Annealing-induced evolution at the LiCoO2 /LiNbO3 interface and its functions in all-solid-state batteries with a Li10GeP2S12 electrolyte

A thin coating layer between the cathode active materials (CAMs) and solid electrolytes (SEs) is indispensable for alleviating the reaction at the CAM/SE interface and thereby enhancing the reversible capacity of all-solid-state Li-ion batteries (ASSLIBs). It is well established that the interfacial resistance in the CAM/coating layer/SE is sensitive to post-annealing processes. However, the underlying mechanism remains unclear; the influence of the electrochemical/physicochemical phenomena on capacity degradation, particularly the impact of the annealing temperature, has not been adequately elucidated. Herein, we applied various annealing temperatures to LiNbO3-coated LiCoO2 (LNO-coated LCO) particles, and initially adopted a double-coating method to investigate the behaviour of the LCO/LNO/Li10GeP2S12 interfaces. We observed that the electronic conductivity of LNO-coated LCO increased with increasing annealing temperature, presumably because of the LNO segregation at the LCO surface and Co diffusion to the LNO layer, both of which occurred during the dynamic LNO phase change from amorphous (350 °C annealed state) to crystalline (700 °C annealed state). The increased electronic conductivity triggers the interfacial reaction between the LNO layer and Li10GeP2S12, which is the primary cause of reversible capacity loss, whereas the ionic conductivity in LNO has minimal effect on the reversible capacity at low C-rates. This study highlights the importance of having a suitable electronic conductivity in the coating layer to improve the performance of bulk-type ASSLIBs.