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Chemical solution deposition (CSD)-based thin-film fabrication techniques for solid electrolytes are important for accelerating the commercialization of all-solid-state lithium-ion batteries. Post-treatments, typically heat treatments at high temperatures, are applied to obtain a pure crystal structure with a dense morphology for high electrochemical performance. However, lithium evaporates at high temperatures, leading to changes in the material composition and decrease in performance. In this study, a novel rapid sintering method, namely flash-light sintering (FLS), was applied as a post-treatment for spin-coated lithium lanthanum titanate (LLTO) thin films to prevent lithium evaporation and obtain fully dense thin films with perovskite structures. The flash-light-sintered LLTO thin film exhibited negligible lithium loss with pore-free morphologies and a pure cubic perovskite structure, whereas the thermally sintered LLTO thin film exhibited significant lithium evaporation. The lithium-preserved cubic LLTO dense thin films prepared by FLS showed an outstanding total ionic conductivity of ∼6 × 10−5 S cm−1, the highest recorded for CSD-based LLTO thin films. We demonstrated that the FLS method could be successfully applied for LLTO thin-film fabrication while maintaining the lithium stoichiometry and ensuring a highly densified film and cubic structure, which has a higher ionic conductivity than those of structures such as tetragonal or orthorhombic unit cells. The rapid FLS method not only simplifies the fabrication of LLTO thin films but also provides opportunities for its wide application to materials containing lithium or other elements with low melting points. [ABSTRACT FROM AUTHOR]