Functionalization-Driven Disorder in Amorphous MOF Separators for Ion-Selective Lithium–Sulfur Batteries

Amorphous metal–organic frameworks (MOFs) combine local coordination order with long-range structural disorder, yet their application as functional separators in electrochemical energy storage systems has remained largely unexplored. Here, we report a one-step, acid-assisted synthesis of amorphous sulfonated UiO-66 as a multifunctional separator coating for lithium–sulfur (Li–S) batteries. Cooperative formate coordination and sulfonic acid functionalization disrupt crystallinity, yielding a defect-rich framework with sulfonate-lined mesopores while preserving short-range Zr–linker correlations, as confirmed by pair distribution function (PDF) analysis. This functionalization-driven disorder enhances electrolyte affinity, enables selective Li⁺ transport (t⁺ = 0.79), and regulates cathode–separator interfacial chemistry by strengthening polysulfide binding via Li⁺–SO₃⁻ coordination, supported by adsorption measurements, zeta potential analysis, XPS, and DFT calculations. Li–S cells employing UiO-66–SO₃H/PP separators exhibit stable long-term cycling with 72.1% capacity retention over 850 cycles at 0.5 C (~0.015% fade/cycle) and robust rate capability from 0.1 to 2 C. Through stepwise increases in sulfur loading and concurrent reduction of electrolyte content, the separator sustains multi-mAh cm⁻² areal capacities under lean and ultra-lean (E/S = 5 µL mg⁻¹) conditions, maintaining reversible operation even at sulfur loadings up to 3.6 mgₛ cm⁻². The amorphous separator further stabilizes Li plating/stripping behavior by suppressing dendrite growth. Compared with crystalline and non-MOF coatings, this work establishes functionalization-driven disorder in MOFs as a scalable design strategy for ion-selective separators bridging laboratory performance and practical requirements for Li–S batteries. Figure 1