Using a new mathematical method, it was possible to virtually unwind electrodes that had been wound into the form of a compact cylinder, and thus actually observe the processes on the surfaces of the electrodes.
Lithium batteries are found everywhere: they power smart phones, 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 into a cylindrical form. This is because the electrodes must nevertheless have large surfaces to facilitate high capacity and rapid charging.
X-ray and neutron-tomography combined
An international team of researchers from the Helmholtz-Zentrum Berlin and University College London has now investigated the electrode surfaces during charging and discharging using for the first time a combination of two complementary tomography methods. Using data provided by the HZB BER II and Institut Laue-Langevin (ILL) neutron sources and employing X-ray tomography at the European Synchrotron Radiation Facility (ESRF) in Grenoble, they were able to analyse the microstructure of the electrodes and detect deformations and discontinuities that develop during the charging cycles.
“Neutron tomography 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 Dr. Ingo Manke, 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 worldwide.
Alessandro Tengattini, instrument scientist at NeXT-Grenoble, a novel imaging station at Institut Laue-Langevin (ILL) said:
"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.”
The NeXT-Grenoble instrument allows for the simultaneous acquisition of x-ray and neutron tomography, developed in collaboration between the Institut Laue-Langevin and the Université Grenoble-Alpes in the last few years. An upgraded version of the instrument is currently under development with an additional partnership with the Helmholtz-Zentrum Berlin.
Virtual unwinding the battery
A new mathematical method developed at the Zuse-Institut in Berlin then enabled physicists to virtually unwind the battery electrodes – because the cylindrical windings of the battery are difficult to examine quantitatively. Only after mathematical analysis and the virtual unwinding could conclusions be drawn about processes at the individual sections of the winding.
“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.”
Dr. Tobias Arlt of HZB continues: “This is the first time we have applied the algorithm to a typical commercially available lithium battery. We modified and improved the algorithm in several feedback steps in collaboration with computer scientists of the Zuse-Institut.”
Characteristic problems with wound batteries were able to be investigated using this method. For example, the inner windings exhibited completely different electrochemical activity (and thus Lithium capacity) than the outer windings. In addition, the upper and lower parts of the battery each behaved very differently. The neutron data also showed areas where a lack of electrolyte developed, which severely limited the functioning of the respective electrode section. It could also be shown 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 analysing where and why performance losses occur. This allows us to develop specific strategies for improving the design of wound batteries”, concludes Manke.
Tengattini adds “as with all finite resources, we can expect the 'electric revolution' to lead to greater demand from lithium-ion batteries, for less resource. To meet this we must understand what is happening at the heart of these cells."
Images: © T.Arlt, I. MankeHZB, R. ZiescheUCL