14 February 2012
Progress toward inexpensive grid-scale batteries
New results from an ongoing research program at MIT, reported in the Journal of the American Chemical Society, show a promising technology that could provide that long-sought way of leveling the load - at far lower cost and with greater longevity than previous methods.
Professor Donald Sadoway (l.) and Materials Processing Center Research Affiliate David Bradwell observe one of their small test batteries. The battery itself is inside the heavily insulated metal cylinder at center, which heats it to 700 degrees Celsius.
The system uses high-temperature batteries whose liquid components, like some novelty cocktails, naturally settle into distinct layers because of their different densities.
The three molten materials form the positive and negative poles of the battery, as well as a layer of electrolyte - a material that charged particles cross through as the battery is being charged or discharged - in between.
"All three layers are composed of materials that are abundant and inexpensive," explains Donald Sadoway, the John F. Elliott Professor of Materials Chemistry at MIT and the senior author of the new paper.
"We explored many chemistries," Sadoway says, looking for the right combination of electrical properties, abundant availability and differences in density that would allow the layers to remain separate.
His team has found a number of promising candidates, he says, and is publishing their detailed analysis of one such combination: magnesium for the negative electrode (top layer), a salt mixture containing magnesium chloride for the electrolyte (middle layer) and antimony for the positive electrode (bottom layer). The system would operate at a temperature of 700 degrees Celsius, or 1,292 degrees Fahrenheit.
The inspiration for the concept came from Sadoway's earlier work on the electrochemistry of aluminum smelting, which is conducted in electrochemical cells that operate at similarly high temperatures.
Many decades of operation have proved that such systems can operate reliably over long periods of time at an industrial scale, producing metal at very low cost. In effect, he says, what he figured out was "a way to run the smelter in reverse."
Over the last three years, Sadoway and his team have gradually scaled up their experiments. Their initial tests used batteries the size of a shot glass; they then progressed to cells the size of a hockey puck, three inches in diameter and an inch thick. Now, they have started tests on a six-inch-wide version, with 200 times the power-storage capacity of the initial version.
The team is continuing to work on optimizing all aspects of the system, including the containers used to hold the molten materials and the ways of insulating and heating them, as well as ways of reducing the operating temperature to help cut energy costs.
"We,ve discovered ways to decrease the operating temperature without sacrificing electrical performance or cost," Sadoway says.
Robert Huggins, a professor emeritus of materials science and engineering at Stanford University, says, "As for any radically different approach, there are a number of new practical problems to solve in order for it to become a practical alternative for use in large-scale energy storage, [including] electrolyte evaporation, and corrosion and oxidation of components, as well as the ever-present issue of cost."
Nevertheless, he says, this is "a very innovative approach to electrochemical energy storage, and it is being explored with a high degree of sophistication."
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