Hydrophobicity Gradient in Ultrathin Cellulose Separators for Durable Seawater‐Based Zinc Batteries

Natural seawater electrolytes enable cost‐effective deployment of aqueous zinc (Zn) ion batteries in maritime environments but face intensified chloride ions (Cl − ) ‐induced corrosion and water‐induced side reaction challenges. Here, an 18 µm‐thick cellulose separator with negative surface charges is developed and gradient hydrophobicity to simultaneously block Cl − penetration and reduce interfacial water activity. The hydrophobicity gradient establishes a stepwise desolvation pathway for solvated Zn ions (Zn 2+ ), forming a high aggregate electrolyte that suppresses parasitic hydrogen evolution, while the electrostatic shielding effect arising from negatively charged functional groups synergistically repels Cl − and homogenizes Zn 2+ flux. Consequently, Zn||Zn symmetric cells achieve record stability in natural seawater electrolyte, sustaining 2900 h at 1 mA cm −2 /1 mAh cm −2 and 1300 h under 50% depth of discharge. Paired with a thin Zn anode (20 µm) and high‐loading NaV 3 O 8 ·1.5H 2 O cathode, full cells deliver a volumetric energy density of 233.1 Wh L −1 at a practical N/P of 2.3, outperforming the majority of the reported aqueous Zn batteries. This work presents a critical advance toward offshore energy storage.