Research into a liquid metal catalyst could be the answer to grid-scale energy storage

Researchers from the US based Massachusetts Institute of Technology (MIT) have developed a liquid battery system that could allow grid-scale electrical energy storage.

The liquid battery catalysts use layers of molten metal separated by layers of molten salt that act as the electrolyte. The different densities of the three materials allow them to naturally separate into layers, like oil floating on water. Work initially began using magnesium and antimony with a salt catalyst. While this was successful, it meant an operating temperature of 700°C. However, by using lithium and then a mixture of lead and antimony, the temperature of the liquid battery has been reduced to 450°C without any drop off in performance. The lead researcher at MIT, Donald Sadoway, says: "The new formula allows the battery to work at a temperature more than 200°C lower than the previous formulation. In addition to the lower operating temperature, which should simplify the battery's design and extend its working life, the new formulation will be less expensive to make." Extensive testing has shown that, even after 10 years of daily charging and discharging, the system should retain about 85% of its initial efficiency, a key factor in making such a technology an attractive investment for electric utility companies as they wrestle with questions around energy storage. Currently, the only widely used system for utility-scale storage of electricity is pumped hydro. This sees water pumped uphill to a storage reservoir when there is excess power and when the power is needed, it is forced down through a turbine to generate power. Such systems can be used to match the intermittent production of power from irregular sources, such as wind turbines and solar power generation, however the inevitable losses of friction in pumps and turbines mean a round trip efficiency of about 70%. Sadoway says his liquid-battery can already deliver the 70% efficiency and with further refinements could do better. And unlike pumped hydro systems, which are only feasible in locations with sufficient water and an available hillside, liquid batteries could be built anywhere and at any size. "The fact that we don't need a mountain, and we don't need lots of water, gives us a decisive advantage," says Sadoway. "Now we understand that liquid metals bond in ways that we didn't understand before. I think there's still room for major discoveries in this field." The biggest surprise for the researchers was that the antimony-lead electrode performed so well. They found that antimony could produce a high operating voltage and that lead gave a low melting point. However, mixing the two combined both advantages with a voltage as high as antimony but with a melting point between the two constituents. The fact there was no drop off in voltage was a genuine surprise. Going forward, the team will continue to search for other combinations of metals that might allow even lower operating temperatures, lower-cost production, and better storage performance.