A novel electron-blocking interlayer composed of potassium fluoride (KF) is introduced to enhance the performance of garnet-based quasi-solid-state lithium-metal batteries. The KF layer, when reacted with molten lithium, transforms into a KF/LiF interlayer that effectively blocks electron leakage and inhibits lithium dendrite growth. This interlayer significantly improves the cycle life of lithium metal batteries, with symmetric cells demonstrating a cycle life of ~3000 hours at 0.2 mA cm⁻² and over 350 hours at 0.5 mA cm⁻². Additionally, an ionic liquid (LiTFSI in C₄mim-TFSI) is used to wet the positive electrode interface, enhancing the battery's specific capacity, lifespan, and capacity retention. The Li|KF-LLZTO|NCM cells achieve a specific capacity of 109.3 mAh g⁻¹, a lifespan of 3500 cycles, and a capacity retention of 72.5% at 25°C and 2 C. The KF/LiF interlayer and ionic liquid combination provides a simple and effective strategy for high-performance quasi-solid-state lithium metal batteries. The study highlights the importance of interface modification in improving the safety and performance of solid-state lithium batteries.A novel electron-blocking interlayer composed of potassium fluoride (KF) is introduced to enhance the performance of garnet-based quasi-solid-state lithium-metal batteries. The KF layer, when reacted with molten lithium, transforms into a KF/LiF interlayer that effectively blocks electron leakage and inhibits lithium dendrite growth. This interlayer significantly improves the cycle life of lithium metal batteries, with symmetric cells demonstrating a cycle life of ~3000 hours at 0.2 mA cm⁻² and over 350 hours at 0.5 mA cm⁻². Additionally, an ionic liquid (LiTFSI in C₄mim-TFSI) is used to wet the positive electrode interface, enhancing the battery's specific capacity, lifespan, and capacity retention. The Li|KF-LLZTO|NCM cells achieve a specific capacity of 109.3 mAh g⁻¹, a lifespan of 3500 cycles, and a capacity retention of 72.5% at 25°C and 2 C. The KF/LiF interlayer and ionic liquid combination provides a simple and effective strategy for high-performance quasi-solid-state lithium metal batteries. The study highlights the importance of interface modification in improving the safety and performance of solid-state lithium batteries.