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

Abstract Iron ion batteries using Fe 2+ as a charge carrier have yet to be widely explored, and they lack high‐performing Fe 2+ hosting cathode materials to couple with the iron metal anode. Here, it is demonstrated that VOPO 4 ∙2H 2 O can reversibly host Fe 2+ 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, VOPO 4 ∙2H 2 O's capacity of hosting Zn 2+ fades precipitously over tens of cycles. VOPO 4 ∙2H 2 O stores Fe 2+ with a unique mechanism, where upon contacting the electrolyte by the VOPO 4 ∙2H 2 O electrode, Fe 2+ ions from the electrolyte get oxidized to Fe 3+ ions that are inserted and trapped in the VOPO 4 ∙2H 2 O structure in an electroless redox reaction. The trapped Fe 3+ ions, thus, bolt the layered structure of VOPO 4 ∙2H 2 O, which prevents it from dissolution into the electrolyte during (de)insertion of Fe 2+ . The findings offer a new strategy to use a redox‐active ion charge carrier to stabilize the layered electrode materials.