Molten-salt storage can enhance EVs with a 12-week battery life

It relies on a new "freeze-thaw" design.
Christopher McFadden

A recent study has just been published by U.S. scientists who have managed to develop an aluminum-nickel molten salt battery that can retain over 90% of its initial capacity over a period of up to 12 weeks. Having an energy density of 260 W/hour per kg, the new battery was built with an aluminum anode and a nickel cathode, immersed in a molten-salt electrolyte.

The study was published in the journal Cell Reports Physical Science.

The breakthrough was developed by scientists a the US Department of Energy’s Pacific Northwest National Laboratory (PNNL), and has been described by its developers as a small prototype "freeze-thaw battery". This battery is able to break off its self-charging functions when it is idling. 

“It’s a lot like growing food in your garden in the spring, putting the extra in a container in your freezer, and then thawing it out for dinner in the winter,” explained researcher Minyuan Miller Li.

The new battery can be charged by heating it to around 356 degrees Fahrenheit (180 degrees Celsius), allowing ions to flow through its liquid electrolyte. It can then be restored to room temperature and the electrolyte becomes solid, thus trapping the ions that transport the stored energy, effectively halting the charge cycle. 

“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 scientists said.

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The battery can be heated again, as the ions restart flowing through the electrolyte when energy is needed. 

To help increase the battery's storage capacity, sulfur was added to the electrolyte too. A ceramic separator was embedded between the anode and the cathode to avoid breakage during the freeze-thaw cycle.

“The PNNL battery uses simple fiberglass, possible because of the battery’s stable chemistry. This cuts costs and makes the battery sturdier when undergoing freeze-thaw cycles,” said the research team.

According to the study, the battery can retain 92% of its initial capacity over a period of 12 weeks while also maintaining a high energy density. 

“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,” study members said. “The team is exploring the use of iron, which is less expensive, in hopes of bringing the materials cost down to around $6 per kilowatt-hour, roughly 15 times less than the materials cost of today’s lithium-ion batteries," they concluded. 

Study abstract: 

"Grid-level storage of seasonal excess can be an important asset to renewable electricity. By applying the freeze-thaw thermal cycling strategy, here, we report Al-Ni molten salt batteries with effective capacity recovery over 90% after a period of 1–8 weeks as a proof-of-concept. We explore three activation methods of the nickel cathode in a molten-salt battery: (1) heat-treating the cathode granules under H2/N2, (2) incorporating a partially charged NiCl2/Ni cathode, and (3) doping the molten salt electrolyte with sulfur. In particular, sulfur doping, a cost-efficient method suitable for large-scale applications, is not only effective in activating the Ni cathode initially but also invaluable for energy retention during thermal cycling. Overall, these Al-Ni molten salt batteries under thermal cycling show high retention in cell capacity over weeks, setting a direction for scalable seasonal storage."

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