Reversible Na⁺/I^- Dual-Ion Chemistry by Electrolyte Engineering Enables High-Rate and Long-Lifespan Energy Storage...

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-I₂) battery system, incorporating a carbon-coated Fe_0.5Ti₂(PO₄)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 H₂O in Na⁺ solvation sheath. Breaking the hydrogen-bond network of H₂O 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-I₂ 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-I₂ batteries.