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.