Natural Anionic Interfacial Reconstruction for Suppressing Pitting Corrosion Enables Durable Seawater Zinc‐Ion Batteries

ABSTRACT Seawater represents a virtually inexhaustible, low‐cost, and environmentally benign electrolyte resource for aqueous batteries, offering great potential for sustainable large‐scale energy storage. However, the chloride‐rich environment poses challenges for the long‐term stability of Zn metal anodes. Herein, we report an in situ electrochemically induced molecular sieve/sodium alginate (MS_SA) composite interfacial layer that enables durable and efficient Zn metal operation in seawater‐derived electrolytes. The MS_SA layer adaptively smooths the anode surface, homogenizes the interfacial electric field, and effectively regulates ion transport, while reversible Zn ²⁺ ‐alginate coordination facilitates uniform Zn nucleation and dense deposition. As a result, Zn||Zn symmetric cells exhibit stable cycling for over 2000 h and maintain long‐term durability beyond 800 h at 40.6% depth of discharge in seawater‐based electrolytes. Meanwhile, iodine‐based full cells deliver a high reversible capacity of 290 mAh g ⁻¹ after more than 4000 cycles at 5 A g ⁻¹ with excellent capacity retention. Notably, the pouch cell sustains a stable capacity of ∼ 280 mAh for over 400 cycles at an N/P ratio of 2.3 with negligible degradation. This work demonstrates a viable strategy for realizing low‐cost and sustainable seawater‐based Zn‐ion batteries.