X-ray Photoelectron Spectroscopy and AC Impedance Spectroscopy Studies of Li-La-Zr-O Solid Electrolyte Thin Film/LiCoO₂Cathode Interface for All-Solid-State Li Batteries

Li ion half-cell electrode-based configuration was prepared by spin-coating using precursor of Li-stuffed garnet with intended composition of Li 7 La 3 Zr 2 O 12 on LiCoO 2 . The main goal of this study was lowering the heat-treatment to 400°C, which is well below the chemical reaction temperature of these phases. Liquid precursor of the nominal compound was prepared and spin-coated on the top surface of MgO(100) and LiCoO 2 pellets until the appropriate thickness was obtained. Electrochemical impedance spectroscopy (EIS) was used to understand the resistance and capacitance behavior of the electrolyte and electrolyte-electrode interface. The ac impedance of half-cell shows semicircles, corresponding to the response from bulk electrolyte, second phase and interface. The interfacial region between LiCoO 2 and Li-La-Zr-O film was deeply studied with X-ray photoelectron spectroscopy (XPS) depth profile analysis. XPS results show that Co diffuses from the substrate toward the solid electrolyte; Li depletes in the interface region, and moves to the top surface, which changes the electronic structure of Co to a multiplicity of reduced forms. The present study shows that even at 400°C, the Li-stuffed garnet precursor and LiCoO 2 cathode seem to form reaction products, suggesting that is critical for applications to develop protecting layers between the cathodes and Li-stuffed garnet electrolytes.