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 I2 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 Zn2+ 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||I2 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.