As the pressure to decarbonize electricity grids mounts, so does the need to have long-term storage options for power generated from renewables—especially for sources like wind and solar, which have discontinuous availability. While rechargeable batteries are the solution of choice for consumer-level use, they are impractical for grid-scale consideration. Scientists have been looking for solutions in gravity energy storage, thermal or geothermal storage, and also molten-salt batteries.
A recent study from the Pacific Northwest National Laboratory (PNNL) looks at molten-salt batteries that can “freeze” their charge for months until required. In their proof of concept, the researchers reported that the battery retained 92 percent of its capacity over three months. “We have some test cases ongoing for six months at this time,” says Minyuan “Miller” Li, first author of the study. He expects the battery to retain over 80 percent of its charge in that period.
Molten-salt batteries, as the name implies, use a liquid, molten-salt electrolyte, which freezes at room temperature, allowing the batteries to be stored in an inactive state. When activated, the cathode, anode, and electrolytes separate, with the molten electrolyte serving as a highly conductive medium for ionic exchange.
“If you can truly freeze out that self-discharge mechanism, could these battery systems sit somewhere for years and…[when needed] be heated up by a readily available means?”
—Vincent L. Sprenkle, Pacific Northwest National Laboratories
Batteries are generally unreliable for seasonal or long-term storage because they discharge when unused. However, in the PNNL team’s demonstration, the freeze-thaw mechanism of the molten salt is able to circumvent that problem. They used nickel and aluminium as materials for the cathode and anode, respectively, with sodium aluminium tetrachloride (NaAlCl4) as the molten-salt electrolyte—all relatively cheap, earth-abundant materials. This electrolyte has a melting point of around 157 ºC, and remains solid over a large spectrum of room temperatures.
“We want to charge this battery when renewables are abundant,” explains lead researcher Guosheng Li, “and then we’re going to keep the battery at ambient temperature, which will freeze [it]…and shut off the self-discharge for long storage.” When it’s time to use the battery, he continues, there are a few different ways to heat it up. For example, “we can use waste heat, or…activate some of the battery in the beginning and then use that electricity to self-heat.”
To demonstrate their concept, they built a relatively small, hockey-puck-size battery. But Li doesn’t see any impediments to scaling up for practical use, given that the materials are easy to source. Even so, he says, “we know [these are] probably not the best materials, [so] our next target is to replace nickel with a lower-cost material.”
Iron is one of the alternatives they are considering, as well as looking at other options for the electrolyte. The current melting point of 157 ºC of NaAlCl4 is higher than the researchers would like. “For future research, we are geared towards low-cost materials as well as relatively low operating temperature, but still above the ambient temperature,” Li says. “We want to freeze the electrolyte at ambient temperature, right? So around 70 or 80 degrees [freezing point] would be ideal, which means we don’t need to so much heat the battery to 180 degrees [as in the study].”
When it comes to potential applications, adds Vincent L. Sprenkle, another coauthor, the lower they can get the operating temperature, the better off they will be. “One of the potential applications we envision for this is resiliency of critical infrastructures,” he says. “If you can truly freeze out that self-discharge mechanism, could these battery systems sit somewhere for years and…[when needed] be heated up by a readily available means?”
The PNNL team are trying to target a 2030 to 2035 time frame for commercial application, particularly for use in systems that are deployed and operational. A long road, in other words, lies ahead. As Li points out, this proof of concept was just to see if the technology was possible. “If we can perform a more elegant experiment, I do believe we can [bring down] the temperature, maybe to right above room temperature…and save a lot of energy.”
Apart from bringing down operating temperatures and replacing the core with lower-cost materials, there are other challenges that the researchers feel will become more visible as they scale up. An important one, Sprenkle adds, is understanding the value of that stored energy. “A large part of our value is just resiliency…[and] we have difficulty accurately valuing resiliency…So there are some broader policy questions that come into play that are out of our control [but] still need to be addressed.”
