<p>The ionic conductivity of solid electrolytes is still insufficient to approach performance promises of solid-state Li metal batteries, suffering from their charged interfaces among phase components and movable Li⁺ concentration. Herein, an anion-capturing interface based on FeF₃ is established on Li_6.5La₃Zr_1.5Ta_0.5O₁₂ surface through a sol-gel method. It promotes Li-salt dissociation and formation of anion aggregated layer before blending with polymer. Coulombic interaction of anion on grains boundary showcases multiple merits, including their weakened built-in electric field, restrained charge gradient layer, spontaneous Li⁺ cross-phase migration, and homogenized interfacial charge distribution. As such, the resulting composite solid electrolytes exhibits an ion conductivity of 1.1×10⁻⁴ S/cm² and Li⁺ migration number of 0.75 at 25°C. Its resulting Li symmetrical batteries maintain Li plating/stripping behaviors for over 1300 h and low polarization at 0.1 mA/cm² current density. When being assembled with LiFePO₄ positive electrode in solid-state batteries, it performs a specific capacity of 152.8 mAh/g at 1.0 C (170 mA/g) with 96% retention after 600 cycles. This work prioritizes the promises of interface engineering for solid electrolytes in solid-state Li metal batteries.</p>