Fe-Ion Bolted VOPO₄ ∙2H₂ O as an Aqueous Fe-Ion Battery Electrode

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 ∙2H₂ O 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 ∙2H₂ O's capacity of hosting Zn²⁺ fades precipitously over tens of cycles. VOPO₄ ∙2H₂ O stores Fe²⁺ with a unique mechanism, where upon contacting the electrolyte by the VOPO₄ ∙2H₂ O electrode, Fe²⁺ ions from the electrolyte get oxidized to Fe³⁺ ions that are inserted and trapped in the VOPO₄ ∙2H₂ O structure in an electroless redox reaction. The trapped Fe³⁺ ions, thus, bolt the layered structure of VOPO₄ ∙2H₂ O, 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.