Separator Modification Strategies for Next-Generation Sodium-Ion Batteries

Sodium-ion batteries (SIBs) are emerging as cost-effective and resource-abundant alternatives for large-scale energy storage, benefitting from structural similarities to lithium-ion systems and the natural abundance of sodium. However, sluggish desolvation kinetics, uneven Na⁺ flux at hard carbon defects, and poor separator–electrolyte compatibility hinder their commercialization. Conventional separators such as polyolefin, glass fiber, and cellulose exhibit limited wettability, irregular pore structures, and poor high-voltage stability. Recent advances in functional separator engineering through interfacial chemistry modulation, multiscale architecture design, and hybrid material integration have significantly improved Na⁺ flux uniformity, electrolyte affinity, and cycling stability. This review summarizes progress in inorganic (ND-GF, Al₂O₃-PVDF), organic (PVDF-Celgard, EVA/PI/EVA), organic–inorganic composite (ZrO₂-PE, cellulose-PAN-Al₂O₃), functional polymer (PEI/PVP, ZIF-8 AAS), and cellulose-based (CP@PPC, CSSA11) separators. A comprehensive electrochemical evaluation framework covering ionic conductivity, Na⁺ transference number, cycling stability and safety performance is also proposed. Furthermore, computational studies and future perspectives on scalable manufacturing (<$5 m⁻²) are discussed to guide the design of next-generation separators for practical, high-performance SIBs.