Lithium metal is sometimes considered 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 build up on the anode, causing short circuits that can ignite the flammable liquid electrolyte of lithium-ion batteries. An international team of materials scientists led by Pang Quanquan of Peking University and Donald Sadoway of MIT has now developed a battery that avoids the problem entirely. Combining some of the most abundant materials on Earth, it is a fast-charging aluminum-sulfur battery that operates using a molten salt electrolyte.

Although iron is the most commercially advantageous metal, it does not possess the electrochemical properties required to make efficient batteries. Aluminum is the most abundant metal on the planet, and it turns out to be the better anode choice. In principle, any element of sulfur can act as a cathode. But the researchers decided to use the cheapest form of sulfur. Between the electrodes, they chose the molten salt NaCl-KCl-AlCl3. (From left to right in the picture are the common constituents of aluminum, sulfur, and salt crystals.)

Chloroaluminates are less volatile than other salts because their melting point (ie, operating temperature) is lower than the boiling point of water. They're also more practical: the researchers' proof-of-concept experiments show that the material combination yields a high energy density (526 Wh/L, predicted from electrochemical data, comparable to that of lithium-ion batteries) and is Hundreds of cycles at exceptionally fast charging rates - fully charged in less than a minute.

Additionally, while the team chose the chloroaluminate molten salt electrolyte primarily because of its low melting point, the new experiments show that at these charging rates, the high AlCl3 concentration in the salt also resists the dendrite shorting problem. The researchers speculate that trace amounts of dissolved sulfide could act as a leveling agent, hindering dendrite growth.

Taking into account the availability of all components, the researchers estimate the cost of an Al-S battery to be as low as $8.99 per kWh. This is 12-16% of the cost of lithium-ion batteries today. And, because the molten salt electrolyte is thermally stable above 500°C and immune to thermal runaway and fire, the researchers believe the battery chemistry could be particularly attractive for electric vehicles. Sadoway has used this research as the basis for Avanti, a spin-off company he founded that has licensed battery technology patents.

The new type of batteries could also be ideal for powering a single home, as they can be profitably expanded with the necessary tens of kilowatt-hours of storage capacity. Today's lithium-ion batteries are still too expensive for this energy storage application.