In situ synchrotron diffraction experiments were performed to study the behaviour of the cathode material LiNi0.8Co0.2O2 at high cycle numbers. The battery-cell was cycled 140 times between 4.2 and 2.7V. The initial cycle and several cycles at proceeded degradation of the cell were investigated. In between the cell was cycled ex situ. The evolution of the lattice parameters during cycling was determined for the different stages of degradation by means of Rietveld refinement. Comparing the initial cycle of the cell with the higher cycle numbers, the mechanism of lithium insertion/extraction does not change significantly. But, the range in which the lattice parameters change during one cycle becomes smaller. This can be attributed to interface layer formation and to slightly less lithium exchange in the same voltage window.