In the pursuit of sustainable energy solutions, the development of advanced materials for energy storage has emerged as a cornerstone of scientific innovation. Among cathode materials for Li-ion batteries, Lithium Manganese Nickel Oxide (LMNO) crystals have garnered significant attention due to their exceptional electrochemical properties, making them promising candidates for high-performance batteries. To harness the full potential of LMNO crystals, a comprehensive understanding of their structural features is paramount. The core principles and nanoscale mechanisms of ion deintercalation in battery cathode materials remain poorly understood. One of the most challenging questions is how crystallographic defects (dislocations, small angle grain boundaries, etc) affect microscopic features of Li deintercalation and degradation mechanisms. Investigation of these fundamental properties is crucial since they govern the macroscopic characteristics of battery such as energy efficiency, capacity and stability. In recent years, X-ray techniques have emerged as a powerful tool for non-destructive, high-resolution imaging of materials, holding immense potential for advancing the understanding of battery materials.
This thesis aims at pushing the boundaries of both ex-situ and operando scanning X-ray nano-diffraction (SXDM) technique available at the European Synchrotron (ESRF-EBS) for the comprehensive characterization of LMNO crystals. SXDM utilizes a nano-focused X-ray beam (sub-100 nm) which is raster-scanned across a crystal, measuring diffraction signals at different positions of the sample. The obtained maps are extremely sensitive to local lattice strain and crystallographic orientation of the measured planes. Combining this novel technique with more conventional methods like micro-focused powder X-ray diffraction (PXRD) and electron microscopy allows to obtain a holistic understanding of the microstructure of LMNO crystals. This work focuses on the determination of complex internal structures, defects, phase transformations and degradation mechanisms within LMNO crystals, crucial for optimizing their performance in energy storage applications.
Operando SXDM measurements help to establish unusually persistent strain gradients inside the single crystals that suggest that the shape and size of solid solution domains are templated by lattice defects, which guide the entire delithiation process. Morphology, strain distribution, and tilt boundaries reveal that the (Ni2+/3+) and (Ni3+/4+) phase transitions proceed through different mechanisms, either a "phase-field" mechanism with more localized delithiation or a homogeneous "core-shell" transition. This offers a solution for reducing structural degradation in high voltage spinel active materials towards commercially useful durability. We also observe a structural plastic deformation of a crystal occurring in some particles during the phase transition that involves splitting of the crystal into domains tilted relative to each other. The structural distortion is required to accommodate two phases with different lattice parameters in a single crystal. This effect is only partially reversible resulting in a permanent structural change of the lattice after the transition phase. Study of aged particles confirms that the persistent defects present in the initial population of particles guide the long-term structural evolution of the material. The observed dynamic reorientation of the lattice leads to a continuous development of permanent low-angle tilt boundaries during consequent cycling. Furthermore, the lattice misorientations are highly heterogeneous over the population of measured particles, and only some particles seem to be affected over time. The deterioration of particles manifests in increased local orientational variance of lattice which is likely related to Mn atom loss. This results in modification of phase transformation mechanisms by delaying or preventing the reaction.

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