Redox effect of Fe²⁺/Fe³⁺ in iron phosphates for enhanced electrocatalytic activity in Li-O₂ batteries

FePO₄ and porous Fe₂P₂O₇ laundry-ball-like nanostructures (FePO₄ LBs and p-Fe₂P₂O₇ LBs, respectively) were prepared to investigate their functionalities as oxygen-electrode (O₂-electrode) electrocatalysts in Li–O₂ batteries. These structures were synthesized in two steps, via hydrothermal and thermal reactions. FePO₄ LBs were synthesized through thermal dehydrogenation of as-prepared FePO₄·2H₂O precursors (FePO₄·2H₂O → FePO₄ + 2H₂O), and p-Fe₂P₂O₇ LBs were synthesized through thermochemical reduction of same precursors under an H₂ atmosphere (2FePO₄·2H₂O + H₂ → Fe₂P₂O₇ + 5H₂O). As an O₂-electrode electrocatalyst in Li–O₂ cells, p-Fe₂P₂O₇ LBs exhibited a higher discharge capacity (30,000 mA h g_catalyst^–1 at a current density of 500 mA g_catalyst^–1), higher reversibility (300 cycles at a current rate of 500 mA g_catalyst^–1), and lower voltage gap, compared to FePO₄ LBs. These superior performances of p-Fe₂P₂O₇ LBs result from the Fe²⁺/Fe³⁺ redox effect and porous structure, which enhance the oxygen reduction or evolution reaction activities.