolyte.
This balances the charge when
electrons flow through an external circuit
from the cathode to the anode, creating
electricity.
In DIBs, both cations and anions (negatively
charged ions) are active and move
in parallel from the electrolyte to the
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anode and cathode, respectively, in an
accordion-like fashion, allowing for po -
tentially high-power applications, like
supercapacitors, while still being able to
use moderately high energy, like batteries.
Furthermore, this mechanism renders the
ions in the electrolyte active, allowing for
further optimization of the battery.
DIBs still perform at only about a
third of the capacity of Li-ion batteries
and Li-ion batteries still have one of the
highest energy densities of any comparable
system, meaning they can provide a
significant amount of energy and still
stay small. This advantage is one of the
main reasons they're used in mobile
applications such as smartphones and
electric cars.
If the researchers can achieve a high
enough voltage for the battery, even if
performance is not on par with Li-ion
batteries, DIBs can be made bigger and
a suitable candidate for grid energy storage
applications.
For more information, contact Elsie Puig
Santana at elsie.puig@pnnl.gov.
Tech Briefs, July 2021

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Tech Briefs Magazine - July 2021

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