This study presents a novel strategy to enhance the performance of garnet-based quasi-solid-state lithium-metal batteries by introducing a potassium fluoride (KF) interlayer on Li6.4La3Zr1.4Ta0.6O12 (LLZTO) solid electrolyte. The KF layer, when reacted with molten lithium, transforms into a KF/LiF interlayer, which effectively blocks electron leakage and inhibits lithium dendrite growth. This modification results in a significant improvement in the cycle life and electrochemical performance of the battery. Symmetric cells with the KF/LZTO interlayer showed a cycle life of over 3000 hours at 0.2 mA cm−2 and over 350 hours at 0.5 mA cm−2. Additionally, a LiTFSI in C4mim-TFSI ionic liquid was used to wet the LLZTO/NCM positive electrode interface, achieving a specific capacity of 109.3 mAh g−1, a cycle life of 3500 cycles, and a capacity retention of 72.5% at 25 °C and 2 C. The integration of the KF/LiF interlayer and ionic liquid demonstrates a promising approach for high-performance garnet-based lithium-metal batteries.This study presents a novel strategy to enhance the performance of garnet-based quasi-solid-state lithium-metal batteries by introducing a potassium fluoride (KF) interlayer on Li6.4La3Zr1.4Ta0.6O12 (LLZTO) solid electrolyte. The KF layer, when reacted with molten lithium, transforms into a KF/LiF interlayer, which effectively blocks electron leakage and inhibits lithium dendrite growth. This modification results in a significant improvement in the cycle life and electrochemical performance of the battery. Symmetric cells with the KF/LZTO interlayer showed a cycle life of over 3000 hours at 0.2 mA cm−2 and over 350 hours at 0.5 mA cm−2. Additionally, a LiTFSI in C4mim-TFSI ionic liquid was used to wet the LLZTO/NCM positive electrode interface, achieving a specific capacity of 109.3 mAh g−1, a cycle life of 3500 cycles, and a capacity retention of 72.5% at 25 °C and 2 C. The integration of the KF/LiF interlayer and ionic liquid demonstrates a promising approach for high-performance garnet-based lithium-metal batteries.