Lithium-ion batteries power nearly everything, from phones and laptops to electric vehicles. But the liquid electrolytes inside them are flammable, and that fundamental chemistry has been behind fires in e-bikes, EVs, and consumer electronics for years. Replacing those liquids with safer solid alternatives has been one of battery science’s longest-running challenges.
Polymeric ionic liquids, or PILs, are one of the most promising candidates. They do not catch fire easily and can conduct lithium ions at room temperature. But they have a persistent problem: the versions that conduct ions well tend to be too soft to hold their shape in a practical battery, while the versions that are mechanically strong tend to conduct ions poorly. It has been a frustrating trade-off that has kept these materials stuck in the lab.
A research team led by Gila Stein has now identified why, and developed a way around it. Their study, published in the journal Macromolecules, reveals that when PIL materials are engineered for structural strength using block copolymers, tiny defects form in their nanoscale architecture. These defects act as dead ends that trap ions, dramatically reducing overall conductivity. The finding explains a performance gap that has puzzled researchers working on solid-state battery electrolytes.
Block copolymers are materials made by connecting two different polymer chains together. Because the two blocks have different chemical properties, they naturally separate at the nanoscale and self-assemble into ordered structures, similar to how oil and water separate but at a molecular level. In battery applications, one block conducts ions while the other provides mechanical rigidity. The idea is to get both properties in a single material.
The problem, the team found, is that this self-assembly process is imperfect. The resulting nanostructures contain defects, places where the ordered layers are disrupted, creating pockets that ions can enter but not easily exit. These dead ends reduce the effective conductivity of the material, even though the individual components should theoretically conduct ions well.
“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.”
To test this hypothesis, the team focused on materials that form lamellar structures, alternating flat sheets of the two polymer blocks. This geometry gave them a clearer view of how ions move through the material and where they get stuck. By systematically varying the chemistry and studying the resulting structures, they confirmed that dead-end defects were the primary bottleneck.
“Even a small change in chemistry can significantly impact how the material organizes and behaves,” said Samuel Adotey, a researcher on the team.
Armed with this understanding, the team developed design guidelines for reducing dead-end defects in PIL block copolymers. Their approach improved ionic conductivity by up to an order of magnitude, a tenfold increase, while maintaining the material’s structural stability. That is a significant step because it breaks the conductivity-versus-strength trade-off that has limited these materials.
The research also has implications beyond batteries. PIL block copolymers are being explored for thin-film electronics and actuators, where the same combination of ionic conductivity and mechanical integrity is needed.
The study does not deliver a finished battery product. These are still laboratory-scale materials, and the path from a materials science publication to a commercial cell involves years of engineering, scaling, and testing. But the identification of a specific, fixable structural flaw, and the demonstration that fixing it produces a tenfold performance improvement, gives battery researchers a concrete design principle to work with rather than another incremental tweak.
