Reconstructed tomograms from neutron and X-ray computed tomography. Clearly visible in the X-ray image is the nickel current-collecting mesh, which appears brighter than the active electrode material. Image: UCL, ILL, HZB.
Reconstructed tomograms from neutron and X-ray computed tomography. Clearly visible in the X-ray image is the nickel current-collecting mesh, which appears brighter than the active electrode material. Image: UCL, ILL, HZB.

Lithium batteries are found everywhere: they power smartphones, laptops, and electric bicycles and cars by storing energy in a very small space. This compact design is usually achieved by winding the thin sandwich of battery electrodes up into a cylindrical form, thereby ensuring they have large surfaces to facilitate high capacity and rapid charging.

An international team of researchers from the Helmholtz-Zentrum Berlin (HZB) in Germany and University College London in the UK has now investigated the surfaces of these wound electrodes during charging and discharging. To do this, they used, for the first time, a combination of two complementary tomography methods: X-ray tomography and neutron tomography. They report their findings in a paper in Nature Communications.

The researchers used X-ray tomography at the European Synchrotron Radiation Facility (ESRF) in Grenoble, France, to analyze the microstructure of the electrodes, and to detect deformations and discontinuities that develop during the charging cycles.

"Neutron tomography, on the other hand, made it possible to directly observe the migration of lithium ions and also to determine how the distribution of the electrolyte in the battery cell changes over time," explains Ingo Manke, a tomography expert at HZB.

The neutron tomography data were obtained mainly at the HZB BER II neutron source at the CONRAD instrument, one of the best tomography stations in the world. Additional data were obtained at the neutron source of the Institut Laue-Langevin (ILL) in Grenoble, where a first neutron imaging station is currently being set up with help from experts at HZB. Following the shutdown of BER II in December 2019, the CONRAD instrument will be transferred to ILL so that it will be available for future research.

The instrument at NeXT-Grenoble is able to simultaneously acquire x-ray and neutron tomography, and was essential to the process of obtaining the images featured in this article. Dr. Alessandro Tengattini, an ILL instrument scientist, had this to say: "We're demanding more power from our consumer electronics all the time. To make them more efficient, and also safe, we need to understand the minor fluctuations occurring inside the batteries throughout their lifetime. The electro-unrolling technique has enabled us to analyse the inside of batteries, while they are in use, to identify such minuscule fluctuations to almost the micrometre. It's hard to analyse Lithium with x-rays because it is a light-weight element, but in combination with high-flux neutrons provided at the Institut Laue-Langevin (ILL) researchers have been able to learn about the electro-chemical and mechanical properties at play simultaneously while these lithium-ion batteries are in use.”

A new mathematical method developed at the Zuse-Institut in Berlin, Germany, then allowed the physicists to virtually unwind the battery electrodes, as the cylindrical windings of the battery are difficult to examine directly. Only after mathematical analysis and the virtual unwinding could the researchers draw conclusions about the processes occurring at the individual sections of the electrodes.

"The algorithm was originally meant for virtually unrolling papyrus scrolls," explains Manke. "But it can also be used to find out exactly what happens in compact densely wound batteries."

"This is the first time we have applied the algorithm to a typical commercially available lithium battery," adds Tobias Arlt from HZB. "We modified and improved the algorithm in several feedback steps in collaboration with computer scientists of the Zuse-Institut."

Characteristic problems with wound batteries could be investigated using this method. For example, the researchers found that the inner windings exhibited completely different electrochemical activity (and thus lithium capacity) to the outer windings. In addition, the upper and lower parts of the battery each behaved very differently.

The neutron data also showed areas that experienced a lack of electrolyte, severely limiting the functioning of the respective electrode section. It also revealed that the anode is not equally well loaded and unloaded with lithium everywhere.

"The process we have developed gives us a unique tool for looking inside a battery during operation and analyzing where and why performance losses occur. This allows us to develop specific strategies for improving the design of wound batteries," concludes Manke.

This story is adapted from material from Helmholtz-Zentrum Berlin, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.