Lattice dimension change. The (de)lithiation of NMC811 cathode materials lead to a dilatation and distortion of their atomic structures. The atomic lattice dimension change during charge and discharge is measured by operando synchrotron X-ray diffraction (XRD) experiments . Fig. shows the evolution of the lattice strain in the c and a direction of the NMC811 cathode obtained from XRD experiments . The symmetry of the layered oxide crystal structure indicates that the cathode exhibits the same lattice strain in the a and b direction. The a and b lattice parameters increase upon lithiation by maximum 2%. The c lattice parameter exhibits a nonmonotonic behaviour that it rapidly increases at the initial stage of discharge and gradually collapses as θ > 0.37. These dimensional changes are attributed to the change of Ni oxidation states, and the modification of the interlayer spacing between the O 2-planes.