Separator Engineering for Sodium Metal Batteries: Challenges, Rational Design and Recent Modification Strategies.

Sodium metal batteries (SMBs) have emerged as promising candidates for next-generation energy storage systems, leveraging their high theoretical capacity and the natural abundance of Na resources. Nevertheless, critical challenges, including dendritic growth, side reactions, and pronounced volume fluctuations during cycling, continue to impede their commercialization. Conventional separators, including polyolefin or glass fiber types, suffer from poor wettability, uneven ion flux, and a rough surface. To tackle these challenges, functional separator engineering encompassing interfacial chemistry regulation, multiscale structural design, and hybrid integration of advanced materials has demonstrated remarkable progress in regulating Na⁺ flux, stabilizing SEI, and suppressing dendrite propagation. This review systematically provides a comprehensive summary of recent developments in separator engineering for SMBs, including modified polyolefin separators, enhanced glass fiber frameworks, cellulose-based separators, MOFs/COFs-based, and emerging separators. In addition, a comprehensive electrochemical evaluation framework encompassing CE in half-cells is proposed, the lifespan of symmetric cells, full cell performance, and safety validation to assess practical applicability. Furthermore, computational simulations for mechanism elucidation and predictive design, as well as future separator perspectives, have been also discussed to guide the development of next-generation separators for high-performance and commercially viable SMBs.