Inhibiting Formation and Reduction of Li₂CO₃ to LiC_x at Grain Boundaries in Garnet Electrolytes to Prevent Li Penetration

Abstract Poor ion and high electron transport at the grain boundaries (GBs) of ceramic electrolytes are the primary reasons for lithium filament infiltration and short‐circuiting of all‐solid‐state lithium metal batteries (ASLMBs). Herein, it is discovered that Li₂CO₃ at the GBs of Li₇La₃Zr₂O₁₂ (LLZO) sheets is reduced to highly electron‐conductive LiC_x during cycling, resulting in lithium penetration of LLZO. The ionic and electronic conductivity of the GBs within LLZO can be simultaneously tuned using sintered Li₃AlF₆. The generated LiAlO₂ (LAO) infusion and F‐doping at the GBs of LLZO (LAO‐LLZOF) significantly reduce the Li₂CO₃ content and broaden the energy bandgap of LLZO, which decreases the electronic conductivity of LAO‐LLZOF. LAO forms a 3D continuous ion transport network at the GB that significantly improves the total ionic conductivity. Lithium penetration within LLZO is suppressed and an all‐solid‐state LiFePO₄/LAO‐LLZOF/Li battery stably cycled for 5500 cycles at 3 C. This work reveals the chemistry of Li₂CO₃ at the LLZO GBs during cycling, presents a novel lithium penetration mechanism within garnet electrolytes, and provides an innovative method to simultaneously regulate the ion and electron transport at the GBs in garnet electrodes for advanced ASLMBs.