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Battery cathode materials are a key focus for improvement of Li-ion battery chemistries due their limits in terms of capacity and cost. During insertion and extraction of Li, many cathode materials exhibit structural changes, ranging from e.g. phase transitions to amorphization. Such structural rearrangements can be unfavourable for electrochemical performance due to the incompatibility between any intermediate phases formed. The crystalline structure can be readily studied using diffraction techniques such as X-ray and neutron diffraction, making them a versatile tool for identifying and understanding these structural changes. The work in this thesis focuses on utilizing diffraction to study equilibrium and non-equilibrium changes in the crystalline structure of the two cathode materials LiNi_0.5Mn_1.5O₄ and Li₂VO₂F. Both of these materials display interesting structural behaviour upon electrochemical cycling, some of it which can be linked to how the cations arrange in the structure. As such, the work presented here aims to elucidate further on the cationic structural features of these materials and how these may impact their use in batteries. 

For both LiNi_0.5Mn_1.5O₄ and Li₂VO₂F, the (re)arrangement of cations in the structure was found to play a crucial role for both equilibrium and non-equilibrium structural transitions occurring. In particular, the formation of phases that could be regarded as disadvantageous from a battery application perspective could be attributed to cationic diffusion. Due to the many structural similarities of transition metal oxide-based cathode materials, structural reorganization following similar mechanisms involving cationic rearrangement could be expected also in other materials similar to those studied here. As such, strategies for mitigating any unwanted redistribution of cations in the structure should be considered for improving the performance of this class of cathode materials.