Garnet electrolyte-based solid-state lithium metal batteries (SLMBs) are attractive owing to their high energy density and high safety. However, garnet electrolytes are sensitive to the humid air, forming Li2CO3-rich contaminants on the surface that trigger high kinetic barrier and premature battery failure. Herein, a straightforward “waste-to-treasure” strategy is proposed to transform Li2CO3 on Li6.4La3Zr1.4Ta0.6O12 (LLZTO) surface into LiF by solvent-free trifluoroacetic acid (TFA) -induced interfacial engineering. The as-formed LiF interfacial layer with electronic insulation nature effectively improves the interface wettability between Li anode and LLZTO, reduces the interface resistance and produces a ultralong-lifespan symmetric cell of up to 10000 h at 0.1 mA cm-2. Furthermore, the SLMB paired with LiFePO4 cathode delivers a high capacity of 155.6 mAh g-1 at 0.5 C and maintains 78% of the initial capacity after 900 cycles. This study offers a facile strategy to overcome the limitations associated with surface impurities, addresses critical interface issues, and accelerates the practical application of garnet-type solid-state electrolytes in high-performance SLMBs.