Researchers
investigate mechanics of lithium
sulfides, which show promise as solid electrolytes.
By David L. Chandler,| MIT News
Office
February 2, 2017 -- Most batteries are composed of two
solid, electrochemically active layers called electrodes, separated by a
polymer membrane infused with a liquid or gel electrolyte. But recent research
has explored the possibility of all-solid-state batteries, in which the liquid
(and potentially flammable) electrolyte would be replaced by a solid
electrolyte, which could enhance the batteries’ energy density and safety.
Now, for the first time, a team at
MIT has probed the mechanical properties of a sulfide-based solid electrolyte
material, to determine its mechanical performance when incorporated into
batteries.
The new findings were published
this week in the journal Advanced Energy Materials, in a paper by
Frank McGrogan and Tushar Swamy, both MIT graduate students; Krystyn Van Vliet,
the Michael (1949) and Sonja Koerner Professor of Materials Science and
Engineering; Yet-Ming Chiang, the Kyocera Professor of Materials Science and
Engineering; and four others including an undergraduate participant in the
National Science Foundation Research Experience for Undergraduate (REU) program
administered by MIT’s Center for Materials Science and Engineering and its
Materials Processing Center.
Lithium-ion batteries have provided
a lightweight energy-storage solution that has enabled many of today’s
high-tech devices, from smartphones to electric cars. But substituting the
conventional liquid electrolyte with a solid electrolyte in such batteries
could have significant advantages. Such all-solid-state lithium-ion batteries
could provide even greater energy storage ability, pound for pound, at the
battery pack level. They may also virtually eliminate the risk of tiny,
fingerlike metallic projections called dendrites that can grow through the
electrolyte layer and lead to short-circuits.
“Batteries with components that are
all solid are attractive options for performance and safety, but several
challenges remain,” Van Vliet says. In the lithium-ion batteries that dominate
the market today, lithium ions pass through a liquid electrolyte to get from
one electrode to the other while the battery is being charged, and then flow
through in the opposite direction as it is being used. These batteries are very
efficient, but “the liquid electrolytes tend to be chemically unstable, and can
even be flammable,” she says. “So if the electrolyte was solid, it could be
safer, as well as smaller and lighter.”
But the big question regarding the
use of such all-solid batteries is what kinds of mechanical stresses might
occur within the electrolyte material as the electrodes charge and discharge
repeatedly. This cycling causes the electrodes to swell and contract as the
lithium ions pass in and out of their crystal structure. In a stiff
electrolyte, those dimensional changes can lead to high stresses. If the
electrolyte is also brittle, that constant changing of dimensions can lead to
cracks that rapidly degrade battery performance, and could even provide
channels for damaging dendrites to form, as they do in liquid-electrolyte
batteries. But if the material is resistant to fracture, those stresses could
be accommodated without rapid cracking.
Until now, though, the sulfide’s
extreme sensitivity to normal lab air has posed a challenge to measuring
mechanical properties including its fracture toughness. To circumvent this
problem, members of the research team conducted the mechanical testing in a
bath of mineral oil, protecting the sample from any chemical interactions with
air or moisture. Using that technique, they were able to obtain detailed
measurements of the mechanical properties of the lithium-conducting sulfide,
which is considered a promising candidate for electrolytes in all-solid-state
batteries.
“There are a lot of different
candidates for solid electrolytes out there,” McGrogan says. Other groups have
studied the mechanical properties of lithium-ion conducting oxides, but there
had been little work so far on sulfides, even though those are especially
promising because of their ability to conduct lithium ions easily and quickly.
Previous researchers used acoustic
measurement techniques, passing sound waves through the material to probe its
mechanical behavior, but that method does not quantify the resistance to
fracture. But the new study, which used a fine-tipped probe to poke into the
material and monitor its responses, gives a more complete picture of the
important properties, including hardness, fracture toughness, and Young’s
modulus (a measure of a material’s capacity to stretch reversibly under an
applied stress).
“Research groups have measured the
elastic properties of the sulfide-based solid electrolytes, but not fracture
properties,” Van Vliet says. The latter are crucial for predicting whether the
material might crack or shatter when used in a battery application.
The researchers found that the
material has a combination of properties somewhat similar to silly putty or
salt water taffy: When subjected to stress, it can deform easily, but at
sufficiently high stress it can crack like a brittle piece of glass.
By knowing those properties in detail,
“you can calculate how much stress the material can tolerate before it
fractures,” and design battery systems with that information in mind, Van Vliet
says.
The material turns out to be more
brittle than would be ideal for battery use, but as long as its properties are
known and systems designed accordingly, it could still have potential for such
uses, McGrogan says. “You have to design around that knowledge.”
“The cycle life of state-of-the-art
Li-ion batteries is primarily limited by the chemical/electrochemical stability
of the liquid electrolyte and how it interacts with the electrodes,” says Jeff
Sakamoto, a professor of mechanical engineering at the University of Michigan
Sakamoto adds that “Lithium metal
anodes exhibit a significant increase in capacity compared to state-of-the-art
graphite anodes. This could translate into about a 100 percent increase in
energy density compared to [conventional] Li-ion technology.”
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