Abstract: A sulfonic-functionalized Zr-type metal–organic framework (UiO-66-SO₃H) was synthesized, and UiO-66-SO₃Li was obtained via subsequent lithiation treatment. The UiO-66-SO₃H framework was incorporated into a polymer matrix as a functional filler to obtain high-performance solid composite electrolytes (SCEs). Structural and morphological analyses, as well as various electrochemical tests, were performed. Owing to well-defined porous channels, abundant lithium-ion transport sites, and effective regulation of polymer crystallinity by \textSO_3^- moieties at 60°C, the optimized SCE-2 (10wt% UiO-66-SO₃Li) exhibited a high ionic conductivity of 5.36 × 10⁻⁴ S·cm⁻¹ (1.86 × 10⁻⁴ S·cm⁻¹ at the room temperature of 25°C), along with a wide electrochemical stability window of 4.7 V and a lithium-ion transference number as high as 0.623. The Li|SCE-2|LiFePO₄ full cell exhibited excellent long-cycle stability at both 60°C and room temperature, as well as an excellent rate capability up to 5 C (1 C is equivalent to 170 mA·g⁻¹). Moreover, SCE-2 can also be coupled with high-voltage LiNi_0.5Co_0.2Mn_0.3O₂ cathodes. This study demonstrates that sulfonic-modified UiO-66-SO₃Li can effectively regulate Li⁺ migration, suppress lithium dendrite growth, and enhance interfacial compatibility, offering a viable approach for designing advanced solid composite electrolytes.