What’s ‘Blocking’ PIL Block Copolymers

Many important technologies, from handheld phones to medical devices and transportation vehicles, rely on rechargeable batteries. Modern top-of-the-line rechargeable batteries transport lithium ions between electrodes to store and deliver energy.

However, as many viral videos have demonstrated, the electrolytes in batteries can pose a significant safety risk, causing overheated laptops, electric bikes, cars, and more to catch fire.

Polymeric ionic liquids (PILs), a nonflammable type of electrolytic (ion-conductive) polymer, offer great promise for lithium ion and lithium metal batteries—as well as other technologies, including thin-film transistors and actuators.

Unfortunately, the PILs that conduct ions best at room temperature also tend to be very soft, reducing the materials’ functionality and making them a challenge to use in devices.

“One strategy to address this mechanical weakness is to connect the PIL to a second, rigid polymer, creating what is called a block copolymer,” said Department of Chemical and Biomolecular Engineering Professor Gila Stein. “As the distinct blocks do not like to mix, this block copolymer will spontaneously self-assemble into ordered nanostructures.”

Those ordered nanostructures—each a billionth of a meter long—are responsible for many of the PIL block copolymers’ useful properties, including ionic transport and mechanical performance.

“Block copolymers are a fascinating blend of chemistry and self-organization,” said Samuel Adotey, a PhD student in Stein’s lab. “Even a small change in chemistry can significantly impact how the material organizes and behaves. That constant potential for discovery makes studying block copolymers extremely engaging.”

However, the block copolymers’ ionic conductivity—a measure of how easily ions can move through the material—can be one to two orders of magnitude lower than the conductivity of otherwise comparable PIL homopolymers (single-material polymers).

To understand why, Stein, Adotey, and Oak Ridge National Laboratory (ORNL) staff scientist Yangyang Wang secured a $170,000 grant from the UT-ORNL Innovation Institute (UTORII) in 2023. Over the last two years, the team created a series of block copolymers and examined how changes to their design influenced their self-assembly and resulting ionic conductivity.

“The self-assembly process has a lot of imperfections,” said Stein. “We thought it was likely that some of these defects were acting like ‘dead ends’ and blocking the movement of ions out of the material.”

The team published its results last fall in Macromolecules, the leading journal for fundamental polymer science.

“Getting my work published in Macromolecules is a significant milestone for me,” said Adotey, the lead author on the paper. “I’ve admired this journal since the beginning of my PhD, so seeing my first paper published there feels surreal and incredibly rewarding. Moments like this make all the long hours spent in the lab feel worthwhile and motivate me to continue pushing forward with my research.”

Searching for ‘Dead Ends’

Stein and Adotey decided to investigate their theory using PIL block copolymers that self-assemble into clearly defined, layered structures known as lamellar grains.

“Unlike irregular or disordered domains, this system forms alternating sheets that clearly demonstrate how the ionic components influence spacing, mobility, and structural stability of the block copolymer,” Adotey explained. “This organized morphology provides a better foundation for understanding how ionic functionality impacts the behavior of the material.”

Stein and Adotey created more than a dozen different PIL-containing materials, including both materials that self-assemble with the desirable lamellar grains and some that create asymmetrical structures.

With Wang’s help, they characterized these block copolymers’ material properties and ion dynamics across a wide range of temperatures and pressures.

The team confirmed that the low conductivity of the lamellar-grained PIL materials is indeed due to the hypothesized ‘dead ends.’ Even more importantly, the researchers developed molecular design guidelines that can be used to control the types of defects that form in nanostructured electrolytes, making it possible to increase block copolymers’ ionic conductivity by an order of magnitude or more.

“This research enhances our understanding of how to design PIL block copolymers that retain their nanoscale structure under practical conditions,” Adotey said. “With appropriate molecular design, these materials could be customized for next-generation battery systems and other technologies that require a combination of ionic conductivity and mechanical strength.”

Contact

Izzie Gall (egall4@utk.edu)