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TechWaTch MIT Unveils Nanolithia Cathode Battery
Cambridge, MA — Lithium-air bat- teries are considered highly promis- ing technologies for electric cars and portable electronic devices because of their potential for delivering a high energy output in proportion to their weight. But such batteries have some pretty serious drawbacks: They waste much of the injected energy as heat and degrade relatively quickly. They also require expensive extra components to pump oxygen gas in and out, in an open-cell configuration that is very different from conven- tional sealed batteries. But a new variation of the bat-
tery chemistry, which could be used in a conventional, fully sealed battery, promises similar theoretical perform- ance as lithium-air batteries, while overcoming all of these drawbacks. The new battery concept, called
a nanolithia cathode battery, is de- scribed in the journal Nature Energy in a paper by Ju Li, the Battelle En- ergy Alliance professor of nuclear sci- ence and engineering at MIT; post- doc Zhi Zhu; and five others at MIT, Argonne National Laboratory, and Peking University in China. One of the shortcomings of lithi-
um-air batteries, Li explains, is the mismatch between the voltages in-
volved in charging and discharging the batteries. The batteries’ output voltage is more than 1.2 volts lower than the voltage used to charge them, which represents a significant power loss incurred in each charging cycle. “You waste 30 percent of the electrical energy as heat in charging. It can actually burn if you charge it too fast,” he says.
Staying Solid Conventional lithium-air bat-
teries draw in oxygen from the out- side air to drive a chemical reaction with the battery’s lithium during the discharging cycle, and this oxygen is then released again to the atmos- phere during the reverse reaction in the charging cycle. In the new variant, the same
kind of electrochemical reactions take place between lithium and oxygen during charging and discharging, but they take place without ever letting the oxygen revert to a gaseous form. Instead, the oxygen stays inside the solid and transforms directly between its three redox states, while bound in the form of three different solid chem-
ical compounds, Li2O, Li2O2, and LiO2, which are mixed together in the form of a glass. This reduces the volt-
age loss by a factor of five, from 1.2 volts to 0.24 volts, so only 8 percent of the electrical energy is turned to heat. “This means faster charging for cars, as heat removal from the battery pack is less of a safety concern, as well as energy efficiency benefits,” Li says. This approach helps overcome
another issue with lithium- air batteries: As the chemical reaction involved in charging and discharging converts oxygen between gaseous and solid forms, the material goes through huge volume changes that can disrupt electrical conduction paths in the structure, severely limiting its lifetime. The secret to the new
formulation is creating mi- nuscule particles, at the nanometer scale (billionths of a meter), which contain both the lithium and the oxygen in the form of a glass, confined tightly within a ma- trix of cobalt oxide. The re- searchers refer to these par- ticles as nanolithia. In this form, the transitions be-
In a new concept for battery cathodes, nanometer-scale particles made of lithi- um and oxygen compounds (depicted in
tween LiO2, Li2O2, and Li2O, can take place entirely in- side the solid material, says Li. The nanolithia particles would
red and white) are embedded in a sponge- like lattice (yellow) of cobalt oxide, which keeps them stable. The researchers pro- pose that the material could be packaged in batteries that are very similar to con- ventional sealed batteries, yet provide much more energy for their weight.
normally be very unstable, so the re- searchers embedded them within the cobalt oxide matrix, a sponge-like material with pores just a few nanometers across. The matrix stabi- lizes the particles and also acts as a catalyst for their transformations. Conventional lithium-air batter-
ies, Li explains, are “really lithium-dry oxygen batteries, because they can’t handle moisture or carbon dioxide,” so these have to be carefully scrubbed from the incoming air that feeds the batteries. “You need large auxiliary systems to remove the carbon dioxide and water, and it’s very hard to do this.” But the new battery, which nev- er needs to draw in any outside air, circumvents this issue.
No Overcharging The new battery is also inher-
ently protected from overcharging, the team says, because the chemical reaction in this case is naturally self- limiting; when overcharged, the reac- tion shifts to a different form that prevents further activity. “With a typical battery, if you overcharge it, it can cause irreversible structural damage or even explode,” Li says. But with the nanolithia battery, “We have overcharged the battery for 15 days, to a hundred times its capacity, but there was no damage at all.” In cycling tests, a lab version of
the new battery was put through 120 charging-discharging cycles, and showed less than a 2 percent loss of ca- pacity, indicating that such batteries could have a long useful lifetime. And because such batteries could be in- stalled and operated just like conven- tional solid lithium-ion batteries, without any of the auxiliary compo-
odes, the new design could store as much as double the amount of energy for a given cathode weight, the team says. And with further refinement of the design the new batteries could ul- timately double that capacity again. All of this is accomplished with-
out adding any expensive compo- nents or materials, according to Li. The carbonate they use as the liquid electrolyte in this battery “is the cheapest kind” of electrolyte, he says. And the cobalt oxide component weighs less than 50 percent of the nanolithia component. Overall, the new battery system is “very scalable, cheap, and much safer” than lithium- air batteries, Li says. The team ex- pects to move from this lab-scale proof of concept to a practical proto- type within about a year. “This is a foundational break-
through, which may shift the para- digm of oxygen-based batteries,” says Xiulei Ji, an assistant professor of chemistry at Oregon State University, who was not involved in this work. “In this system, commercial carbonate- based electrolyte works very well with solvated superoxide shuttles, which is quite impressive and may have to do
with the lack of any gaseous O2 in this sealed system. All active masses of the cathode throughout cycling are solid, which presents not only large energy density but compatibility with the current battery manufacturing in- frastructure.” The research team included
MIT research scientists Akihiro Kushima and Zongyou Yin; Lu Qi of Peking University; and Khalil Amine and Jun Lu of Argonne National Laboratory in Illinois. The work was supported by the National Science Foundation and the U.S. Depart- ment of Energy. r
nents needed for a lithium-air battery, they could be easily adapted to exist- ing installations or conventional bat- tery pack designs for cars, electronics, or even grid-scale power storage. Because these “solid oxygen”
cathodes are much lighter than con- ventional lithium-ion battery cath-
October, 2016
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