Combinatorial Synthesis and Ionic Conductivity of Amorphous Oxynitrides in a Pseudo-ternary Li3PO4-Li4SiO4-LiAlO2 System

A combinatorial synthesis system consisting of co-sputtering multiple radio frequency (RF) cathodes was developed to investigate the presence of new amorphous solid electrolytes in a pseudo-ternary Li3PO4-Li4SiO4-LiAlO2 system. Oxynitride solid-electrolyte films were synthesized under an N2 atmosphere by reactive RF sputtering with cathodes made of different materials, such as Li3PO4, Li4SiO4, and LiAlO2 installed at different positions in the chamber. The formation of amorphous films with no grains and a continuous atomic distribution was confirmed using scanning electron microscopy and energy-dispersive X-ray spectroscopy. In a single synthesis, we fabricated an oxynitride solid-electrolyte film, in which the composition range of the components was approximately one-eighth that in the corresponding pseudo-ternary system. The highest Li-ion conductivity of 3.1 × 10−6 S cm−1 was obtained for the LiPSiAlON film with a composition ratio of LiPON : LiSiON : LiAlON = 48.5 : 33.9 : 17.6. The different bonding states of O and N in the LiPSiAlON film doped with P, Si, and Al were examined using X-ray photoemission spectroscopy. The ionic conductivity was improved by the mixed anion effect. The combinatorial synthesis enables the efficient optimization of the chemical composition and facilitates the development of highly conductive amorphous solid electrolytes.