Toward Rational Design of Aqueous Battery Membranes Beyond the Selectivity-Conductivity Trade-Off.

Electrochemical energy-storage systems increasingly require membranes that can simultaneously deliver fast ion transport, suppressed active-species crossover, chemical and dimensional stability, and scalable manufacturability. Yet conventional membrane platforms still struggle to reconcile conductivity with selectivity, while durability and processability remain important practical constraints. Sulfonic-acid-based membranes are particularly attractive because they combine strong acidity, fixed-charge character, and tunable hydrated ionic domains. Together, these features make sulfonic acid groups a versatile design element for regulating transport, exclusion, swelling, and membrane processability. Although these features have been most extensively explored in redox flow batteries, the resulting insights are also highly relevant to the broader design of next-generation energy-storage membranes. This review therefore uses sulfonic-acid-based membranes in redox flow batteries as a representative platform to extract transferable design principles. It covers proton-transport mechanisms, synthetic and microstructural design strategies, representative fluorinated and non-fluorinated membrane classes with flexible and rigid architectures, and their roles in vanadium, zinc, and emerging organic redox flow batteries. It further discusses cost, durability, dimensional stability, scale-up, and future opportunities in localized sulfonic-acid placement, biomass-derived membranes, and data-driven design. By clarifying structure-transport-manufacturing relationships, this review extracts general design principles for advanced membranes across electrochemical energy-storage systems.