Batteries with solid-state electrolyte materials, rather than liquids or gels, are a goal of technology developers because of their advantages for battery safety, durability and reduced charge times. Certain ceramic-based electrolytes have shown the ability to transport ions quickly, but are brittle and difficult to process into working batteries. Polymer-based electrolytes overcome these challenges, but exhibit poor ionic conductivity.
Now, researchers at the Oak Ridge National Laboratory (ORNL; Oak Ridge, Tenn.; www.ornl.gov) have found a path to avoiding that tradeoff by boosting the ionic conductivity of polymer electrolytes using zwitterions — neutral molecules that have both positive and negative charges simultaneously at different points on their structure.
Beginning with the polymer poly-(dimethyl aminoethyl acrylate), the ORNL team functionalized the polymer backbone with a precisely tuned amount of zwitterionic molecules. The zwitterions increase local polarity, but maintain a zero charge on the macromolecule overall. The presence of a specific amount of zwitterions resulted in a self-organizing behavior — specifically the formation of channel-like structures that allowed the ion transport to happen with minimal resistance. When doped with a lithium salt, these channels “provide a high-mobility path for the ions to move through,” according to Tomonori Saito, an distinguished researcher in the ORNL Chemical Sciences Division.
The ORNL researchers developed careful chemical processes that allowed them to tailor the number of zwitterionic groups attached to the polymer backbone. After testing various levels, the team found that functionalizing 80% of the polymer units with zwitterionic groups achieved room-temperature ion transport comparable to the best ceramic electrolyte materials.
While solid-state batteries are a clear application for this new type of solid electrolyte, the ORNL project has relevance to other energy technologies that rely on effective ion transport, such as flow batteries, fuel cells, grid-level energy storage and others applications.
The research was described in a recent issue of Materials Today. The research team plans additional investigations into the fundamental mechanisms that enable the superionic nature of the polymer.