We investigate materials that can serve as electrode materials for rechargeable Li-ion batteries. The aim is to clarify the electrochemical reaction pathways that occur during insertion and extrusion of Li. Besides standard battery testing we use solid-state NMR spectroscopy and diffraction methods to study the changes in the local structure of the host materials and the mobility of the Li ions. New in-situ methods that are applied during charging and discharging of the batteries include in-situ NMR, in-situ XRD/XAS, and in-situ Mössbauer spectroscopy. Finally the results of these fundamental studies are used for optimization of the battery performance.