Organic mixed ion-electron conductors (OZIEP) is among the most promising materials for new generation batteries and electronic devices. These flexible, soft polymer semiconductors exhibit excellent electrochemical properties. However, their molecular microstructures and the mechanisms by which electrons move through them are not fully understood; This is an important knowledge gap that needs to be filled before OMIECs can be commercialized.

To fill this gap, Stanford materials scientists recently applied a specialized electron microscopy technique that works with so-called “radiation-sensitive” soft materials, such as biomolecules, to get a clearer picture of the structural interior of OMIECs and why they have such favorable electrochemical properties. features.

Like water in a car battery, a liquid electrolyte leaks between the OMIEC polymer layers. Electrolyte is a medium in which ions move between positive and negative poles, creating electric current.

“OMIEC polymers swell like an accordion when immersed in a liquid electrolyte but retain their electronic functions. Hong Seh and Vivian WM Lim professor Alberto Salleo said, “We learned that the long molecular chains of the polymer material can stretch and bend slightly, forming a continuous path even when the material is 300% swollen in the electrolyte.” ” said the School of Engineering and senior author of a paper published in the journal Nature Materials.

“The research represents a conceptual breakthrough in visualizing the microstructure of these materials. Previously we could only develop theory, now we can see what makes OMIECs work so well,” said Salleo, a graduate student in Salleo’s lab who performed most of the electron microscopic observations on the paper. Written by Yael Tsarfati. “Learning how a material works on a structural level is the key to creating more perfect materials.”

It’s a difficult process

Salleo and Tsarfati worked on this study for three years. They are the first to use cryo-electron microscopy (Cryo 4D-STEM) to image an OMIEC polymer impregnated with an aqueous electrolyte while containing electrical charges. This type of microscope uses powerful electron beams instead of light to image and requires the sample to be very cold to prevent the electrons from damaging the material.

Salleo says the dual stress from wetting and electrical charge leads to complex but significant changes in the polymer’s structure. Visualizing how the performance of a polymer is maintained despite these stresses is a puzzle of interest to the community. However, imaging these polymers using conventional electron microscopes has been quite challenging.

If OMIECs were solid semiconductors, researchers would quickly turn to electron microscopy to examine their crystal structures. But OMIECs are so soft that the powerful electron beams used to illuminate their internal structure damage it during observation.

Using this new microscopy technique, Salleo and Tsarfati can now see how a soft, plastic polymer maintains its structural integrity when expanded. The team now believes that the soft liquid crystal polymer structure of OMIECs stretches and bends, creating a continuous path of electrons around the electrolyte bubbles that form between the folded strands of the polymer.

soft touch

Cryo 4D-STEM essentially freezes the material during operation. The electrolyte does not solidify like water turns into ice. Instead, it transitions to a different, glassy state, allowing Salleo and his team to peer into the microstructure at work.

“The polymer forms a kind of gel that can be bent and stretched,” explains Salleo. “It can swell so much, sometimes as much as 300 percent, that it completely destroys the electronic properties of most materials. However, electronic features are still preserved in OMIECs.”

Tsarfati notes that after swelling, the polymer chains undergo minimal structural changes even during charging and discharging. This leads to a more efficient ion exchange with minimal charge on the material itself, making OMIECs electronically attractive.

“The polymers show impressive resistance to physical changes and ion entry, compared to other materials we have studied, a desirable property for future electronics,” Tsarfati said, pointing to new directions for the team’s research. he added.