X-ray Photoelectron Spectroscopy and AC Impedance Spectroscopy Studies of Li-La-Zr-O Solid Electrolyte Thin Film/LiCoO2 Cathode Interface for All-Solid-State Li Batteries

Abstract

Li ion half-cell electrode-based configuration was prepared by spin-coating using precursor of Li-stuffed garnet with intended composition of Li7La3Zr2O12 on LiCoO2. 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 LiCoO2 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 LiCoO2 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 LiCoO2 cathode seem to form reaction products, suggesting that is critical for applications to develop protecting layers between the cathodes and Li-stuffed garnet electrolytes.