Lithium-intercalating materials such as graphite are of great interest, especially for application in lithium-ion batteries. In this work we present an investigation of the electrochemical performance of mesocarbon microbeads (MCMB) modified with copper to reveal the basic electrochemical mechanisms. Copper-modified graphite is known to have better long-term cycling behavior as well as higher capacity compared to the pristine material. Several reasons for these effects were postulated but not proven. Solid-state nuclear magnetic resonance (NMR) spectroscopy provides structural and dynamic information on lithium in ionic conductors. To elucidate the changes in structure and dynamics for the pristine and the modified material, we have employed multi-nuclear solid-state NMR spectroscopy as well as 7Li spin-lattice relaxation measurements and were able to clarify some reasons for the improved characteristics of copper-modified graphite compared to the pristine material, which include increased solid-electrolyte interface (SEI) formation, a facilitated diffusion of lithium ions through the SEI, and reduced moisture.