Li-ion batteries have a huge potential for use in electrification of the transportation sector. The major challenge to be met is the limited energy storage capacity of the battery pack: both the amount of energy which can be stored within the space available in the vehicle (defining its range), and the aging of the individual battery cells (determining how long a whole pack can deliver sufficient energy and power to drive the vehicle). This thesis aims to increase our knowledge and understanding of structural changes induced by aging and usage of the Li-ion battery materials involved.

Aging processes have been studied in commercial-size Li-ion cells with two different chemistries. LiFePO₄/graphite cells were aged under different conditions, and thereafter examined at different points along the electrodes by post mortem characterisation using SEM, XPS, XRD and electrochemical characterization in half-cells. The results revealed large differences in degradation behaviour under different aging conditions and in different regions of the same cell. The aging of LiMn₂O₄-LiCoO₂/Li₄Ti₅O₁₂ cells was studied under two different aging conditions. Post mortem analysis revealed a high degree of Mn/Co mixing within individual particles of the LiMn₂O₄-LiCoO₂ composite electrode.

Structural changes induced by lithium insertion were studied in two negative electrode materials: in Li_0.5Ni_0.25TiOPO₄ using in situ XRD, and in Ni_0.5TiOPO₄ using EXAFS, XANES and HAXPES. It was shown that Li_0.5Ni_0.25TiOPO₄ lost most of its long-range-order during lithiation, and that both Ni and Ti were involved in the charge compensation mechanism during lithiation/delithiation of Ni_0.5TiOPO₄, with small clusters of metal-like Ni forming during lithiation.

Finally, in situ XRD studies were also made of the reaction pathways to form LiFeSO₄F from two sets of reactants: either FeSO₄·H₂O and LiF, or Li₂SO₄ and FeF₂. During the heat treatment, Li₂SO₄ and FeF₂ react to form FeSO₄·H₂O and LiF in a first step. In a second step LiFeSO₄F is formed. This underlines the importance of the structural similarities between LiFeSO₄F and FeSO₄·H₂O in the formation process of LiFeSO₄F.