As a promising candidate of current solid-state electrolytes with various advantages, such as broad electrochemical stability window, high thermal stability, and chemical stability, garnet-type solid-state electrolyte Li7La3Zr2O12 (LLZO) attracts extensive research attention. Researchers usually use the element doping method to improve the lithium ion conductivity of LLZO and to stabilize the cubic phase. However, the improvements either increase the cost of raw materials or need complex process. In this work, the preparation process of Al-doped LLZO is optimized by a simple post-doping method (LLZO-Al), and the as-prepared LLZO-Al shows a high lithium ion conductivity of 5.331 × 10−4 S cm−1 and an activation energy of 0.25 eV at room temperature. In addition, the LLZO-Al exhibits excellent stability against Li in LiǁLLZO-AlǁLi symmetric cell, and the LiNi0.8Co0.1Mn0.1O2 (NCM811)ǁLLZO-AlǁLi solid-state cell delivers a stable cycle of 120 cycles at 0.1C. This work offers a low-cost preparation method of LLZO-based solid-state electrolytes with high ionic conductivities for high-energy solid-state lithium metal batteries.

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