Tailored separators for diverse batteries: design, modification, and prospects

With the diversification of energy storage technologies and the pressing demands for high-performance batteries, separators have transcended their conventional role as inert barriers to emerge as critical enablers of performance, safety, and longevity. To address the unique operational challenges across emerging battery chemistries - including lithium-ion, lithium-metal, and multivalent-ion systems - innovative separator design has become a key research focus. This review begins by outlining the fundamental properties and essential functions of separators to provide guiding principles for functional engineering. It then critically surveys recent advances in separator development tailored for diverse battery systems, systematically analyzing their critical roles, performance improvements, and persistent challenges. Emphasis is placed not only on modification strategies and implementation techniques, but also on the underlying working mechanisms and design rationales behind emerging functional separators. The realized functions are classified and discussed in detail, offering insights into how separators can be engineered to meet specific electrochemical and interfacial requirements. Finally, forward-looking perspectives on future research directions are provided, aiming to bridge fundamental understanding with practical application. By synthesizing current knowledge and highlighting future opportunities, this review seeks to foster the rational design of high-performance separators and accelerate their deployment in next-generation energy storage systems.