Scientists find solution to the sudden death common to these batteries
From: Pacific Northwest National
Laboratory (PNNL)
By Tom Rickey, PNNL
June
28, 2021 – RICHLAND, Wash.— Researchers have increased the lifetime of a
promising electric vehicle battery to a record level, an important step toward
the goal of lighter, less expensive and long-lasting batteries for future
electric vehicles. The work is reported June 28 in the journal Nature Energy.
Such batteries—the goal of research
groups the world over—are seen as an important part of the solution to reduce
the effects of climate change, and scientists are exploring a dizzying array of
options.
One solution on the horizon is a
lithium-metal battery for electric vehicles. These batteries hold almost twice
the energy of their widely used lithium-ion counterparts, and they’re lighter.
That combination offers the enticing prospect of an electric vehicle that would
be lighter and go much farther on a single charge. But lithium-metal batteries
in the laboratory have been plagued by premature death, lasting only a fraction
of the time of today’s lithium-ion batteries.
Now, a team of scientists at the U.S.
Department of Energy’s Pacific Northwest
National Laboratory has created a lithium-metal battery that lasts for
600 cycles, far longer than other reported results. That means it can be fully
charged and discharged 600 times before it dies.
It’s a big step forward for a promising
technology, but lithium-metal technology is not yet ready for prime time. While
the lithium-ion batteries used in electric vehicles today hold less energy,
they last longer, typically at least 1,000 cycles. But vehicles won’t go as far
on one charge as they would with an effective lithium-metal battery.
The new research was done through
DOE’s Innovation
Center for Battery500 Consortium, a multi-institution effort led by PNNL to
develop electric vehicle batteries that are lighter, more energy intensive and
less expensive than those currently used. PNNL leads the consortium and is
responsible for integrating the latest advances from partner institutions into
devices known as high-energy pouch cells and demonstrating improved performance
under realistic conditions.
Lithium metal:
thin strips of lithium translate to longer lifetime
The PNNL team found a way to increase
the battery’s lifetime by taking a surprising approach. Instead of using anodes
with more lithium, the team used incredibly thin strips of lithium, just 20
microns wide, far thinner than the width of a human hair.
“Many people have thought that thicker
lithium would enable the battery to cycle longer,” said Jie Xiao, who along
with Jun Liu, the director of the Battery500 Consortium, is a corresponding
author of the paper. “But that is not always true. There is an optimized
thickness for each lithium-metal battery depending on its cell energy and
design.”
The lithium-metal battery created by the
Battery500 team has an energy density of 350 watt-hours per kilogram
(Wh/kg)—very high but not unprecedented. The value of the new findings has more
to do with the battery’s lifetime. After 600 cycles, the battery retained 76
percent of its initial capacity.
Just four years ago, an experimental
lithium-metal battery could operate for 50 cycles. That has increased rapidly;
two years ago the PNNL team achieved 200 cycles—and now 600. Moreover, the PNNL
battery is a pouch cell, which more closely mirrors real-world conditions than
does a coin cell, a less realistic type of device used in many battery research
projects.
Lithium metal:
why thickness matters
The team’s decision to try thinner
lithium strips was based on its detailed understanding of the molecular
dynamics of the anode as explained in the Nature Energy paper.
The scientists found that thicker strips
contribute directly to battery failure. That’s due to complex reactions around
a film on the anode known as the solid electrolyte interphase, or SEI. The SEI
is the byproduct of side reactions between lithium and the electrolyte. It acts
as an important gatekeeper that allows certain molecules to go from the anode
to the electrolyte and back again while keeping other molecules at bay.
It’s an important job. An SEI working
effectively allows certain lithium ions to pass through but limits unwanted
chemical reactions that reduce battery performance and accelerate cell failure.
A primary goal for researchers has been to reduce unwanted side reactions
between the electrolyte and the lithium metal—to encourage vital chemical
reactions while restraining unwanted ones.
The team found that thinner lithium
strips are adept at creating what one might call good SEI, while the thicker
strips have a higher chance of contributing to what one might call harmful SEI.
In their paper, the researchers use the terms “wet SEI” and “dry SEI.” The wet
version retains contact between the liquid electrolyte and the anode, making
important electrochemical reactions possible.
But in the dry version, the liquid
electrolyte doesn’t reach all of the lithium. Simply, because the lithium
strips are thicker, the electrolyte needs to flow into deeper pockets of the
lithium, and as it does so, it leaves other portions of the lithium dry. This
stops important reactions from occurring, effectively smothering necessary
electrochemical reactions, and contributes directly to the early death of the
battery.
It’s an important issue, especially
in realistic
batteries like pouch cells, where the amount of electrolyte available
is 20 to 30 times less than that used in experimental coin cells.
Consider how a frying pan gradually
builds up a layer of grease if not cleaned thoroughly after each time it is
used. Over time, the layer builds up and acts as a barrier, reducing the flow
of energy and making the surface less effective. In the same way, an unwanted,
dry SEI layer prevents the effective transfer of energy needed inside a
battery.
Progress
thanks to Battery500
The progress on lithium-metal batteries
has been substantial, thanks to the Battery500 Consortium. The goal is to
increase the amount of energy packed into a long-duration, safe, affordable
battery. More energy per pound of material translates to a lighter vehicle that
can go farther on one charge. Today’s electric vehicle batteries are in the
neighborhood of 200-250 Wh/kg; Battery500 is aiming for a cell level of 500
Wh/kg.
“The Battery500 Consortium has made
great progress in increasing the energy density and extending the cycle life,”
said Distinguished Professor M. Stanley Whittingham of Binghamton University, the
2019 Nobel Prize laureate in chemistry and a coauthor of the paper. “But much
more needs to be done. In particular, there are safety issues with
lithium-metal batteries that must be addressed. That’s something that the
Battery500 team is working hard to resolve.”
The work was funded by DOE’s Office of
Energy Efficiency and Renewable Energy’s Vehicle Technologies Office. Much of
the microscopy to evaluate the battery was done at EMSL, the Environmental
Molecular Sciences Laboratory, a DOE Office of Science user facility
located at PNNL.
In addition to Liu, Xiao and
Whittingham, authors include PNNL scientists Chaojiang Niu, Dianying Liu,
Joshua Lochala, Cassidy Anderson, Xia Cao, Mark Gross, Wu Xu and Ji-Guang
(Jason) Zhang. Xiao and Liu also have appointments at the University of
Washington.
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