Analysis for Reaction Layer Growth Mechanism at Cathode/Solid Electrolyte Interface via Operando X-Ray Reflectometry

To examine the growth mechanism of a reaction layer formed at the cathode/solid electrolyte interface under high potential, a LiCoO2/Li1+xAlxTi2−x(PO4)3(LATP)/Fe2(MoO4)3 model cell was constructed by pulsed laser deposition method and the structure of the LiCoO2/LATP interface was analyzed using operando X-ray reflectometry. The thickness of a reaction layer formed between LiCoO2 and LATP increased slowly with time up to 2.6 h under holding the voltage of 1.2 V, which corresponds to 4.2 V versus Li+/Li. In holding the voltage above 2.6 h, the thickness increased rapidly with increment of time, following the parabolic law. The roughness of the interface between the reaction layer and LATP increased once after 1.2 V holding and then decreased. These results suggest that the reaction layer formed between LiCoO2 and LATP grows the solid electrolyte, changing its morphology from island-like to film-like with time, driven by the difference in chemical potential of lithium.