Atomic level changes during capacity fade in highly oriented thin films of cathode material LiCoPO4

High-quality thin films with well-defined fast lithium ion diffusion pathways and minimal structural discontinuities are needed to develop high-performance electrodes for all-solid-state Li-ion batteries. Achieving this requires an understanding of the electrochemical processes and structural changes that take place within an electrode during charge/discharge. Here we report the successful synthesis of highly oriented olivine-structured LiCoPO4 thin films by chemical solution deposition onto Au(111)/Al2O3(0001) substrates. State-of-the-art scanning transition electron microscopy (STEM) and theoretical simulations are used to examine surface structures and the changes that occur during electrochemical cycling. As-synthesised films are found to be composed of nearly defect-free domains that are predominantly aligned with (210), (010) or (101) surfaces parallel to the substrate. High-angle annular dark field (HAADF)-STEM revealed the formation of considerable numbers of cation exchange (antisite) defects in the surface regions after only three cycles. Upon further electrochemical cycling, significant capacity fade, together with an increase in the concentration of cation exchange defects, was observed. Electron energy loss spectroscopy (EELS) revealed that formation of these defects is associated with oxygen loss and deformation of PO4 tetrahedra, leading to structural degradation detrimental to the electrochemical performance of thin-film LiCoPO4 electrodes.