Bridging papermaking and zinc-ion batteries: Engineering partial Janus separators from hierarchically porous softwood fibers

Zinc-ion batteries are emerging as promising candidates for grid and large-scale energy storage due to their intrinsic safety, low cost, and aqueous chemistry. However, challenges related to zinc anode instability—such as corrosion, side reactions, and dendrite formation—continue to hinder practical deployment. Here, we present a papermaking-based strategy for separator engineering, grounded in the principles and industrial platform of papermaking wet-end chemistry and additive technology. Chemically pulped softwood fibers, with lignin and hemicellulose removed to some extent, exhibit hierarchically porous walls enriched with hydroxyl groups and interfibrillar gaps—providing an ideal substrate for post-treatment using dilute polymer–solvent systems. Using poly (vinylidene fluoride) and N -methyl-2-pyrrolidone as a model system, we demonstrate the formation of a spatially heterogeneous, partially hydrophobic network—a partial Janus structure—within the cellulosic paper. This treatment introduces hydrophobic domains while preserving ion-conductive pathways, achieving a favorable balance between Zn 2+ transport and water activity regulation. The resulting modified paper exhibits enhanced wet mechanical strength, corrosion resistance, interfacial stability, and long-term cycling performance. Full-cell and pouch cell tests further confirm the mechanical robustness and electrochemical reliability of the engineered separator. In addition, a green chitosan-based treatment is also demonstrated, highlighting the potential for sustainable additive systems to achieve similar structural and electrochemical benefits. This work shows how papermaking-derived materials, revisited through the lens of interfacial chemistry, can offer scalable and adaptable solutions to longstanding challenges in aqueous battery separator design. • Papermaking wet-end chemistry & additives applied to engineer battery separators • Multiscale porous structure of softwood fiber walls retained after interfacial engineering • Partial Janus interfacial structure formed via polymer–solvent self-assembly • Partial Janus paper modulates Zn 2+ transport while suppressing water-induced reactions. • Concept generalizable beyond current additives via fiber–additive interactions