YSM Issue 94.3

NEWS
Chemistry
BUILDING
A BATTERY
FUTURE
Sodium batteries in
a lithium-dominated
world
BY JORDAN SAHLY
IMAGE COURTESY OF WIKIMEDIA COMMONS
Every day around the world, modern luxuries are plugged in,
charged, and drained—and the cycle begins anew. Critical
to this energy dependence is the lithium-ion battery, the
electrochemical backbone behind cell phones, laptops, electric
cars, and most other battery-powered devices. In the US alone,
electronics consume some two thousand metric tons of lithium
annually, of which over fifty percent arrives as imports from other
countries such as China, Chile, and Australia.
Yiren Zhong, a postdoctoral associate in Yale’s Department of
Chemistry, understands the need to find a substitute for lithiumion
batteries. “We all know that lithium is a very very limited
resource, not only in the Earth’s crust but also in the oceans, in the
lakes,” Zhong said. Resource scarcity and related environmental
concerns have inspired chemists—including Zhong—to look
for candidates no further than a row down in the periodic table.
One promising candidate is sodium, an alkali metal like lithium.
Sodium and lithium have many similar qualities due to periodic
trends, with the notable difference that sodium is larger in atomic
size and has less electric potential overall. However, sodium is
also far more abundant naturally on Earth. Would this periodic
similarity make sodium a prime candidate for battery production?
This is what Zhong set out to study with his research, published in
August of 2021 in the Journal of the American Chemical Society.
Despite its natural abundance, sodium has a long way to go
before it can replace lithium as a primary battery component. There
are pros and cons for sodium metal as an electrode. Compared to
lithium, sodium has good reversibility—the ability to return the
electrochemical reaction to its original reactants, meaning that
batteries with good reversibility can be recharged and reused.
However, sodium also cannot be charged or discharged very quickly.
Intrinsic elemental properties stand in the way of sodium’s potential.
Zhong’s research group investigated these properties through rigorous
experiments testing sodium batteries at varying power levels, then
examining the electrode’s physical and chemical structure after
both charging and discharging. The research team performed the
experiments at high currents, which were closer to those of lithium
batteries, and at far lower currents for comparison. When the sodium
electrode was discharged at the higher currents, it performed with only
zero to sixty percent Coulombic efficiency—the ability of a battery to
output the usable electrons, or electricity, that it produces.
An interesting physical reaction indicated a key elemental difference
between lithium and sodium. When built through charging, sodium
metal electrodes naturally form in dendritic structures, which
are long, thin columns of metal that become porous, microscopic
forests on the surface of the electrode. On electrodes charged at high
current densities, these dendritic structures form with non-metallic
impurities. When discharged at high current densities, these impure
porous surfaces reduce the reversibility of the battery overall by
allowing fast-moving current to react unevenly—especially at the base
of the electrode—causing electrode erosion and eventual electrical
disconnection. Thus, the low performance of sodium batteries likely
stems from their elemental characteristics, namely their atomic size:
electrodes made from sodium metal have more spread-out dendritic
structures due to the larger atomic size. This creates the porous surface
that allows for erosion of the electrode foundation layer at high
currents, like waves washing away the base of a sandcastle.
Zhong’s findings, however, also suggest a favorable future for sodium.
At low power levels, the sodium battery did not decay and performed
favorably with Coulombic efficiencies as high as 99.5 percent. At these
low current densities, sodium batteries may demonstrate commercial
usefulness in technologies like short-range transportation tools.
Having observed sodium’s intrinsic characteristic limiting
its potential in batteries, Zhong’s group laid the foundation
for future sodium battery technology. One of his newest ideas
involves the electrode shape itself. “My current thinking
is trying to use a three-dimensional electrode,” Zhong said.
He theorized that a three-dimensional electrode may reduce
local current density across a larger surface area, which could
improve the electrochemical reaction in the battery.
As our society’s energy dependence grows each year, more
environmentally friendly batteries become a necessity rather than
a goal. “We need to develop a battery future,” Zhong said. “By
the year 2050, I would envision that sodium would be one of the
major components in the battery market.” Sodium metal has the
potential to help build a sustainable battery future, and thanks to
the continued work of innovative chemists, that future is in reach. ■
10 Yale Scientific Magazine October 2021 www.yalescientific.org