"White Graphene" Helps Rechargeable Batteries Withstand Extreme Heat

Rice University materials scientists have demonstrated the use of a combined electrolyte and separator for rechargeable lithium-ion batteries that reliably supplies energy at usable voltages in extremely high-temperature environments.

The Rice team, led by materials scientist Pulickel Ajayan, says batteries made with a composite incorporating hexagonal boron nitride (h-BN), i.e., “white graphene,” functioned perfectly in conditions ranging from room temperature to 150 degrees Celsius for more than a month with negligible loss of efficiency—in the process recording one of the widest temperature ranges ever reported for such devices. “We tested our composite against benchmark electrodes and found that the batteries were stable for more than 600 cycles of charge and discharge at high temperatures,” says researcher and Rice graduate student Marco-Túlio Rodrigues.

Boron nitride helps make the battery electrolyte more stable under high heat/high voltage. Image credit: Rice University/Jeff Fitlow.

Batteries with the new electrolyte would have great utility in industrial and aerospace applications, the researchers say. Oil and gas companies, in particular, require robust batteries to power sensors on drill bits that experience extreme temperatures in downhole environments.

“At present, non-rechargeable batteries are heavily used for the majority of these applications, which pose practical limitations on changing batteries on each discharge and also for disposing their raw materials,” says fellow researcher Leela Mohana Reddy Arava, now an assistant professor of mechanical engineering at Wayne State University.

Hexagonal boron nitride is not a conductor, and is not known to be an ionic conductor, so it was not expected to be of any obvious help to battery performance. But the Rice researchers felt a material that is chemically and mechanically resistant, even at very high temperatures, might give some stability to the electrolyte layer.

Boron nitride is a common component in ceramics for high-temperature applications and is fairly inert, so it doesn’t react with chemicals, expand or contract significantly, or react poorly to high temperatures. Additionally, use of the material eliminates the need for conventional plastic or polymer separators—membranes that keep a battery’s electrodes apart to prevent short circuits—which can shrink or melt at high temperatures.

Testing went better than the researchers anticipated. Though inert, the mix of h-BN, piperidinium-based ionic liquid and a lithium salt seemed to catalyze a better reaction from all the chemicals around it.

“It took almost two years to confirm that even though the boron nitride, which is a very simple formulation, is not expected to have any chemical reaction, it’s giving a positive contribution to the way the battery works,” says Rice postdoctoral researcher Hemtej Gullapalli. “It actually makes the electrolyte more stable in situations when you have high temperature and high voltages combined.”

"Pushing the boundaries of working temperature ranges is very interesting," adds Ajayan. "There is no commercial battery product that works above about 80 degrees Celsius. Our interest is to break this barrier and create stable batteries at twice this temperature limit or more.”