Synergistic Dual-Polar-Functionalized Metal–Organic Framework-Modified Separator for Stable and High-Performance Sodium Metal Batteries

Sodium metal, regarded as an ideal anode material for high-energy-density rechargeable sodium metal batteries (SMBs), faces critical challenges, such as sluggish Na⁺ transport kinetics and uncontrolled dendritic growth, which severely hinder its cycling stability and practical applications. Herein, the well-designed, multifunctional separator, UFS2@GF, constructed using metal-organic frameworks functionalized with fluorinated (-F) and sulfonic acid (-SO₃H) groups, synergistically provides more nucleation sites for Na⁺ deposition, thereby reducing the nucleation overpotential and achieving uniform deposition. The inorganic-rich solid electrolyte interphase induced by UFS2 facilitates a uniform Na⁺ flux and enhances charge transfer efficiency. Structural characterization and density functional theory calculations further demonstrate that the introduction of abundant sodiophilic sites provided by -F and -SO₃H significantly enhances Na⁺ transport kinetics by reducing the energy barriers for Na⁺ migration within the UFS2 framework, leading to a higher Na⁺ transference number, superior ionic conductivity, and accelerated ion transport. Because of these synergistic effects, the symmetric cell with UFS2@GF achieves stable performance, enabling stable cycling for over 2500 h at 0.25 mA cm^-2 while delivering an excellent specific capacity of 87.3 mA h g^-1 at 10C in Na∥Na₃V₂(PO₄)₃ cells. These results highlight the critical role of synergistic functional group strategies in addressing the limitations of SMBs.