Fe‐Ion Bolted VOPO₄∙2H₂O as an Aqueous Fe‐Ion Battery Electrode

Abstract Iron ion batteries using Fe²⁺ as a charge carrier have yet to be widely explored, and they lack high‐performing Fe²⁺ hosting cathode materials to couple with the iron metal anode. Here, it is demonstrated that VOPO₄∙2H₂O can reversibly host Fe²⁺ with a high specific capacity of 100 mAh g⁻¹ and stable cycling performance, where 68% of the initial capacity is retained over 800 cycles. In sharp contrast, VOPO₄∙2H₂O's capacity of hosting Zn²⁺ fades precipitously over tens of cycles. VOPO₄∙2H₂O stores Fe²⁺ with a unique mechanism, where upon contacting the electrolyte by the VOPO₄∙2H₂O electrode, Fe²⁺ ions from the electrolyte get oxidized to Fe³⁺ ions that are inserted and trapped in the VOPO₄∙2H₂O structure in an electroless redox reaction. The trapped Fe³⁺ ions, thus, bolt the layered structure of VOPO₄∙2H₂O, which prevents it from dissolution into the electrolyte during (de)insertion of Fe²⁺. The findings offer a new strategy to use a redox‐active ion charge carrier to stabilize the layered electrode materials.