In situ studies of rechargeable batteries using neutron and hard X-ray diffraction

Global battery demand is set to increase tenfold within the next decade. To achieve the performance gains in power density, energy density and long-term cyclability demanded by future applications, it is crucial to continue to improve our understanding of fundamental degradation mechanisms in batteries. The work described in this thesis seeks to broaden the search for improvements by unifying characterisation techniques primarily used for the material-level study of structural degradation with the study of commercial-sized cells under realistic cycling conditions. Neutrons and high-energy X-rays offer a unique way of quantifying spatially resolved structural changes on the atomic level in real-time.

Macro-scale inhomogeneity in Li diffusion and Li inventory has been shown to be an important factor in multilayer pouch cells and wound cylindrical cells. By using high-energy X-ray diffraction with short exposure times, detailed operando 1D and 2D map sequences of Li inventory in cathodes and anodes have been made. Furthermore, Li inhomogeneity has been found to be different in cathodes and anodes and depends heavily on the degradation state, state of charge, Crate, and relaxation steps.

Neutron diffraction has been valuable in elucidating lithiation mechanisms in blended silicongraphite anodes. 5 Ah multilayer pouch cells with highnickel NMC 811 cathodes and Si-Gr anodes were manufactured to investigate the interplay between those two state-of-the-art materials, where degradation mechanisms are nevertheless not fully understood. Studying these materials in large cells made it possible to investigate the effect the increased volume expansion of Si-Gr has on Li inhomogeneity and thickness variations across the pouch cells.

Another avenue to increase specific energy density is to increase the loading of individual electrodes. Maintaining sufficient power capabilities in 4.5 mAh/cm² electrode loadings is very challenging, so a thorough study of the drying mechanisms necessary to achieve this was done. To this effect, 63 electrodes were made on a pilot scale at a continuous coating line at Cidetec. The electrochemical performance of electrodes resulting from those 63 different drying configurations was studied, with a view to in the future coupling these data with X-ray computed micro tomography measurements to characterise the resulting microstructure of those electrodes and the effect different drying parameters have on particle aggregation, binder migration and pore formation.

Next-generation rechargeable Zn-air batteries were also investigated. This was done using time-resolved X-ray diffraction with continuous vertical scanning. A big challenge for achieving viable rechargeable Zn-air batteries is improving the reversibility of phase transformations in Zn/ZnO anodes. Here it was possible to quantify the mechanisms leading to morphology changes and internal material migrations within the anode.