A toothpaste-like composite containing hexagonal boron nitride makes an effective electrolyte and separator in lithium-ion batteries intended for high-temperature applications. Photo: Jeff Fitlow/Rice University.
A toothpaste-like composite containing hexagonal boron nitride makes an effective electrolyte and separator in lithium-ion batteries intended for high-temperature applications. Photo: Jeff Fitlow/Rice University.

Materials scientists at Rice University have developed a combined electrolyte and separator for rechargeable lithium-ion batteries that can supply energy at usable voltages and high temperatures. An essential component of the non-flammable, toothpaste-like composite is hexagonal boron nitride (h-BN), the atom-thin compound often called ‘white graphene’.

According to senior researcher Pulickel Ajayan, batteries made with the composite functioned perfectly well at temperatures of 150°C 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 working temperature ranges ever reported for such devices, the researchers said.

"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," said Marco-Túlio Rodrigues, a Rice graduate student. The results were reported in a paper in Advanced Energy Materials.

Last year, members of a team from Rice University and Wayne State University reported an electrolyte made primarily of common bentonite clay that operated at 120°C. This year the team confirmed its hunch that h-BN would be able to perform even better.

Rodrigues said batteries made with the new electrolyte would be geared more toward industrial and aerospace applications than cell phones. 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."

"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," said Rice alumnus and co-author Leela Mohana Reddy Arava, now an assistant professor of mechanical engineering at Wayne State.

Hexagonal boron nitride is not a conductor and is not known to be an ionic conductor. "So we didn't expect it to be any obvious help to battery performance," said Rodrigues. "But we thought a material that is chemically and mechanically resistant, even at very high temperatures, might give some stability to the electrolyte layer."

He added that boron nitride 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."

The material also eliminated the need for conventional plastic or polymer separators, which are membranes that keep a battery's electrodes apart to prevent short circuits. "They tend to shrink or melt at high temperatures," said Rice postdoctoral researcher Hemtej Gullapalli.

Tests 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," Gullapalli said. "It actually makes the electrolyte more stable in situations when you have high temperature and high voltages combined."

He noted that all the electrolyte's components are non-flammable. "It's completely safe. If there's a failure, it's not going to catch fire," he added.

"Our group has been interested in designing energy storage devices with expandable form factors and working conditions," Ajayan said. "We had previously designed paper and paintable battery concepts that change the fundamental way power delivery can be imagined. Similarly, 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."

This story is adapted from material from Rice University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.