Configuration of gradient-porous ultrathin FeCo2S4 nanosheets vertically aligned on Ni foam as a noncarbonaceous freestanding oxygen electrode for lithium–oxygen batteries

Abstract

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.