Lithium metal is sometimes touted as the ultimate anode material because it provides the highest voltage and energy density for a given cathode. However, if steps aren’t taken to avoid this, lithium dendrites can accumulate at the anode, causing a short circuit that ignites the flammable liquid electrolyte of a lithium-ion battery. An international team of materials scientists led by Quanquan Pang of Peking University and Donald Sadoway of MIT has now developed a battery that completely eliminates this problem. It’s a rapidly charging aluminum-chalcogen cell that uses a molten salt electrolyte and incorporates some of the world’s most abundant materials.
Although iron is the most commercially dominant metal, it doesn’t possess the right electrochemical properties to make an efficient battery. Aluminum, the most abundant metal in the world, emerges as a better anode choice. In principle, any chalcogen element can serve as a cathode. However, the researchers decided to use sulfur, the cheapest option. And they chose the molten salt NaCl-KCl-AlCl3 as the electrolyte. (The figure visually juxtaposes these common components of aluminum, sulfur, and salt crystals from left to right.)
With a melting point (and therefore operating temperature) below the boiling point of water, chloroaluminate salts are less volatile than other high-temperature salts. They are also more practical: Researchers’ proof-of-concept experiments showed that the material combination produces a high energy density (526 watt-hours/liter, estimated from electrochemical data, on par with lithium-ion batteries) and withstands hundreds of battery cycles at extremely fast charging rates—reaching full charge in less than a minute.
Furthermore, although the team primarily chose the chloroaluminate molten salt electrolyte due to its low melting point, new experiments showed that the high AlCl3 concentration in the salt resists the dendrite short-circuiting problem at these charging rates. The researchers hypothesize that dendrite growth is inhibited by trace amounts of dissolved sulfide, which is known to act as a leveling agent.
Given the availability of all components, researchers estimate the cost of the Al-S battery to be as low as $8.99 per kilowatt-hour. This is 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, researchers argue the battery chemistry could be particularly attractive for electric vehicles. Sadoway has already based the research on Avanti, the company he co-founded that licenses patents for the battery technology.
The new batteries could also be ideal for powering a single home, as they scale profitably to the required 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.)
R. Mark Wilson, 29 Aug 2022. Physics Today
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