Silicon is a promising material for negative electrodes in lithium-ion batteries. The degradation of silicon was investigated using thin films between 5 and 50 nm deposited on substrates of varying roughness. We used differential capacity plots to examine the degradation behavior of thin films during cycling. We found that the delamination of the thin films from the substrate depends on the surface roughness and film thickness. The delamination changes the morphology of the thin film and is characterized through the formation of Li15Si4. The morphology change is also accompanied by reduced coulombic efficiency and an increasing cell polarization. The transient occurrence of Li15Si4 is explained by the processes during cycling. It may be prevented by stress induced potential suppression which was observed in very thin films as the polarization increases with decreasing layer thickness.