vehicles, and even grid scale energy storage applications with high energy, high output voltage, low self-discharge, and long cycling life. [1] Currently, the safe operation of LIBs relies on the physical separation of cathode and anode electrodes using polymer separators, which are immersed in lithium salt solution with carbonate solvents for transporting lithium ion. For fast lithium-ion transfer, high ionic conductivity and Li-ion diffusion coefficient of electrolyte are essential. During the last few decades, through intensive investigations, there has been significant progress in the performance of liquid electrolytes because of their promising features, such as high bulk ionic conductivity and excellent interface with solid electrodes surfaces wetting. [2] However, there are some limitations, such as narrow ion selectivity, [3] high flammability with low thermal stability, which can lead to an explosion if overcharged or short circuits occur. [4] To overcome the critical drawbacks of liquid electrolytes, solid-state electrolytes have been intensively investigated since the 1990s. The solid-state electrolytes provide several advantages in LIB, such as high simplicity of design, nonflammability, diverse ionic selectivity, and stability with lithium metal. [5,6] Recently, there have been significant improvements on high ionic conductivity solid electrolyte with metal oxides and metal sulfides. [7-10] Particularly, oxide-based solid electrolyte possesses appropriate chemical stability, high thermal stability, [11-13] and wide electrochemical window, [4,14,15] which could potentially resolve the safety issues in LIBs. [16,17] Among them, garnet-type of Li 7 La 3 Zr 2 O 12 (LLZO) has been intensively investigated as a promising solid electrolyte with high ionic conductivity (≈10 −3 S cm −1) at room temperature and superior electrochemical stability. [18-22] However, there are several remaining issues, such as low ionic conductivity at grain boundary, anisotropic ion transport, high processing temperature, and limited composition tunability. Amorphous materials could be an alternative to resolve these issues. For example, the amorphous Li-La-Zr-O thin films, based on primitive stoichiometric ratio of LLZO, have demonstrated the advantages of amorphous materials, such as the lack of grain boundary, intimate contact with electrode materials, and low processing temperature. [23-27] Lithium-rich amorphous Li-La-Zr-O (a-Li-La-Zr-O) electrolyte is successfully synthesized using sol-gel processing method. With unlimited compositions of the amorphous structure, lithium-rich compositions are systematically investigated to determine optimal composition and optimized processing conditions of a-Li-La-Zr-O coatings. There is an improvement in ionic conductivity of a-Li-La-Zr-O with Li content increasing, specifically from 3.0 × 10 −8 S cm −1 (Li 8 La 2 Zr 2 O 11) to 1.18 × 10 −6 S cm −1 (Li 18 La 2 Zr 2 O 16), thereby resulting in low activation energy. The high-ionic-conductivity a-Li-La-Zr-O is implemented as an artificial s...
