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The use of Li-ion batteries is increasing rapidly. Understanding the processes behind active material aging helps to enhance the materials, and therefore, development of new in situmethods for structural studies is important. In addition, understanding the effect of different synthesis methods on the active material properties is necessary to optimize the material cycle life. In this work, the performance of LiCoO2doped with Mg during the lithiation step is compared to LiCoO2prepared using an Mg-doped Co3O4precursor. In situlaboratory-scale X-ray absorption near-edge spectroscopy is used to analyze the Co valence changes in LiCoO2to understand the electrochemical behavior of the investigated materials. The maximum reachable Co valence state is found to decrease upon aging, a small decrease indicating a good cycle-life, and this is attributed to the enhanced stacking order, better Mg distribution in the lattice, and fine primary particle size in the material. In the synthesis conditions used in this study, Mg doping during the lithiation step is shown to perform better compared to the precursor doping. Overlithiation is shown to reduce the electrochemical performance of nondoped and precursor-doped LiCoO2materials but not to affect the cyclability of lithiation-doped LiCoO2.