Regulating Interlayer‐Spacing of Vanadium Phosphates for High‐Capacity and Long‐Life Aqueous Iron‐Ion Batteries

Although the research on aqueous batteries employing metal as the anode is still mainly focused on the aqueous zinc-ion battery, aqueous iron-ion batteries are considered as promising aqueous batteries owing to the lower cost, higher specific capacity, and better stability. However, the sluggish Fe²⁺ (de)intercalation leads to unsatisfactory specific capacity and poor electrochemical stability, which makes it difficult to find cathode materials with excellent electrochemical properties. Herein, phenylamine (PA)-intercalated VOPO₄ materials with expanded interlayer spacing are synthesized and applied successfully in aqueous iron-ion batteries. Owing to enough diffusion space from the expanded interlayer, which can boost fast Fe²⁺ diffusion, the aqueous iron-ion battery shows a high specific capacity of 170 mAh g^-1 at 0.2 A g^-1 , excellent rate performance, and cycle stability (96.2% capacity retention after 2200 cycles). This work provides a new direction for cathode material design in the development of aqueous iron-ion batteries.