Structure changes of LiNi0.5Mn0.5O2 were detected at the electrode/electrolyte interface of lithium cell using synchrotron X-ray scattering and two-dimensional model electrodes. The electrodes were constructed by an epitaxial film of LiNi0.5Mn0.5O2 synthesized by pulsed laser deposition (PLD) method. The orientation of the film depends on the substrate plane; the 2D layer of LiNi0.5Mn0.5O2 is parallel to the SrTiO3(110) substrate ((110) LiNi0.5Mn0.5O2//(110) SrTiO3), while the 2D layer is perpendicular to the SrTiO3(111) substrate ((003) LiNi0.5Mn0.5O2//(111) SrTiO3). The in situ X-ray diffraction of LiNi0.5Mn0.5O2(003) confirmed three-dimensional lithium diffusion through the two-dimensional transition meal layers. The intercalation reaction of LiNi0.5Mn0.5O2 will be discussed.