Integrating intermittent energy from renewable resources into the grid supply by energy storage technology is significant in driving a more sustainable energy future. Aqueous batteries based on water receive renewed interest in this aspect. Natural seawater (NS) offers a promising alternative to distilled water electrolytes since NS accounts for ~96.5% of Earth’s total water reserves. Paring seawater electrolyte with zinc negative electrode has emerged as one of the most sustainable solutions for offshore stationary energy storages such as those for offshore wind farms or floating photovoltaic, owing to the intrinsic safety, extremely low cost, and unlimited water source (Fig. 1a). Still, direct use of NS in the electrolyte poses greater challenges for the zinc metal negative electrode. The chloride environment (0.54 M of Cl^-) and the ionic complexity (e.g., Na⁺, Ca²⁺, Mg²⁺) in NS significantly influence the electrode reactions.

To address this challenge, we first unveil the corrosion mechanism of the Zn negative electrode in the NS, revealing that Cl^- induces pitting and spontaneously forms by-products with other components as a reactant. These substantial corrosion behaviors further deteriorate the interface environment, thus aggravating dendritic deposition and causing rapid battery failure (Fig. 1b). We then propose a charge gradient interface (CGI) strategy to modulate ion transport at the interface. The gradually strengthened negative charges formed via diffusion-controlled electrostatic complexation of biomass-derived polysaccharide precursors serve to propel surface accumulation of Cl^- while simultaneously accelerating the diffusion of Zn²⁺, thus facilitating uniform deposition, and mitigating corrosion and side reactions (Fig. 2a).

The CGI enables prolonged cycling of the zinc negative electrode beyond 1300 h in NS, more than 40 times that of the unprotected Zn negative electrode (Fig. 2b). The NS electrolyte is also revealed to stabilize the positive electrodes, for example, the positive contribution of Na⁺ in NS to the NaV₃O₈ cathode. The seawater-based Zn//NaV₃O₈ full cell delivers a practical areal capacity of 5 mAh cm^-2 and operates stably for over 500 cycles (Fig. 2c). This work provides viable guidelines for stabilizing the Zn negative electrode in the NS system and constructing sustainable NS-based energy storage.