Surface and bulk structural changes of LiNi0.5Mn0.5O2 were investigated during electrochemical reaction using synchrotron X-ray scattering and a restricted reaction plane consisting of two-dimensional epitaxial-film electrodes. The changes in bulk structure confirmed lithium diffusion through the (110) surface, which was perpendicular to the two-dimensional (2D) edges of the layered structure. No (de) intercalation reaction was observed through the (003) surface at voltages of 3.0-5.0 V. However, intercalation did proceed through the (003) plane below 3.0 V, indicating unusual three-dimensional (3D) lithium diffusion in the over-lithiated 2D structure. During the electrochemical process, the surface of the electrode showed different structure changes from those of the bulk structure. The reaction mechanism of the intercalation electrodes for lithium batteries is discussed on the basis of surface and bulk structural changes.