Engineers develop separator that stabilizes gaseous electrolytes, making ultra-low temperature batteries safer

According to foreign media reports, nanoengineers at the University of California San Diego have developed a battery separator that acts as a barrier between the cathode and anode to prevent the gaseous electrolyte in the battery from evaporating. The new separator prevents pressure from accumulating inside the battery, thereby preventing the battery from swelling and exploding.

“By trapping gas molecules, the separator can act as a stabilizer for the volatile electrolyte,” said Zheng Chen, a professor of nanoengineering at the UC San Diego Jacobs School of Engineering and leader of the research.

The new separator can improve battery performance at ultra-low temperatures. Battery cells using the separator can operate at temperatures of minus 40 degrees Celsius and have a capacity of up to 500 milliamp hours per gram, while batteries using commercial separators have a capacity of almost zero in this condition. The researchers said that even if the battery cells were idle for two months, the capacity was still high. This performance shows that the separator can also extend the storage life. The discovery can bring the researchers one step closer to the goal of producing batteries that can provide power for vehicles in extremely cold environments, such as spacecraft, satellites and deep-sea vessels.

(Image credit: University of California, San Diego)

The research is based on a study conducted by the laboratory of Ying Shirley Meng, a professor of nanoengineering at the University of California, San Diego. The study used a special liquefied gas electrolyte to develop a battery that can maintain good performance in an environment of minus 60 degrees Celsius for the first time. Among them, the liquefied gas electrolyte is a gas that is liquefied by applying pressure and is more resistant to low temperatures than traditional liquid electrolytes.

But this electrolyte has a flaw: It can easily change from liquid to gas. "This is the biggest safety issue with this electrolyte," Chen said. To use this electrolyte, pressure must be increased to condense the liquid molecules and keep the electrolyte in liquid form.

To address this problem, Chen's lab collaborated with Meng and Tod Pascal, a professor of nanoengineering at UC San Diego. By combining the expertise of computational experts like Pascal with that of researchers like Chen and Meng, all of whom are affiliated with the Materials Research Science and Engineering Center (MRSEC) at UC San Diego, they developed a way to easily liquefy this vaporized electrolyte without applying too much pressure.

The method draws on a physical phenomenon in which gas molecules spontaneously condense when they are trapped in tiny nanoscale spaces. This phenomenon, called capillary condensation, allows gases to become liquids at lower pressures. The research team used this phenomenon to build a battery separator that stabilizes the electrolyte in ultra-low temperature batteries, a liquefied gas electrolyte made of fluoromethane gas. The researchers used a porous crystalline material called a metal-organic framework (MOF) to create the separator. What's special about MOF is that it is full of tiny pores that can capture fluoromethane gas molecules and condense the molecules at relatively low pressures. For example, fluoromethane usually condenses at minus 30 degrees Celsius and a pressure of 118 psi; but using MOF, the condensation pressure of porous at the same temperature only requires 11 psi.

Chen表示：“这种MOF显著降低了电解质工作所需的压力。因此，我们的电池在低温下可提供大量容量，并且不会出现退化。”研究人员在锂离子电池中测试了基于MOF的隔膜。该锂离子电池由碳氟化物阴极和锂金属阳极组成，并可在70 psi的内部压力下填充氟甲烷气态电解质，远远低于液化氟甲烷所需的压力。电池在零下40℃下仍可保持其室温容量的57%。相比之下，在相同温度和压力下，使用含氟甲烷气态电解质的商用隔膜电池的容量几乎为零。

The micropores of MOF-based separators are key because they keep more electrolyte flowing in the battery even under reduced pressure. Commercial separators have larger pores and cannot retain gaseous electrolyte molecules under reduced pressure. But micropores are not the only reason why the separator works well under these conditions. The researchers also designed the separator so that the pores form a continuous path from one end to the other, ensuring that lithium ions can flow freely through the separator. In tests, the ionic conductivity of batteries using the new separator at minus 40 degrees Celsius was 10 times that of batteries using commercial separators.

Chen's team is currently testing MOF-based separators on other electrolytes. "We saw similar effects," Chen said. "By using this MOF as a stabilizer, we can adsorb a variety of electrolyte molecules and improve battery safety, including traditional lithium batteries with volatile electrolytes."