Reversible Na ⁺ /I ⁻ Dual‐Ion Chemistry by Electrolyte Engineering Enables High‐Rate and Long‐Lifespan Energy Storage Device

Abstract Aqueous battery energy storage systems have driven extensive attention owing to safety, low‐cost, and environmental friendliness. However, the dendrite growth, poor reversibility, and hydrogen evolution reactions (HER) hinder their commercial utilization. To address these issues, we first developed a dual‐ion sodium‐iodine (Na‐I 2 ) battery system, incorporating a carbon‐coated Fe 0.5 Ti 2 (PO 4 ) 3 anode coupled with an activated carbon cathode. This system demonstrates dendrite‐free behavior, superior durability, and excellent reversibility. Impressively, a Na‐ion solvation engineering strategy was performed through electrolyte optimization of sodium trifluoromethanesulfonate (NaOTf), in which OTf − replaces the H 2 O in Na + solvation sheath. Breaking the hydrogen‐bond network of H 2 O molecules and reducing the desolvation energy barrier by approximately 40% is confirmed by MD simulations, as well as enhancing ionic conductivity and charge transfer kinetics. Accordingly, the Na‐I 2 batteries achieve a high capacity of 105 mAh g −1 for 20,000 cycles with 79.2% capacity retention at 5 A g −1 . Pouch cells with a capacity of 56 mAh achieve a long cycling lifespan up to 500 cycles with the retention of 92.3% at the current of 50 mA. Extended to 1.1 Ah, it also presents outstanding charging–discharging performance. This work offers a transformative framework for the development of high‐performance aqueous Na‐I 2 batteries.