Sadoway and his students developed liquid metal batteries, which can store large amounts of energy and thus even out the ups and downs of power production and power use, a decade ago. The technology is being commercialized by a Cambridge-based startup company, Ambri. (Earlier post.)

All the active components in a liquid metal battery (LMB) are liquid: a low-density liquid metal negative electrode; an intermediate-density molten salt electrolyte; and a high-density liquid metal positive electrode. Because of density differences and immiscibility, these active components self-segregate into three distinct layers.

On discharge, the negative electrode is oxidized to form an ion which migrates across the molten salt electrolyte to the positive electrode, where the ion is electrochemically reduced to neutral metal, alloying with the positive electrode. This process is reversed upon charging.

In earlier works, Sadoway and his colleagues noted that the LMB is well-positioned to satisfy the demands of grid-scale energy storage due to its ability to vitiate capacity fade mechanisms present in other battery chemistries and to do so with earth-abundant materials and easily scalable means of construction.

In this latest work, Sadoway and postdoc Takanari Ouchi, along with Hojong Kim (now a professor at Penn State University) and PhD student Brian Spatocco at MIT showed that calcium, an abundant and inexpensive element, can form the basis for both the negative electrode layer and the molten salt that forms the middle layer of the three-layer battery.

Schematic of cell with the negative current collector consisting of a stainless steel rod and Fe–Ni foam and the positive current collector made of mild steel or graphite. The foam contains the negative electrode. Current collectors are electrically isolated by means of an alumina insulator. Ouchi et al. Click to enlarge.

That was a highly unexpected finding, Sadoway says, due to the known issues with the element such as its easily dissolving in salt.

It was the seeming impossibility of making calcium work in a liquid battery that attracted Ouchi to the problem, he says. “It was the most difficult chemistry” to make work but had potential benefits due to calcium’s low cost as well as its inherent high voltage as a negative electrode.

First, the researchers tackled the problem of the high melting point (almost 900 ˚C) by alloying the calcium with magnesium—another inexpensive metal—which has a much lower melting point. The resulting mix provides a lower operating temperature—about 300 degrees 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 electrolyte that the charge-carrying ions must cross as the battery is used.

The new salt formulation consists of a mix of lithium chloride and calcium chloride; 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.

Charge–discharge voltage time traces of Ca–Mg (20–80 mol%)||Bi, Ca–Mg (90–10 mol%)||Bi, and Ca–Mg (90–10 mol%)||Sb operated at current density 200 mA cm−2 and temperature 650 °C. The theoretical capacities of Ca–Mg (20–80 mol%)||Bi, Ca–Mg (90–10 mol%)||Bi, and Ca–Mg (90–10 mol%)||Sb cells were 0.569, 1.33 and 1.08 Ah, respectively. The results of measurements were replicated more than five times, four times and twice, respectively. Ouchi et al. Click to enlarge.

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 says.

There is another potential big bonus in this new battery chemistry, Sadoway says. “If you’re trying to find high-purity ore bodies, magnesium and calcium are often found together,” he says. 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.

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 research was supported by the US Department of Energy’s Advanced Research Projects Energy (ARPA-E) and by the French energy company Total SA.

Resources

• Takanari Ouchi, Hojong Kim, Brian L. Spatocco & Donald R. Sadoway (2016) “Calcium-based multi-element chemistry for grid-scale electrochemical energy storage” Nature Communications 7, Article number: 10999 doi: 10.1038/ncomms10999