For recent lithium-ion batteries (LIBs), there have been lots of researches about convert liquid-phase electrolyte to solid-state electrolyte (i. e. all-solid-state battery) to improve safety and stability. Solid-state electrolyte materials have to satisfy some criteria including high ionic conductivity at room-temperature, chemical stability against reducing environment, and mechanical strength. Many solid-materials for electrolyte have been researched, including perovskite, garnet, and sulfide-type of materials. Among them, garnet-type based lithium lanthanum zirconium oxide (LLZO, Li₇La₃Zr₂O₁₂) has been received attention due to its high ionic conductivity (~10^-4 S/cm at room temperature) and wide electrochemical stability window (~6 V versus Li/Li⁺). The main issue of LLZO is the high interfacial resistance with anode and cathode materials such as Li metal and LiCoO₂. Many researchers have tried to understand the cause of this interfacial resistance for LLZO. K. Park et al. researched about the cathode/electrolyte (LLZO) interface and revealed co-diffusion and second-phase formation of LLZO at high-temperature [1]. M. Zarabian et al. reported that spin-coated LLZO on the LiCoO₂ pellet fabricated at low-temperature (~400^oC) and co-diffusion of La and Co at the interface by using X-ray photoelectron spectroscopy (XPS) [2]. They commonly suggested that elevated temperature make chemical interaction and this make high interfacial resistance.

In this study, we adopted novel sintering method, flash light annealing (FLA) process, to fabricate LLZO solid-electrolyte. This method is also called rapid, cool sintering because of its unique characteristics compared to conventional sintering methods [3]. FLA uses instantaneous high current to generate intensive pulsed light (IPL), and irradiate IPL toward the specimen to sinter the materials. Unlike the conventional sintering methods, this novel method has lots of advantages including availability of treatment at STP and fast sintering time (~ a few ms). Rapid sintering process gives no time for the diffusion at the interface, thus the interfacial resistance between cathode and LLZO solid-electrolyte must be reduced. We fabricated LLZO solid-electrolyte on the commercial LiCoO₂ pellet by using solution-based spin-coating method and sintered the film by using FLA method. XPS and energy-dispersive X-ray spectroscopy (EDX) analysis were conducted to observe interface and diffusion of La and Co elements. Also, X-ray diffraction (XRD) method was used to investigate sintering ability of FLA through crystallinity development. The interfacial resistance between LiCoO₂ and LLZO solid-electrolyte was analyzed by electrochemical impedance spectroscopy (EIS) through cross-plane set-up.