”Hybrid” cathodes could provide more power
for a given weight and volume.
David L. Chandler, MIT News Office
David L. Chandler, MIT News Office
March 25, 2019 -- Researchers around the globe have been on a
quest for batteries that pack a punch but are smaller and lighter than today’s
versions, potentially enabling electric cars to travel further or portable
electronics to run for longer without recharging. Now, researchers at MIT and
in China
The team describes their concept as
a “hybrid” cathode, because it combines aspects of two different approaches
that have been used before, one to increase the energy output per pound
(gravimetric energy density), the other for the energy per liter (volumetric
energy density). The synergistic combination, they say, produces a version that
provides the benefits of both, and more.
The work is described today in the
journal Nature Energy, in a paper by Ju Li, an MIT professor of
nuclear science and engineering and of materials science and engineering;
Weijiang Xue, an MIT postdoc; and 13 others.
Today’s lithium-ion batteries tend
to use cathodes (one of the two electrodes in a battery) made of a transition
metal oxide, but batteries with cathodes made of sulfur are considered a
promising alternative to reduce weight. Today, the designers of lithium-sulfur
batteries face a tradeoff.
The cathodes of such batteries are
usually made in one of two ways, known as intercalation types or conversion
types. Intercalation types, which use compounds such as lithium cobalt oxide,
provide a high volumetric energy density — packing a lot of punch per volume
because of their high densities. These cathodes can maintain their structure
and dimensions while incorporating lithium atoms into their crystalline
structure.
The other cathode approach, called
the conversion type, uses sulfur that gets transformed structurally and is even
temporarily dissolved in the electrolyte. “Theoretically, these [batteries]
have very good gravimetric energy density,” Li says. “But the volumetric
density is low,” partly because they tend to require a lot of extra materials,
including an excess of electrolyte and carbon, used to provide conductivity.
In their new hybrid system, the
researchers have managed to combine the two approaches into a new cathode that
incorporates both a type of molybdenum sulfide called Chevrel-phase, and pure
sulfur, which together appear to provide the best aspects of both. They used
particles of the two materials and compressed them to make the solid cathode.
“It is like the primer and TNT in an explosive, one fast-acting, and one with
higher energy per weight,” Li says.
Among other advantages, the
electrical conductivity of the combined material is relatively high, thus
reducing the need for carbon and lowering the overall volume, Li says. Typical
sulfur cathodes are made up of 20 to 30 percent carbon, he says, but the new
version needs only 10 percent carbon.
The net effect of using the new
material is substantial. Today’s commercial lithium-ion batteries can have
energy densities of about 250 watt-hours per kilogram and 700 watt-hours per
liter, whereas lithium-sulfur batteries top out at about 400 watt-hours per
kilogram but only 400 watt-hours per liter. The new version, in its initial
version that has not yet gone through an optimization process, can already
reach more than 360 watt-hours per kilogram and 581 watt-hours per liter, Li
says. It can beat both lithium-ion and lithium-sulfur batteries in terms
of the combination of these energy densities.
With further work, he says, “we
think we can get to 400 watt-hours per kilogram and 700 watt-hours per liter,”
with that latter figure equaling that of lithium-ion. Already, the team has
gone a step further than many laboratory experiments aimed at developing a
large-scale battery prototype: Instead of testing small coin cells with
capacities of only several milliamp-hours, they have produced a three-layer
pouch cell (a standard subunit in batteries for products such as electric
vehicles) with a capacity of more than 1,000 milliamp-hours. This is comparable
to some commercial batteries, indicating that the new device does match its
predicted characteristics.
So far, the new cell can’t quite
live up to the longevity of lithium-ion batteries in terms of the number of
charge-discharge cycles it can go through before losing too much power to be
useful. But that limitation is “not the cathode’s problem”; it has to do with
the overall cell design, and “we’re working on that,” Li says. Even in its
present early form, he says, “this may be useful for some niche applications,
like a drone with long range,” where both weight and volume matter more than
longevity.
“I think this is a new arena for
research,” Li says.
No comments:
Post a Comment