Separator Modification Strategies for Next‐Generation Sodium‐Ion Batteries

ABSTRACT 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, Al 2 O 3 ‐PVDF), organic (PVDF‐Celgard, EVA/PI/EVA), organic–inorganic composite (ZrO 2 ‐PE, cellulose‐PAN‐Al 2 O 3), 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.