The underlying liquid sodium electrode battery chemistry was first described in 1968, but needs a thin membrane of a britle, fragile ceramic to separate its molten components. These membranes made the batteries too easily damaged in real-world operating conditions out in the grid and so have struggled with adoption.
Instead, Prof Donald Sadoway and his team at MIT took a different approach by using a specially coated metal mesh. This provides a much stronger and more flexible material that could be used in industrial-scale storage systems. "The cost was kept high because of the fragility of the ceramic membranes," said Sadoway, the John F. Elliott Professor of Materials Chemistry. "Nobody's really been able to make that process work." He cites GE, which spent nearly 10 years working on the technology before abandoning the project.
"I consider this a breakthrough," he added, "because for the first time in five decades, this type of battery -- whose advantages include cheap, abundant raw materials, very safe operational characteristics, and an ability to go through many charge-discharge cycles without degradation -- could finally become practical."
The development came from a fortunate accident. The team was investigating lead compounds in the battery cell. Opening the cell, they found found droplets of molten lead acting as an electrode rather than membrane. "That really opened our eyes to a completely different technology," said Sadoway. This uses the material as an electrical membrane to allow certain molecules to pass through while blocking others, rather thana physical selective barrier.
After experimenting with various compounds, the team found that an ordinary steel mesh coated with a solution of titanium nitride could perform all the functions of the previously used ceramic membranes, but without the brittleness and fragility. The results could make possible a whole family of inexpensive and durable materials practical for large-scale rechargeable batteries.