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Lithium-metal battery anodes can store 10-fold more energy by weight than those in today’s best batteries, but they have been too unstable to be practical. Now, researchers have borrowed technology from the semiconductor industry to coat lithium-metal anodes with a thin protective layer that may finally allow them to achieve their promise (ACS Nano 2015, DOI: 10.1021/acsnano.5b02166). New types of batteries made with a stable lithium-metal anode could allow electric vehicles to drive farther between charges or offer a more compact power source for implanted medical devices.
Today’s rechargeable lithium-ion batteries use carbon anodes. They were adopted instead of lithium metal because companies simply could not make the energy-dense anodes work, says Gary W. Rubloff, director of the Maryland NanoCenter at the University of Maryland.
Lithium-metal anodes have two serious problems: They form dendrites that reach toward the cathode, causing electrical shorts and fires, and they are highly reactive. Lithium metal reacts with most of the organic chemicals used in battery electrolytes, and it tarnishes in water and air, causing problems during battery production. To address these problems, researchers have tried many kinds of coatings made from polymers and other materials, but the thickness and reactivity of these layers have been difficult to control, and the coatings can interfere with the battery’s function.
Rubloff had a new idea. He specializes in a fabrication technology called atomic layer deposition (ALD), used widely in the semiconductor industry to layer films during computer chip production. ALD deposits chemicals onto surfaces under ultrahigh vacuum, resulting in thin, smooth coatings of precise thickness. Rubloff wanted to bring this precision method to making batteries. He and his University of Maryland colleague, chemical engineer Malachi Noked, used ALD to coat lithium anodes with a protective layer of aluminum oxide and then tested their performance.
The team deposited aluminum oxide coatings in a range of thicknesses on samples of lithium-metal foil and exposed them to three substances that corrode unprotected lithium metal: ambient air; an organic solvent, propylene carbonate; and dissolved sulfur. The team found that a coating as thin as 14 nm protected the foil samples from corrosion.
Next, the group made a battery with a coated lithium-metal anode and a sulfur cathode, which also has the potential to store a lot of energy. Ordinarily, lithium and sulfur react badly. However, with the coated anode, the lithium-sulfur batteries retained their storage capacity and showed no signs of dendrites or degradation after 100 charge cycles.
Adopting ALD “would be a significant change in how we make batteries, but if this coating actually enables the use of lithium-metal anodes, it would be worth it,” says Kevin G. Gallagher, a chemical engineer working on next-generation batteries at Argonne National Laboratory. This work, he says, is an important first step in demonstrating the concept.
Rubloff and Noked are now trying other coating materials and have already found some that work better than aluminum oxide. The group is also developing methods to monitor the anode and track its performance during hundreds of charging and discharging cycles.
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