Surface Structure of LiNi_0.8Co_0.2O₂: a New Experimental Technique Using in Situ X-ray Diffraction and Two-Dimensional Epitaxial Film Electrodes

Surface and bulk structural changes in LiNi 0.8 Co 0.2 O 2 were observed during electrochemical reactions using synchrotron X-ray scattering and a restricted reaction plane of two-dimensional (2D) epitaxial-film electrodes. The bulk structural changes confirmed lithium diffusion through the (110) surface, which is perpendicular to the 2D edges of the layered structure. No (de)intercalation reaction was observed through the (003) surface in the voltage range of 3.0−5.0 V. However, intercalation proceeded below 3.0 V, which indicates unusual three-dimensional lithium diffusion in the 2D structure in the overlithiated state. Structural changes at the electrode surface were different from those in the bulk. Contact with the liquid electrolyte caused an abrupt deformation of the surface crystal lattice, followed by restructuring upon applying a cell voltage. These slow reconstruction processes were dependent on the crystal lattice orientations. The reaction mechanism of the intercalation electrodes for lithium batteries is discussed on the basis of the surface and bulk structural changes that occur during electrochemical reactions.