Ally Winning, European Editor, PSD
Lithium is expensive, hard to extract in the quantities that will be required to meet demand for EV batteries and renewable energy storage, and often dangerous. Formulating a battery that doesn’t need lithium, even for some applications, would help ensure the lithium supply we have now lasts longer and more new installations are viable. Researchers at MIT look to have taken inspiration from the concept of a molten-salt battery and made it more viable for uses that would include storage from renewable sources. The new battery made entirely from abundant and inexpensive materials, using aluminum and sulfur as its two electrode materials, with a molten salt electrolyte in between
“I wanted to invent something that was better than lithium-ion batteries for small-scale stationary storage, and ultimately for automotive uses,” explained MIT Professor Donald Sadoway, John F. Elliott Professor Emeritus of Materials Chemistry. Sadoway looked for cheap, abundant metals that could replace lithium. Aluminium is the second-most-abundant metal in the marketplace, and the most abundant metal on Earth. The research team chose sulfur for the electrode as it is the cheapest non-metal. For the electrolyte, they attempted to use some polymers, but eventually decided that a molten salt solution would be the best choice. They looked at a variety of molten salts that have melting points close to the boiling point of water. “Once you get down to near body temperature, it becomes practical to make batteries that don’t require special insulation and anticorrosion measures,” Sadoway continued. A chloro-aluminate molten salt was chosen because of its low melting point, but it also had another advantage in that it was good at preventing dendrites from forming. Dendrites are narrow spikes of metal that build up on one electrode and eventually grow across to contact the other electrode, causing a short-circuit and hampering efficiency.
The team then showed that the new battery cells could endure hundreds of cycles at high charging rates. The cells have a projected cost per cell of about one-sixth of comparable lithium-ion cells. The team demonstrated that the charging rate was highly dependent on the working temperature, with 110oC charging 25 times faster rates than 25oC.
The battery requires no external heat source to maintain its operating temperature, as heat is naturally produced electrochemically by the charging and discharging of the battery. Sadoway says, “In a typical installation used for load-levelling at a solar generation facility, electricity is stored when the sun is shining, and drawn after dark. That charge-idle-discharge-idle is enough to generate enough heat to keep the battery at temperature.”
The new battery formulation will be ideal for installations of around the size needed to power a single home or small to medium business, producing a few tens of kilowatt-hours of storage capacity. The smaller scale of the aluminum-sulfur batteries would also make them practical for uses such as electric vehicle charging stations. When several cars want to charge up at once, the current demand can be so high that there isn’t enough to feed the facility. A battery system like this would allow stored power to be released quickly, instead of installing new power lines.
The new technology will be commercialized by a new company called Avanti, which has licensed the patents to the system. The company was co-founded by Sadoway and Luis Ortiz. It will now attempt to demonstrate that the technology works at scale and then subject it to a series of stress tests, including running through hundreds of charging cycles.
The battery is described in the journal Nature, in a paper by Sadoway and 15 others at MIT and in China, Canada, Kentucky, and Tennessee. The research team included members from Peking University, Yunnan University and the Wuhan University of Technology, in China; the University of Louisville, in Kentucky; the University of Waterloo, in Canada; Oak Ridge National Laboratory, in Tennessee; and MIT. The work was supported by the MIT Energy Initiative, the MIT Deshpande Center for Technological Innovation, and ENN Group.