Solvation Engineering via Cation‐Polyanion Synergy for Ultra‐Stable Seawater Zinc Batteries

ABSTRACT Seawater‐based zinc‐ion batteries (ZIBs), in terms of integration with high safety, environmental friendliness, and low cost, have been regarded as one of the most promising next‐generation large‐scale electrochemical storage systems. However, the complex ionic environment of seawater‐based electrolytes, which leads to chloride pitting corrosion, zinc (Zn) dendrite growth, and hydrogen evolution reactions (HER), impedes their practical applications. Herein, we design a multi‐functional binary additive electrolyte incorporating carboxymethyl cellulose (CMC) and magnesium sulphate (MS) into seawater‐based electrolytes to tackle these challenges. The addition of CMC with strong Zn/Zn 2+ affinity can significantly modify the solvation structures within the seawater‐based electrolyte and facilitate the reversible deposition behaviour of Zn 2+ ions. Moreover, the cation‐binding properties of CMC 2− enable it to effectively induce Mg 2+ ions and seawater cations toward the Zn anode. Simultaneously, Mg 2+ ions, possessing the highest charge density among seawater ions, show strong electrostatic forces toward free‐water. Thus, it can induce the multiple seawater cations to form a water‐poor layer on the Zn anode surface, thereby inhibiting the HER and the corrosion reactions. Notably, the intercalation reactions of multiple cations effectively suppress the structural collapse of cathode materials. Equally important, we innovatively introduced a unidirectional gas valve to resolve gas generation issues of the pouch cell, effectively resolving the key issue of gas accumulation in practical applications. Benefiting from the superior modification effect of the binary additive, the Zn//Zn symmetric cells in seawater‐based electrolyte exhibit a long‐term stability of over 5197 h at 5 mA cm −2 . Furthermore, the Zn//Cu asymmetric cells possess an average CE of 99.74% over stable cycling for 4000 cycles at 5.0 mA cm −2 . The Zn//NVO full cells retain 150 mAh g −1 after 1000 cycles at 2 A g −1 . This novel strategy of combining organic/inorganic additives with seawater‐based electrolytes offers an inspired and straightforward approach to solving the fundamental challenges in ZIBs, driving progress in seawater energy storage applications.