In the subsequent discharge process, however, the Mg-doped material shows a high voltage plateau quite similar to the other two materials, but with a smaller capacity contribution (see Figure ). This indicates similar redox processes although they are not active to the same degree in all materials. The 'O bound to Mg' redox, however, appears to be largely irreversible. Therefore, the Mg-doped sample shows a high voltage plateau rather than a sloping profile on discharge. In the following cycles, the undoped and Sc-doped materials also show the sloping potential profiles (see Figure ), we believe that structural degradation and polarization should be responsible for the shape change of the potential profiles in the long-term cycling. Although the sloping potential contributes less to the cycling stability, it nevertheless has a greater effect on the average potential compared to potential profiles with plateaus in this region.