Synthesis, structure and electrochemical performance of hydrated zinc Iron phosphate as low-cost cathode material for aqueous zinc-ion batteries

Aqueous zinc-ion battery (AZIB) is one of the most promising candidates for large-scale energy storage, so it is critical to explore low-cost cathode materials with practical prospects. Iron-based phosphate cathodes have been shown to be very important in lithium/sodium-ion batteries, but have rarely been applied in AZIBs. Herein, hydrated zinc iron phosphate Zn₂Fe(PO₄)₂·xH₂O (ZFP) is first proposed as potential cathode material for AZIBs due to its advantage of layered structure with lubricative interlayer water. Carbon coated Zn₂Fe(PO₄)₂·xH₂O (ZFP@C) can be prepared by using a liquid-phase method combined with a hydrothermal process. Both the specific capacity and rate ability of ZFP@C are more superior than those of the raw ZFP. After 25 cycles of electrochemical activation, the ZFP@C cathode delivers a peak capacity of 137 mAh g⁻¹ with two charge and discharge platforms at 1.82 V/1.89 V and 0.65 V/0.26 V vs. Zn²⁺/Zn, respectively. Finally, it can be proved that the ZFP@C cathode smoothly experiences Zn²⁺ extraction-intercalation reaction in the bulk, and the conversion reaction to Zn₃(PO₄)₂·xH₂O and Fe(OH)₂ from the surface. We believe that these findings will open the application of iron-based phosphate as a kind of low-cost cathode materials for rechargeable AZIBs.