New technology could dramatically improve the safety of lithium-ion batteries that run on gaseous electrolytes at ultra-low temperatures. Nanotechnology engineers at the University of California at San Diego have developed a separator – the part of the battery that acts as a barrier between the anode and the cathode – that prevents vaporization of gas-based electrolytes in these batteries. This new separator could, in turn, help prevent pressure build-up inside the battery that leads to swelling and explosions.
“By trapping gas molecules, this separator can function as a stabilizer for volatile electrolytes,” said Zheng Chen, professor of nanotechnology at the Jacobs School of Engineering at UC San Diego who led the study.
The new splitter also improved battery performance at ultra low temperatures. Battery cells built with the new separator operated with a high capacity of 500 milliampere-hours per gram at -40 ° C, while those built with a commercial separator had almost no capacity. The battery cells still exhibited a high capacity even after being idle for two months – a promising sign that the new splitter could extend life as well, the researchers said.
The team published its findings on June 7 in Nature Communication.
This breakthrough brings researchers closer to building lithium-ion batteries capable of powering vehicles in extremely cold weather, such as spacecraft, satellites, and ocean-going vessels.
This work builds on a previous study published in Science by the lab of UC San Diego nanotechnology professor Ying Shirley Meng, who was the first to report the development of lithium-ion batteries that perform well at temperatures as low as -60 ° C. Which makes these batteries particularly resistant to cold is that they use a special type of electrolyte called liquefied gas electrolyte, which is a gas that is liquefied by applying pressure. It is much more resistant to freezing than a conventional liquid electrolyte.
But there is a downside. Liquefied gaseous electrolytes have a strong tendency to change from liquid to gas. “This is the biggest safety issue with these electrolytes,” Chen said. To use them, strong pressure must be applied to condense the gas molecules and keep the electrolyte in liquid form.
To combat this problem, Chen’s lab has teamed up with Meng and UC San Diego nanotechnology professor Tod Pascal to develop a way to easily liquefy these gaseous electrolytes without having to apply so much pressure. . The advance was made possible by combining the expertise of computer experts like Pascal with experimenters like Chen and Meng, all of whom are part of UC San Diego’s Materials Science and Engineering Research Center (MRSEC). .
Their approach uses a physical phenomenon in which gas molecules spontaneously condense when trapped in tiny nanometer-sized spaces. This phenomenon, called capillary condensation, allows a gas to become liquid at a much lower pressure.
The team took advantage of this phenomenon to build a battery separator that would stabilize the electrolyte in their ultra-low temperature battery – a liquefied gaseous electrolyte made up of fluoromethane gas. The researchers constructed the separator from a porous crystalline material called a metal-organic framework (MOF). The peculiarity of MOF is that it is filled with tiny pores capable of trapping fluoromethane gas molecules and condensing them at relatively low pressures. For example, fluoromethane typically condenses under a pressure of 118 psi at -30 C; but with MOF it condenses to only 11 psi at the same temperature.
“This MOF greatly reduces the pressure required to operate the electrolyte,” Chen said. “As a result, our battery cells offer high capacity at low temperatures and show no degradation. “
The researchers tested the MOF-based separator in lithium-ion battery cells – built with a carbon fluoride cathode and a metallic lithium anode – filled with fluoromethane gaseous electrolyte under an internal pressure of 70 psi, which is well below the pressure required to liquefy fluoromethane. The cells retained 57% of their capacity at room temperature at -40 ° C. In contrast, cells with a commercial separator exhibited almost no capacity with a fluoromethane gaseous electrolyte at the same temperature and pressure.
The tiny pores of the MOF based separator are essential as they keep more electrolyte in the battery, even under reduced pressure. The commercial separator, on the other hand, has large pores and cannot retain gaseous electrolyte molecules under reduced pressure.
But tiny pores aren’t the only reason the separator works so well under these conditions. The researchers designed the separator so that the pores form continuous paths from one end to the other. This ensures that lithium ions can still flow freely through the separator. In testing, battery cells with the new separator had 10 times higher ionic conductivity at -40 C than cells with the commercial separator.
Chen’s team is currently testing the MOF-based separator on other electrolytes. “We are seeing similar effects. We can use this MOF as a stabilizer to adsorb various types of electrolyte molecules and improve safety even in traditional lithium batteries, which also contain volatile electrolytes.”
Article: “Sub-nanometric containment allows easy condensation of gaseous electrolyte for low temperature batteries.” Co-authors include Guorui Cai *, Yijie Yin *, Dawei Xia *, Amanda A. Chen, John Holoubek, Jonathan Scharf, Yangyuchen Yang, Ki Kwan Koh, Mingqian Li, Daniel M. Davies and Matthew Mayer, UC San Diego; and Tae Hee Han, Hanyang University, Seoul, Korea.
* These authors have contributed equitably to this work
This work was supported by NASA’s Space Technology Research Grants Program (ECF 80NSSC18K1512), the National Science Foundation through the UC San Diego Materials Research Science and Engineering Center (MRSEC, grant DMR-2011924) and seed funds from the Jacobs School of Engineering at UC San Diego. This work was done in part at the San Diego Nanotechnology Infrastructure (SDNI) at UC San Diego, a member of the National Nanotechnology Coordinated Infrastructure, which is supported by the National Science Foundation (grant ECCS-1542148). This research used resources from the National
Energy Research Scientific Computing Center, a user facility of the DOE Office of Science supported by the Office of Science of the United States Department of Energy under Contract No. DE-AC02-05CH11231. This work also used the Extreme Science and Engineering Discovery Environment (XSEDE) and Comet and Expanse supercomputers from the San Diego Supercomputing Center, which is supported by the National Science Foundation (grant ACI-1548562).