Next-generation rechargeable batteries are promising candidates for state-of-the-art lithium-ion batteries, owing to their high energy density and preferred cost efficiency. For instance, lithium-sulfur batteries theoretically offer 10 times higher capacity and five times higher energy density. Shu Lei Chou and colleagues from the University of Wollongong have published a review article in National Science Review proposing a new concept of 4S (stable, safe, smart, sustainable) batteries. They reviewed the development of functional membrane separators in liquid-electrolyte next-generation batteries, based on which they report four important criteria for guiding the advancement of novel battery systems.