The increasing demand for efficient energy storage and conversion in a low-cost, safe and sustainable manner has attracted considerable research efforts. In particular, the research community investigates candidate materials suitable for energy applications such as Li – ion batteries and solid oxide fuel cells. As many key factors of energy systems depend on the properties of their constituent materials, the relation between the materials properties and their applicability in devices is important. The aim of the present thesis is to examine promising systems and provide a deeper understanding on their atomic-scale mechanisms with respect to their potential performance. Density Functional Theory (DFT) calculations are employed to investigate the defect chemistry, doping processes, electronic structure and ionic diffusion in Li2
and anatase TiO2
. Theoretical modelling provides advanced insights on the intrinsic processes and relative energetics, governed by point defects. Overall the dominant mechanisms are identified as well as the structural modifications under doping and / or mechanical strain. The presented results are expected to motivate further research through both theoretical and experimental techniques.