Garnet‐Induced Amorphization of Fluoride Solid Electrolyte Interlayer Enables High‐Voltage and Large‐Capacity Lithium Metal Solid‐State Batteries

ABSTRACT Garnet‐type solid‐state electrolyte Li 6.5 La 3 Zr 1.5 Ta 0.5 O 12 (LLZTO) has garnered significant attention owing to its high Li‐ion conductivity at room temperature (RT) and good electrochemical stability with Li metal. However, it presents the challenges in terms of high resistance at electrolyte/electrode interfaces, coupled with the growth of lithium dendrites along grain boundaries within garnet. These challenges have been proven to be related to a thick passivation layer of Li 2 CO 3 formed on LLZTO surface. Here, we propose a garnet‐induced amorphization strategy of fluoride solid electrolyte (Li 2 SiF 6 ) interlayer with enhanced ionic conductivity (1.32 × 10 −6 S/cm at RT) and electronic insulation to address these challenges. The Li metal symmetric cells exhibit negligible interfacial resistance (≈1 Ω cm 2 ) and achieve a record‐high critical current density of 4.4 mA/cm 2 at RT. The outstanding high‐voltage stability of Li 2 SiF 6 contributes to enhance the oxidation capacity of the LLZTO system. The batteries coupled with LiNi 0.8 Co 0.1 Mn 0.1 O 2 with a loading of 4.3 mg/cm 2 exhibit stable long‐term cycling up to 4.5 V. The all‐solid‐state conversion reaction batteries, with a high loading of FeF 3 cathode up to 5 mg/cm 2 , achieve a high reversible capacity close to 500 mAh/g. This study inspires the positive modulation of ceramic oxides on fluoride solid electrolytes and their multi‐functions on both anode and cathode.