MIT Reports Breakthrough In Solid-State Lithium Battery Development
MIT Reports Breakthrough In Solid-State Lithium Battery Development
By Brian Westenhaus
of OilPrice.com November 24, 2022
Massachusetts Institute
of Technology’s new discovery could finally usher the
development of solid-state lithium batteries, which would be more lightweight,
compact, and safe than current lithium batteries. The growth of metallic filaments called
dendrites within the solid electrolyte has been a longstanding obstacle, but
the new study explains how dendrites form and how to divert them. This is a
goal that’s been pursued by labs around the world for years.
The key to this
potential leap in battery technology is replacing the
liquid electrolyte that sits between the positive and negative electrodes with
a much thinner, lighter layer of solid ceramic material, and replacing one of
the electrodes with solid lithium metal. This would greatly reduce the overall
size and weight of the battery and remove the safety risk associated with
liquid electrolytes, which are flammable.
But that quest has
been beset with one big problem: dendrites.
Dendrites, whose name
comes from the Latin for branches, are projections of metal that can build up
on the lithium surface and penetrate into the solid electrolyte, eventually
crossing from one electrode to the other and shorting out the battery cell.
Researchers haven’t been able to agree on what gives rise to these metal
filaments, nor has there been much progress on how to prevent them and thus
make lightweight solid-state batteries a practical option.
The
new research published in the journal Joule in a paper by MIT Professor
Yet-Ming Chiang, graduate student Cole Fincher, and five others at MIT and
Brown University, seems to resolve the question of what causes
dendrite formation. It also shows how dendrites can be prevented from crossing
through the electrolyte.
Chiang said in the
group’s earlier work, they made a “surprising and unexpected” finding, which
was that the hard, solid electrolyte material used for a solid-state battery
can be penetrated by lithium, which is a very soft metal, during the process of
charging and discharging the battery, as ions of lithium move between the two
sides.
This shuttling back
and forth of ions causes the volume of the electrodes to change. That
inevitably causes stresses in the solid electrolyte, which has to remain fully
in contact with both of the electrodes that it is sandwiched between. “To
deposit this metal, there has to be an expansion of the volume because you’re
adding new mass,” Chiang said. “So, there’s an increase in volume on the side
of the cell where the lithium is being deposited. And if there are even
microscopic flaws present, this will generate a pressure on those flaws that can
cause cracking.”
Those stresses, the
team has now shown, cause the cracks that allow dendrites to form. The solution
to the problem turns out to be more stress, applied in just the right direction
and with the right amount of force.
While previously, some
researchers thought that dendrites formed by a purely electrochemical process,
rather than a mechanical one, the team’s experiments demonstrate that it is
mechanical stresses that cause the problem.
The process of
dendrite formation normally takes place deep within the opaque materials of the
battery cell and cannot be observed directly, so Fincher developed a way of
making thin cells using a transparent electrolyte, allowing the whole process
to be directly seen and recorded. “You can see what happens when you put a
compression on the system, and you can see whether or not the dendrites behave
in a way that’s commensurate with a corrosion process or a fracture process,”
he said.
The team demonstrated
that they could directly manipulate the growth of dendrites simply by applying
and releasing pressure, causing the dendrites to zig and zag in perfect
alignment with the direction of the force.
Applying mechanical
stresses to the solid electrolyte doesn’t eliminate the formation of dendrites,
but it does control the direction of their growth. This means they can be
directed to remain parallel to the two electrodes and prevented from ever
crossing to the other side, and thus rendered harmless.
In their tests, the
researchers used pressure induced by bending the material, which was formed
into a beam with a weight at one end. But they say that in practice, there
could be many different ways of producing the needed stress. For example, the
electrolyte could be made with two layers of material that have different amounts
of thermal expansion, so that there is an inherent bending of the material, as
is done in some thermostats.
Another approach would
be to “dope” the material with atoms that would become embedded in it,
distorting it and leaving it in a permanently stressed state. This is the same
method used to produce the super-hard glass used in the screens of smart phones
and tablets, Chiang explained. And the amount of pressure needed is not
extreme: The experiments showed that pressures of 150 to 200 megapascals were
sufficient to stop the dendrites from crossing the electrolyte.
The required pressure
is “commensurate with stresses that are commonly induced in commercial film
growth processes and many other manufacturing processes,” so should not be
difficult to implement in practice, Fincher added.
Fischer explained that
in fact, a different kind of stress, called stack pressure, is often applied to
battery cells, by essentially squishing the material in the direction
perpendicular to the battery’s plates – somewhat like compressing a sandwich by
putting a weight on top of it. It was thought that this might help prevent the
layers from separating. But the experiments have now demonstrated that pressure
in that direction actually exacerbates dendrite formation. “We showed that this
type of stack pressure actually accelerates dendrite-induced failure,” he said.
What is needed instead
is pressure along the plane of the plates, as if the sandwich were being
squeezed from the sides. “What we have shown in this work is that when you
apply a compressive force you can force the dendrites to travel in the
direction of the compression,” Fincher said, and if that direction is along the
plane of the plates, the dendrites “will never get to the other side.”
That could finally
make it practical to produce batteries using solid electrolyte and metallic
lithium electrodes. Not only would these pack more energy into a given volume
and weight, but they would eliminate the need for liquid electrolytes, which
are flammable materials.
Having demonstrated
the basic principles involved, the team’s next step will be to try to apply
these to the creation of a functional prototype battery, Chiang said, and then
to figure out exactly what manufacturing processes would be needed to produce
such batteries in quantity. Though they have filed for a patent, the
researchers don’t plan to commercialize the system themselves, he said, as
there are already companies working on the development of solid-state
batteries. “I would say this is an understanding of failure modes in
solid-state batteries that we believe the industry needs to be aware of and try
to use in designing better products,” he said.
The research team
included Christos Athanasiou and Brian Sheldon at Brown University, and Colin
Gilgenbach, Michael Wang, and W. Craig Carter at MIT. The work was supported by
the U.S. National Science Foundation, the U.S. Department of Defense, the U.S.
Defense Advanced Research Projects Agency, and the U.S. Department of Energy.
***
Assuming the press
release has adequate data for not being certain this work will yield a
prototype battery, the odds are that there will be a successful prototype
built. How many models are tried and what works in the end is very much in the
air for now.
On the other hand the
mechanical formation research result looks quite compelling and actually makes
reasoned sense now that it is explained. That raises questions. Does the
dendrite formation greatly impede the battery capacity and function or does
that added dendrite surface area increase it? Then one wonders how the dendrite
formation impacts overall lifespan?
This effort isn’t over yet. But this is a significant milestone with lots of clues and hints on where further research might go. It looks like solid state lithium metal batteries are just a matter of innovation, insight and creativity away from the market.
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