Skip to main content
Materials for energy

Materials for energy

Freeze–thaw battery could help store solar and wind energy

27 Apr 2022
Freeze-thaw battery figure

A battery that “freezes” its stored chemical energy for several months has been developed by Minyuan Li and colleagues at Pacific Northwest National Laboratory in the US. Their battery uses a molten salt electrolyte, which remains solid at room temperatures, and thaws out when heated.

Our capacity to generate electrical energy using wind and solar energy is growing in leaps and bounds, but our ability to store this energy has not kept pace. This is a problem because seasonal trends in wind and solar generation often do not coincide with the energy needs of consumers. At higher latitudes, for example, abundant solar energy can be harvested in the summer, when requirements for heating and lighting are low. Conversely, solar energy can be scarce in the winter when energy demand can be high.

This problem could be solved by storing energy for several months and releasing it when demand begins to outpace supply. While lithium-ion batteries could do the job (while losing as much as 5% of their energy in the first month), there are supply and geopolitical concerns regarding materials used in their manufacture. Other battery types have different challenges that would have to be overcome.

Reduced mobility

To develop a better storage technology, Li and colleagues explored the use of molten salt as a battery electrolyte. When their aluminium-nickel salt was heated to 180 °C, its ions were allowed to flow freely between electrodes immersed in the liquid. But when cooled to room temperature, the salt froze into a solid. This drastically reduced the mobility of its ions – which locks in the chemical energy of the battery. After an indefinite period, the salt could then be heated and thawed, allowing the battery to be discharged.

To ensure that their battery could be a practical storage system, a key concern for Li’s team was to use low-cost, widely available materials wherever possible. This involved choosing a suitable material for their battery’s inert, porous separator – which separates the anode and cathode while allowing ions to pass through. So far, separators have typically been made from ceramics, but these materials are costly, and could be easily damaged during the battery’s freeze-thaw cycle.

As an alternative, the researchers used porous fibreglass as a separator, which fares far better at widely varying temperatures. In addition, they doped their electrolyte with sulphur – another easily attainable material. This addition both boosted the battery’s energy retention further, and activated its nickel cathode.

So far, Li’s team has developed a small prototype of the battery, around the size of a hockey puck. After a storage period of up to eight weeks, the device retained over 90% of its stored energy, following a single freeze-thaw cycle. In the future, the researchers hope that the low cost and simplicity of their design will allow them to ramp up its size and capacity. If achieved, this could allow electricity grids to store energy and eliminate differences between supply and demand.

The battery is described in Cell Reports Physical Sciences.

Copyright © 2024 by IOP Publishing Ltd and individual contributors