Configuration of gradient-porous ultrathin FeCo2S4 nanosheets vertically aligned on Ni foam as a noncarbonaceous freestanding oxygen electrode for lithium–oxygen batteries†
The degradation of oxygen electrodes caused by oxygen species in lithium–oxygen (Li–O2) batteries deteriorates their energy efficiency and cyclability and seriously hinders their commercial application. To achieve high energy efficiency and long-term cycle life, gradient-porous ultrathin FeCo2S4 nanosheets on Ni foam (FeCo2S4@Ni) were deliberately designed as a noncarbonaceous freestanding oxygen electrode for Li–O2 batteries. Notably, the gradient-porous structure in FeCo2S4@Ni can offer sufficient active sites as well as mitigate polarization caused by the mass transfer during discharge and charge. The synergistic effect of the two transition metals, Fe2+ and Co3+, optimizes their d-band electronic structure and enhances the intrinsic activity of the oxygen electrode. Benefiting from the above merits, the FeCo2S4@Ni based Li–O2 battery demonstrates greatly increased discharge capacity (8001 mA h g−1), improved rate capability (with a high capacity of 4401 mA h g−1 at 500 mA g−1), and enhanced cycling stability (with a low overpotential of below 1 V after 109 cycles). Our work demonstrates that the battery performance can be improved by regulating the structure and composition of the oxygen electrode and provides a promising strategy for developing high performance Li–O2 batteries.