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An aluminum–sulfur battery may become an inexpensive alternative to lithium-ion cells

29 August 2022

The newly developed device also suppresses the buildup of spiky metallic dendrites that can short-circuit a battery and catch fire.

Three clear cups showing silvery metal lumps, a brown liquid, and white crystals from left to right
Credit: Rebecca Miller

Lithium metal is sometimes touted as the ultimate anode material because it provides the highest voltage and energy density for a given cathode. But unless steps are taken to avoid them, lithium dendrites can accumulate on the anode and induce a short circuit that ignites a lithium-ion cell’s flammable liquid electrolyte. An international team of materials scientists led by Peking University’s Quanquan Pang and MIT’s Donald Sadoway has now developed a battery that avoids that problem entirely. Incorporating some of Earth’s most abundant materials, it’s a rapidly charging aluminum–chalcogen cell that operates with a molten-salt electrolyte.

Although iron is the most commercially dominant metal, it does not have the right electrochemical properties to make an efficient battery. Aluminum, the most abundant metal on Earth, turns out to be a better anode choice. In principle, any chalcogen element could stand in as a cathode. But the researchers decided to use sulfur, which is the least expensive of them. And between electrodes, they chose the molten salt NaCl-KCl-AlCl3. (The figure visually juxtaposes those common ingredients of, from left to right, aluminum, sulfur, and salt crystals.)

With a melting point (and hence operating temperature) below the boiling point of water, the chloroaluminate salts are less volatile than other higher-temperature ones. They are also more practical: The researchers’ proof-of-concept experiments demonstrated that the material combination produces high energy density (526 watt-hours/liter, projected from electrochemical data, on par with that of lithium-ion batteries) and endured hundreds of cycles at exceptionally rapid charging rates—reaching full charge in less than one minute.

What’s more, although the team chose the chloroaluminate molten-salt electrolyte primarily because of its low melting point, the new experiments showed that the high AlCl3 concentration in the salt resists the dendrite-shorting problem at those charging rates. The researchers speculate that dendrite growth is impeded by the presence of trace amounts of dissolved sulfide, which is known to act as a leveling agent.

Given the availability of all the components, the researchers estimate the cost of the Al–S battery to be as low as $8.99 per kilowatt-hour. That’s 12–16% of the cost of today’s lithium-ion batteries. And because the molten-salt electrolyte is thermally stable above 500 °C and immune to thermal runaway and fire, the researchers argue that the battery chemistry is likely to be especially attractive for electric vehicles. Sadoway has already made the research the basis for Avanti, the spin-off company he cofounded that has licensed the patents for the battery technology.

The new batteries could also be ideal for powering a single home, as they profitably scale at the requisite tens of kilowatt-hours of storage capacity. Today’s lithium-ion batteries are still too expensive for such energy storage applications. (Q. Pang et al., Nature 608, 704, 2022.)

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