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 (O-2-electrode) electrocatalysts in Li-O-2 batteries. These structures were synthesized in two steps, via hydrothermal and thermal reactions. FePO4 LBs were synthesized through thermal dehydrogenation of as-prepared FePO4 center dot 2H(2)O precursors (FePO4 center dot 2H(2)O. FePO4 + 2H(2)O), and p-Fe2P2O7 LBs were synthesized through thermochemical reduction of same precursors under an H-2 atmosphere (2FePO(4)center dot 2H(2)O + H-2 -> Fe2P2O7 + 5H(2)O). As an O-2-electrode electrocatalyst in Li-O-2 cells, p-Fe2P2O7 LBs exhibited a higher discharge capacity (30,000 mA h gcatalyst-1 at a current density of 500 mA g(catalyst)(-1)), higher reversibility (300 cycles at a current rate of 500 mA gcatalyst-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.