Liquid metal batteries the key to renewables?

Yes, according to research from the Massachusetts Institute of Technology (MIT). Professor Donald Sadoway and his students invented the batteries, which can store large amounts of energy and therefore even out fluctuations in power production and power use, over a decade ago. Now electricity storage company Ambri is in the process of commercialising the technology.

According to a new report published in the journal Nature Communications, which draws on a paper developed by Sadoway and his team, research has uncovered another set of chemical constituents that can improve the technology. Sadoway is the John F Elliott Professor of Material Chemistry and is working with postdoc Takanari Ouchi, Hojong Kim (now a professor at Penn State University) and PhD student Brian Spatocco at MIT. The research has shown that calcium can form the basis for both the negative electrode layer and the molten salt that forms the middle of a three-layer battery.

According to Science Daily, the finding was highly unexpected, as the chemical properties of calcium make it unlikely to be suitable. For starters, it is easily dissolved in salt. A crucial feature of the liquid battery is that each constituent forms a separate layer based on a different density. Additionally, the inherent high melting point of calcium would mean that the battery needed to operate at almost 900°C, which the team readily admits “is ridiculous”. The seeming impossibility of these challenges is essentially what drove Sadoway’s team to find an answer.

The researchers tackled the temperature problem by alloying the calcium with another inexpensive metal, magnesium, which has a much lower melting point. The resulting mix provides a lower operating temperature — about 300° less than that of pure calcium — while still keeping the high-voltage advantage of the calcium.

The other key innovation was in the formulation of the salt used in the battery’s middle layer, called the electrolyte, that charge carriers, or ions, must cross as the battery is used. The migration of those ions is accompanied by an electric current flowing through wires that are connected to the upper and lower molten metal layers, the battery’s electrodes.

The new salt formulation consists of a mix of lithium chloride and calcium chloride, and it turns out that the calcium-magnesium alloy does not dissolve well in this kind of salt, solving the other challenge to the use of calcium.

But solving that problem also led to a big surprise: normally there is a single ‘itinerant ion’ that passes through the electrolyte in a rechargeable battery, for example, lithium in lithium-ion batteries or sodium in sodium-sulfur. But in this case, the researchers found that multiple ions in the molten-salt electrolyte contribute to the flow, boosting the battery’s overall energy output. That was a totally serendipitous finding that could open up new avenues in battery design, Sadoway said.

And there’s another potential big bonus in this new battery chemistry, Sadoway said. “There’s an irony here. If you’re trying to find high-purity ore bodies, magnesium and calcium are often found together,” he said. It takes great effort and energy to purify one or the other, removing the calcium ‘contaminant’ from the magnesium or vice versa. But since the material that will be needed for the electrode in these batteries is a mixture of the two, it may be possible to save on the initial materials costs by using ‘lower’ grades of the two metals that already contain some of the other.

“There’s a whole level of supply-chain optimisation that people haven’t thought about,” he said.

Sadoway and Ouchi stress that these particular chemical combinations are just the tip of the iceberg, which could represent a starting point for new approaches to devising battery formulations. And since all these liquid batteries (including the original liquid battery materials from his lab and those under development at Ambri) would use similar containers, insulating systems and electronic control systems, the actual internal chemistry of the batteries could continue to evolve over time. They could also adapt to fit local conditions and materials availability while still using mostly the same components.

“The lesson here is to explore different chemistries and be ready for changing market conditions,” Sadoway said. What they have developed “is not a battery; it’s a whole battery field. As time passes, people can explore more parts of the periodic table” to find ever-better formulations, he said.

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