Organic-Solvent-Free Weakly Solvating Electrolytes Enable Durable Ah-Scale Zn–Iodine Batteries under Diverse Harsh Operating Conditions

Aqueous zinc–iodine batteries represent a promising sustainable energy-storage technology, offering intrinsic safety, cost-effectiveness, and competitive energy density. However, their practical deployment has been hindered by the instability of both the I 2 cathode and Zn anode. While traditional weakly solvating electrolytes (WSEs) partially alleviate these issues, they inherently rely on high-viscosity organic cosolvents that impede ion transport, limiting performance under demanding conditions. Here, we overcome this trade-off by pioneering an organic-solvent-free WSE, achieved by incorporating guanidine sulfate, a salt-type diluent with kosmotropic–chaotropic properties, at an optimized concentration. The formulated electrolyte simultaneously facilitates Zn 2+ desolvation and transport while improving electrolyte stability. Furthermore, it promotes the formation of a robust electrode–electrolyte interphase on both electrodes, effectively suppressing polyiodide shuttling and stabilizing Zn plating/stripping. Consequently, the Zn||I 2 cells achieve exceptional performance across diverse harsh operating conditions, including ultralong cycling (92% capacity retention after 10000 cycles at 30 C), high-rate capability (134 mAh g –1 at 200 C), and durable cycling with a low negative/positive capacity ratio (≈1.09), lean electrolyte condition (7.5 mL Ah –1 ), and a wide temperature range (−10 to 60 °C). These metrics are consistently preserved in Ah-scale pouch cells (0.5–2 Ah). This work redefines WSE design through salt-type diluents, offering a scalable pathway to durable aqueous batteries for sustainable energy storage.