Seeing how batteries degrade

Materials World magazine
,
4 Mar 2020

Combining imaging techniques has helped researchers see activity inside working battery cells for the first time, which could improve manufacturing. Ceri Jones reports.

Existing imaging technologies have been combined to produce a picture of what happens inside a lithium battery while it is working. Normally, batteries are cut open after being discharged and examined to understand how they function and degrade, but this new technique allows them to be observed while operating to better understand the processes involved.

Two tomography tools and mathematics were used to achieve this – neutron imaging, X-ray imaging and data analysis. Institut Laue-Langevin, France, Scientist, Dr Alessandro Tengattini, explained that while X-ray images are sensitive to density and the proportional atomic number of individual atoms, neutron imaging reveals what X-rays cannot, including hydrogen and lithium elements. He said that using these highly complimentary imaging tools in sequence has enabled the team to build a detailed picture of the interior workings of a battery, which could improve their design and, therefore, safety.

Inside look

‘The neutron information regards the lithium, specifically, the movement of the electrolytes within the batteries, while the X-rays are predominantly about variations in density, and eventual damages in some cases,’ Tengattini told Materials World. ‘In this case, we used it as a 4D dataset called spatial information, but because we acquired a number of images across time as we discharged the batteries, we have the dimension of time as well, hence it is 4D.’

As the team used a commercially available CR2 battery, which is cylindrical, it required them to employ virtual unrolling techniques to analyse the lithium layer, which is tightly coiled inside the casing. To do this, they wrote an algorithm that could re-order the key points and digitally map them to display the information as a strip instead of a scroll. However, Tengattini said this additional step would not be unnecessary if the technique was applied to another battery format, such as a flat button battery.

‘We tried to follow the evolution of the lithium in the battery and there were some notable cases where we observed acceleration or isolations of some regions of the batteries, because of an accelerated consumption of the lithium,’ said Tengattini. ‘The isolation of some areas of the batteries, even in its earliest stages, was somewhat surprising.’

Making batteries better

By recording electrochemistry issues such as inconsistent lithium loading and irregular degrading during discharge, the team was able to gain insight into causes of battery failures and how these could be overcome through improved design and manufacturing.

‘In general, we gathered a large amount of data regarding the electrochemistry, which is an essential element for the modelling part to try to improve the feasibility and safety, and commercial viability of these batteries,’ said Tengattini.

‘A better understanding is hopefully reflected in a better design. The more we understand the microscopic sources at fault, we can avoid faults and make them more resilient and to have a better performance, but also make them safer.’

Future battery analysis

Each tomography point was taken approximately 30 minutes apart, so while it is not a real-time representation, Tengattini said it ‘comes quite close’ to capturing the full process without losing any intermediate steps. To advance the process further, the team is working on closing this time gap by running the imaging techniques concurrently rather than sequentially, as at present.

‘There is much missing to get a complete understanding of the finer points of batteries,’ said Tengattini. ‘We are hoping in the future to use one of the instruments, or the two of them at the same time, to acquire a truly five-dimensional dataset in which we not only have the three dimensions plus time, but also the information from both the X-rays and the neutrons for the same exact voxel and the same exact point in time and space.’

Read the full paper, 4D imaging of lithium-batteries using correlative neutron and X-ray tomography with a virtual unrolling technique, published in Nature Communications, here: go.nature.com/3cdnOLg