American nanoengineers have developed a combined electrolyte and separator for rechargeable lithium-ion batteries that supplies energy at usable voltages and in high temperatures.
The key ingredient of the composite is hexagonal boron nitride (h-BN), which is sometimes called 'white graphene'. Although not a conductor, h-BN is a common component in ceramics for high-temperature applications.
“It’s fairly inert, so it shouldn’t react with any chemicals, it won’t expand or contract a lot and the temperature isn’t a problem. That made it perfect,” said Marco-Túlio Rodrigues, a researcher at Rice University in Houston.
He said batteries made with the composite functioned perfectly in temperatures of 150 degrees Celsius for more than a month with negligible loss of efficiency. Test batteries consistently operated from room temperature to 150 C, setting one of the widest temperature ranges ever reported for such devices.
Rodrigues said batteries with the new electrolyte are geared more toward industrial and aerospace applications than cellphones. In particular, oil and gas companies require robust batteries to power sensors on wellheads.
“They put a lot of sensors around drill bits, which experience extreme temperatures,” he said. “It’s a real challenge to power these devices when they are thousands of feet downhole.”
Tests went better than the researchers anticipated. Though inert, the mix of h-BN, piperidinium-based ionic liquid and a lithium salt seemed to catalyse 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,” said fellow researcher Hemtej Gullapalli. “It actually makes the electrolyte more stable in situations when you have high temperature and high voltages combined.”
He noted all the electrolyte’s components are nonflammable. “It’s completely safe. If there’s a failure, it’s not going to catch fire,” Gullapalli said.
Leader of the research group Pulickel Ajayan said pushing the boundaries of working temperature ranges is very interesting.
“There is no commercial battery product that works above about 80 C. Our interest is to break this barrier and create stable batteries at twice this temperature limit or more,” said Ajayan.
Researchers Hemtej Gullapalli (standing) and Marco-Túlio Rodrigues are developing new battery technologies at Rice University. Photo: Jeff Fitlow/Rice University