Cylindrical lithium-ion cells with a lithium–nickel–cobalt–aluminum oxide (LiNi0.8Co0.15Al0.05O2) and a non-graphitizable carbon (hard carbon) as the positive and negative electrodes, respectively, were degraded by the storage tests with 50% state-of-charge (SOC). The degraded cells were disassembled and positive electrodes obtained were examined. The cation distribution of lithium and nickel in the positive electrode was clarified using neutron and synchrotron X-ray diffraction measurements. The degree of lithium and nickel disordering varied with the storage condition. X-ray absorption near-edge structure (XANES) analysis demonstrated that the structural change was mainly located near the surface of the positive electrode and the valence state of Ni and Co ions did not change with the storage test. The disordering of the cations near the surface was closely related to the power fade of the cell. The storage condition is an important factor in the power fade of lithium-ion cells.