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 Fe 2+ (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 VOPO 4 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 Fe 2+ 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.