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Sodium-Ion Cells for Cheap Energy Storage

DOE funds the development of low-cost sodium-ion batteries.
December 2, 2009

A new type of sodium-ion battery could prove to be a practical option for storing power from wind and solar farms, says Jay Whitacre, a professor of materials science and engineering at Carnegie Mellon University. Whitacre’s startup, 44 Tech, based in Pittsburgh, PA, will receive $5 million from the U.S. Department of Energy, as part of the 2009 Recovery Act, to develop the technology. The funding, announced last week, is part of a $620 million package for improving the electricity grid.

The startup’s batteries could be not only cheaper but also longer-lasting than existing batteries, Whitacre says. This would make them particularly useful for storing large amounts of electricity cheaply–something that will be essential for making renewable energy the primary source of energy in the U.S., rather than just the supplemental source it is now. Such storage will make it practical to store energy from wind turbines and solar farms for use when the wind isn’t blowing and the sun isn’t shining.

Whitacre’s sodium-ion cells are similar in some ways to lithium-ion cells–the type used in portable electronics and in some electric vehicles. In both types of cell, ions are shuttled between the battery’s positive and negative electrodes during charging and discharging, with an electrolyte serving as the medium for moving those ions. But because sodium is orders of magnitude more abundant than lithium, it is cheaper to use. To make the cells cheaper still, Whitacre plans to operate them at lower voltages, so that water-based electrolytes can be used instead of organic electrolytes. This should further decrease manufacturing costs, since water-based electrolytes are easier to work with.

The change to water-based electrolytes could also make it possible to eliminate much of the supporting material needed in conventional lithium-ion cells, again reducing costs. This is because increasing the ionic conductivity makes it possible to use thicker electrodes with fewer layers of separating and current-collecting materials inside the cell.

“In principle, a sodium-ion system can be low-cost, and with aqueous electrolytes, it could be really low-cost,” says Jeff Dahn, a professor of physics and chemistry at Dalhousie University in Nova Scotia, Canada.

Researchers have looked into sodium-ion batteries in the past, although typically they have used high voltages and organic electrolytes. Using lower voltages reduces the amount of energy the batteries can store–a problem for electric vehicles, where space and weight are limited. But for stationary applications like storing renewable energy, “it’s all about cost,” Whitacre says.

Dahn argues that sodium-ion cells shouldn’t be developed just for large-scale electricity storage. Higher-voltage sodium-ion batteries may eventually prove a much better solution than lithium-ion batteries for electric vehicles, he says. So far, however, very little research has been done on them relative to lithium-ion batteries. Factors that have kept researchers away–such as the large size of sodium ions and the effect this has on the amount of power the batteries could deliver–have been addressed by recent advances in materials manufacturing. The abundance of sodium could also make these batteries extremely attractive. “It’s amazingly more abundant than lithium,” Dahn says. “I think it’s something that’s really important to work on going forward. I hope [the] DOE funds the nonaqueous work, too.”

So far, Whitacre’s work is at an early stage. He has demonstrated small battery cells in the lab and has filed for a patent covering the technology. He has not disclosed what materials he will use for the electrodes and the electrolyte, and it’s too early to provide specific figures about cost, he says. The next steps include making larger prototype batteries. Part of the $5 million award will go to Carnegie Mellon for fundamental research.

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