The small prototype device, developed by scientists at the US Department of Energy's (DOE) Pacific Northwest National Laboratory, could be especially useful for storing energy from intermittent sources, like solar and wind energy.
"Longer-duration energy storage technologies are important for increasing the resilience of the grid when incorporating a large amount of renewable energy," said Imre Gyuk, director of energy storage at DOE's Office of Electricity, which funded the work. "This research marks an important step toward a seasonal battery storage solution that overcomes the self-discharge limitations of today's battery technologies."
Renewable sources ebb and flow with natural cycles, which can make it difficult to include them in a reliable, steady stream of electricity.
In the Pacific Northwest in the spring, for instance, rivers heavy with run-off power hydroelectric dams to the max just as winds blow fiercely down the Columbia Gorge. All that power must be harnessed immediately or stored for a few days at most.
Using the technology, energy firms could store that springtime energy in large batteries and then release it later in the year, when the region's winds are slow, the rivers are low, and demand for electricity peaks.
The batteries would also enhance utilities' ability to weather a power outage during severe storms, making large amounts of energy available to be fed into the grid after a hurricane, a wildfire or other calamity.
The battery is first charged by heating it up to 180°C, allowing ions to flow through the liquid electrolyte to create chemical energy. The battery is then cooled to room temperature, essentially locking in the its energy. The electrolyte becomes solid and the ions that shuttle energy become nearly stationary. When the energy is needed, the battery is reheated and the energy flows.
The freeze-thaw phenomenon is possible because the battery's electrolyte is molten salt - a molecular cousin of ordinary table salt. The material is liquid at higher temperatures but solid at room temperature. The freeze-thaw battery was shown to retain 92 per cent of its capacity over 12 weeks.
The team avoided rare, expensive and highly reactive materials, and made the anode and cathode out of easily available aluminium and nickel. They're immersed in the molten-salt electrolyte which is solid at room temperature but flows as a liquid when heated. The team added sulphur, another low-cost element, to the electrolyte to enhance the battery's energy capacity.
"Reducing battery costs is critical. That is why we've chosen common, less-expensive materials to work with, and why we focused on removing the ceramic separator," said corresponding author Guosheng Li, who led the study.
The battery's energy is stored at a materials cost of about $23 per kilowatt-hour, measured before a recent jump in the cost of nickel. The team is exploring the use of iron, which is less expensive, in hopes of bringing the materials cost down to around $6/kWh, roughly 15 times less than the materials cost of today's lithium-ion batteries.
The battery's theoretical energy density is 260 watt-hours per kilogram—higher than today's lead-acid and flow batteries.
