Researchers from the University of Chicago and their colleagues have figured out how the electron microscope - an instrument that makes it possible to see materials at the level of individual atoms - affects sodium batteries. The work was published in the journal Microscopy and Microanalysis. The main conclusion: a powerful electron beam not only images the sample but also changes it, and this must be taken into account to avoid erroneous results.
"The electron microscope lets us look at the boundaries between battery components and understand what is going wrong inside," explains James Burrow, deputy director of the ESRA (Energy Storage Research Alliance) at the U.S. Department of Energy. According to him, electrons interact with the sample and simultaneously damage it - and this effect is often overlooked.
Thomas Marchese, a graduate student at the Pritzker School of Molecular Engineering (UChicago PME) and the first author of the paper, compares the radiation dose to the water pressure in a hose: the stronger the "pressure," the prettier the image, but the higher the risk of damaging the material. "We can get a beautiful image, but if the pressure is too high, the sample is no longer what we wanted to study, and the conclusions may be wrong," he says.
The focus is on sodium batteries. Lithium-ion batteries are powerful, but lithium and minerals such as copper and cobalt are limited in resources. That is why scientists are looking for alternatives, including sodium and sulfur. The study was part of a cooperation agreement between UChicago PME, Argonne National Laboratory and Thermo Fisher Scientific.
Through joint efforts, a modern electron microscope with efficient detectors has been created to study the structure and chemical composition of materials. Usually, most samples require a dose of tens or hundreds of thousands of electrons per square angstrom (an angstrom is a unit of length equal to one ten-millionth of a millimeter). New detectors make it possible to obtain a lot of information already at 10-50 electrons per square angstrom - before the material begins to change.
During the experiments, the team transferred samples into the microscope at room temperature in an inert atmosphere to avoid unwanted reactions. It turned out that cooling, which is often used to protect against radiation damage, does not help in this case - meaning the damage mechanism is different. Electrostatic charge was named the dominant one. The side effect turned out to be useful: the charge "drove" sodium ions out of the conductive framework, and by scanning only part of the sample with the beam, the scientists were able to grow crystals of metallic sodium on the opposite side.
"We see a fresh boundary between the solid electrolyte and sodium, which reflects the first deposition in anode-free batteries," Marchese notes. This confirms that the so-called SEI - interphase film (a layer of side-reaction products at the electrode-electrolyte boundary) - does not form between the components, and that is precisely why the system operates for a long time without capacity loss. However, the authors emphasize, this concerns laboratory samples, not finished devices. The next task is to study the crystal structure of the sodium growing at the boundary, which could be useful in developing new fuel cells.