South SFV Today

Researchers at Stanford have made significant progress in understanding organic mixed ionic-electronic conductors (OMIECs), a promising class of materials for next-generation batteries and electronic devices. These soft, flexible polymer semiconductors have favorable electrochemical properties, but their molecular microstructure and electron movement remain poorly understood.

To address this gap, Stanford materials scientists used a special electron microscopic technique designed for "beam-sensitive" materials to gain insights into the structural workings of OMIECs. This technique is akin to how biomolecules are studied and helps reveal why OMIECs possess advantageous electrochemical properties.

"When OMIEC polymers are immersed in liquid electrolyte, they swell like an accordion yet maintain electronic functionality," said Alberto Salleo, the Hong Seh and Vivian W. M. Lim Professor in the School of Engineering at Stanford and senior author of the paper published in Nature Materials. "We’ve learned that the long molecular chains of the polymer material are able to stretch and gently curve, creating a continuous path even as the material swells by 300% with the electrolyte."

"The research represents a conceptual breakthrough in visualizing the microstructure of these materials," added Yael Tsarfati, a postdoctoral scholar in Salleo’s lab and first author of the paper. "Learning how a material works at a structural level is key to designing ever-better materials."

Salleo and Tsarfati's study marks the first use of cryo-electron microscopy (Cryo 4D-STEM) to image an OMIEC polymer soaked in an aqueous electrolyte while hosting electrical charges. This microscope uses powerful beams of electrons instead of light and requires extremely cold samples to prevent damage from electron exposure.

The dual stress from being soaked and electrically charged alters the polymer structure in complex ways. Traditional electron microscopes could not image these polymers without causing damage due to their softness. However, Cryo 4D-STEM allows researchers to see how these polymers maintain structural integrity while expanding.

The team discovered that OMIECs' soft liquid crystal polymer structure stretches and bends to form continuous electronic paths around bubbles of electrolyte within folded ribbons of polymer. Cryo 4D-STEM essentially freezes the material during study, keeping it in a vitrified state rather than turning solid like ice.

"The polymer forms a sort of gel that can bend and stretch," explained Salleo. "It can swell a lot, sometimes 300 percent, which would completely destroy the electronic properties of most materials. But in OMIECs, the electronic properties are still preserved."

Tsarfati noted that once swollen, polymer chains experience minimal structural change even during charging and discharging cycles. This results in efficient ion exchange with minimal strain on the material itself, making OMIECs appealing for future electronics.

"The polymers exhibit impressive resilience to physical changes and ion insertion compared to other materials we’ve studied," Tsarfati said, highlighting new research directions for their team.

Other Stanford co-authors include Colin Ophus; Luke Balhorn; Tyler Quill; lab manager Adam Marks; postdoctoral scholar Alexander Giovannitti; along with collaborators from Lawrence Berkeley National Laboratory; University of California, Berkeley; SLAC National Accelerator Laboratory; and University of Oxford.

Funding for this research came from sources including the U.S. Department of Energy, National Science Foundation, Toyota Research Institute, Zuckerman-CHE STEM Leadership Program. Part of this work was conducted at facilities such as Stanford Synchrotron Radiation Lightsource at SLAC National Accelerator Laboratory.

For further information or media inquiries: Jill Wu at jillwu@stanford.edu

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