Taming Free Water Activity via Ethylene Glycol for Durable and Fast‐Charging Rechargeable Seawater Batteries

ABSTRACT Aqueous seawater‐based batteries (ASWBs) are promising for low‐cost, sustainable energy storage but are limited by highly reactive free water in pure seawater (PSW), which leads to electrode degradation and poor cycling stability. Herein, we address this bottleneck by an entropy‐enhanced cosolvent approach using an eco‐friendly, low‐cost modified seawater (MSW) electrolyte (ethylene glycol as the cosolvent) for ASWBs. EG molecules increase the short‐range entropy of the MSW, forming discrete, small clusters that increase Na ⁺ mobility and reduce free water activity by disrupting the native H‐bonding network of PSW. Combining this MSW electrolyte with a high‐entropy hexacyanoferrate (HEHCF) cathode results in exceptional cycling lifespan (>20,000 cycles) at a high current density of 7000 mA g ⁻¹ , with 96.50% capacity retention and full charge–discharge in 35–39 s. This outstanding performance stems from a pseudocapacitive‐dominant charge‐storage mechanism, reversible monoclinic↔cubic phase transitions in HEHCF, and suppressed parasitic reactions. This work provides a cost‐effective, scalable electrolyte‐engineering strategy to unlock stable ASWBs, advancing their potential for grid‐scale and marine energy‐storage applications.