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Scanning method captures ionic movement in electrochemical materials

By Meg Marquardt September 26, 2016

The more scientists characterize the properties of electrochemical materials, such as those used in Li-ion batteries, fuel cells, and supercapacitors, the clearer it becomes that many materials are driven by variations on the nanoscale. As materials are designed to carry out more precise functions, understanding these variations becomes key to having a material that works at its highest capability. One aspect of materials that remains difficult to characterize is the movement of ions in a material. A recent study published in Journal of Applied Physics shows how a research team has now used thermo-ionic imaging in order to trace where and how ions move when a material is stressed.

“Important rate properties of electrochemical materials are often dominated by local variations in materials properties, which can often vary by orders of magnitude near surfaces or at solid-solid interfaces,” says Stuart Adler, professor of chemical engineering at the University of Washington and co-author on the article. Rate properties include surface reactions, interfacial charge transfers, and surface diffusion of ions.

Because electrochemistry happens on the nanoscale, it is often difficult to understand how performance is linked to the structure or properties of a given material. Methods often look at a material’s response in terms of electrochemical strain. However, dynamic local strain-based methods are not always useful for precise measurements. For example, Adler says, electrochemical strain microscopy (ESM) creates strain by altering the probe tip’s conductive potential. That potential is so sensitive that other electronic sources, such as the global voltage applied to the device as a whole, can interfere and make it difficult to interpret direct responses to strain.

The research team, led by Jiangyu Li, a professor of mechanical engineering at the University of Washington, took a different approach. The researchers created a probe that uses thermal perturbations instead of electrical ones. They named the method scanning thermo-ionic microscopy, or STIM.

A STIM probe creates temperature oscillations as it scans a material. Unlike its ESM counterpart, the tip is not used to apply electric fields. Rather, it images thermally induced Vegard strain. The Vegard strain measures the shifts in a material’s lattice as defects increase or decrease in concentration, an indication that ions and electronic species are flowing in and out of certain areas. This allows measurement of ion movement separate from other types of electrostatic interactions, like responses to global voltage. STIM also provides more localized information, allowing researchers to connect the performance of a material to its structure and properties.

The probe tip itself is mounted on a resistive heater. It emits sinusoidal current—the tip alternatively heats and cools the material locally as it passes over. The material is heated at twice the frequency of the applied current. Because the heating and cooling is targeted to a small area, the surrounding material does not expand or contract. This causes a high-stress field right beneath the tip, driving ionic and electronic defects right below the tip. And the defects occur at a faster rate than the thermal expansion itself, making it easy to distinguish ionic response from thermomechanical response.

To test the probe, the researchers studied samples of Sm-doped Ceria and LiFePO4. Ceria is an ideal candidate to test because it has been predicted that its nanocrystal form has high conductivity due to mobile electrons on the surface and at grain boundaries. STIM was able to image much higher responses at the grain boundaries, supporting the mobile electron theory.

STIM also showed that response level changed based on the type of boundary. This may be an indication that mobility is dependent on grain type and size.

“This is a novel scanning technique,” says AndreaCentrone, a nanoscale scientist at the National Institute of Standards and Technology. “It’s quite impressive that it can measure electrochemistry and topography. These are usually difficult characteristics to read in materials. So this technique is quite promising.”

Though promising, Adler notes there are still challenges in the method, notably the fact that the heating and cooling cycle takes microseconds, which may be too slow to see all electrochemical responses, some of which occur at faster timescales.

“However, electrochemical processes in the solid state often involve significant transport distances and oxidation/reduction of the material, which have dynamic response times of fractions of a millisecond to seconds,” Adler says. “These are timescales for which STIM is ideally suited.”

Read the abstract in Journal of Applied Physics.