New Liquid Metal Battery Will Make Renewables Competitive

Liquid metal battery at room temperature staging to convey design. Antimony-lead alloy bottom layer (positive electrode), mixed molten salt middle layer (electrolyte), and a metal mesh top layer (negative electrode) to collect the current / Felice Frankel
Janet Fang 22 Sep 2014, 19:32

Researchers have spent years developing an affordable battery that can store energy from erratic, renewable sources like wind and solar in the electrical grid. Now, an MIT team have announced a cheap, liquid-metal battery that’s suitable for the job: The molten electrolyte and liquid-metal electrodes combine a high-performance metal called antimony with low-cost lead. The findings, published in Nature this week, might finally allow intermittent renewable energy sources to compete with conventional power plants.

Batteries with an all-liquid construction have layers of molten material that automatically separate because of their different densities, like oil and vinegar. Two layers of molten metal (one positive electrode, one negative electrode) are separated by a layer of molten sodium chloride (or salt) that acts as the battery’s electrolyte—the layer that charged particles pass through as the battery is charged and discharged. 

Compared to conventional solid-state batteries, these all-liquid ones are potentially advantageous in several ways: They have a longer life cycle and are simpler to manufacture as large-scale storage systems, for example. MIT’s Donald Sadoway and colleagues have previously produced such a battery using an antimony-magnesium electrode. It had good efficiency, but because of the high melting temperature of the antimony-magnesium alloy, the system required temperatures of 700 degrees Celsius.

To improve on their liquid battery system, Sadoway’s team developed a new battery that substitutes magnesium with lead—which is not only cheaper, but also has a lower melting temperature. The new formulation allows the battery to work at 450 to 500 degrees Celsius. A physical model is pictured above: The positive electrode (bottom) is a molten alloy of antimony and lead, the negative electrode (top) is liquid lithium, and the electrolyte between them is mixture of molten salts.

The reduced operating temperature (and hence, cost) simplifies the design and extends the battery’s working life, without compromising its desirable performance characteristics. The antimony produces a high operating voltage, and the system returns about 70 percent of the power that’s put into it. Also, testing has shown that after a decade of daily charging and discharging, the system should retain 85 percent of its initial efficiency. 

Sadoway tells Nature that a large-scale, molten-metal unit might cost around $500 per kilowatt-hour of electricity produced. “Now we understand that liquid metals bond in ways that we didn’t understand before,” he adds in a news release. The team is looking into other metal combinations that might provide even lower-temperature, lower-cost, and higher-performance systems. 

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