Doping is a popular method to modify layered transition metal oxides used as positive electrode materials in Li-ion batteries. By replacing a small amount of the metals in the structure with other elements, for example the crystal structure can be stabilized, the transition metal dissolution prevented or the Li-ion diffusion through the material enhanced, the effect depending on the used dopant. Simultaneously, the demand for Li-ion batteries and the materials used in them are growing rapidly due to electrification of the transportation. With it, the amount of generated battery waste is increasing as well. To answer the need for raw materials and to protect the environment from the harmful chemicals in the Li-ion batteries, the recycling of the Li-ion batteries and their components should be investigated, and the current processes enhanced. While processes to recycle Li-ion batteries do exist, they are not sufficient, and currently a large part of the batteries end up in landfills. In addition, the batteries are often recycled as a whole, with casing, electrodes, and all other components together, which easily leads to mixing of impurities with the active electrode materials. In this paper, the effect of dopants and impurities on the electrochemical performance of recycled LiCoO2 materials is investigated. A new method to electrochemically re-lithiate aged LiCoO2 electrodes without removing the active material from the current collector is presented, and the doping is observed to improve the re-cycling of the electrode compared to non-doped material. In addition, the use of doped and non-doped Co3O4 precursors to synthesize Mg-doped LiCoO2 is investigated to understand how a dopant or impurity might affect the electrochemical performance of recycled layered transition metal oxides containing impurities due to the elements mixing during the recycling process. The Mg doping, which usually affects LiCoO2 favorably, is observed to decrease the capacity retention when it is added to the precursor independently. However, when the Mg doping is added simultaneously with the Li precursor as is common, it improves the LiCoO2 performance as expected. This suggests that the recycling of layered transition metal oxides has to be done very carefully, and even the typically performance enhancing elements should be removed as impurities during the recycling processes. On the other, if the original structure of the LiCoO2 is not lost during the recycling, the doping has a positive effect on the reusability of the material.