Rechargeable Molten Salt Battery Freezes Energy in Place for Long-Term Storage

During the spring season in the Pacific Northwest, the wind blows hard and meltwater rushes down the rivers. At a time of mild temperatures and relatively low energy demand, these forces spin the region's many power turbine and generate a bounty of electricity. The batteries can't store it long enough to power the air conditioners.

A battery that might solve this problem is being developed by researchers at the Pacific Northwest National Laboratory. In a recent paper published in Cell Reports Physical Science, they demonstrated how freezing and thawing molten salt solution can create a battery that can store energy for weeks or months at a time. It's crucial to shift the U.S. grid away from fossil fuels that release greenhouse gases. President Joe Biden wants to cut U.S. carbon emissions in half by the year 2030.

Conventional batteries store energy in chemical reactions. When the battery is connected to an external circuit, electrons travel from one side of the battery to the other. To compensate for the change, charged particles called ion move through the fluid, paste or solid material that separates the two sides of the battery. When the battery is not in use, the ion diffuses across the electrolyte. The battery loses energy when that happens. A third of the stored charge can be lost in a single month.

The researcher who led the project said they tried to stop the self-discharge in the battery. The electrolyte is made of a salt solution that is solid at ambient temperatures but becomes liquid when heated to 180 degrees Celsius. The ion are locked in place when the electrolyte is solid. The ion can't flow through the battery until the liquifies.

Creating a battery that can endure repeated cycles of heating and cooling is no small task. The researchers had to find materials that could tolerate the changes in temperature that cause the battery to expand and contract.

It is a great example of a promising long-duration energy-storage technology.

In a place like Alaska, where there is relatively low rates of energy use, the technology could be useful. Rob Roys, chief innovation officer at Launch Alaska, says that the freeze-thaw battery is attractive because it can store energy for months. Roys wants to pilot the battery in a remote part of his state.

In cold places, heating the battery may be a challenge. The heating process requires between 10 and 15 percent of the battery's capacity, Li says. The project will look at ways to lower the temperature requirements and incorporate a heating system into the battery. It would be possible to make the battery suitable for home or small-scale use with this feature.

The technology is intended for utility-scale and industrial uses. Sprenkle thinks of something like tractor-trailer truck containers with huge batteries parked next to wind farms or solar panels. The batteries would be charged on-site, allowed to cool and be driven to facilities where the energy could be distributed through power lines.

The DOE's job is to derisk new technologies, and the team at the PNNL plans to continue developing the technology.

The lag between initial research demonstrations and commercialization of energy technologies is something the DOE is trying to shrink. The batteries were not incorporated into electrical grids until the late 2000s, despite scientists developing them in the 1970s. Sprenkle says that artificial intelligence and machine learning can help speed up the validation and testing process for new technologies, allowing researchers to model and predict a decade of battery performance without needing 10 years to collect the data.

It's not clear if adoption will be enough to meet decarbonization targets.