Mesoporous MnCo₂S₄ nanosheet arrays as an efficient catalyst for Li-O₂ batteries

Ternary metal sulfides and ternary metal oxides have received much attention as potential electrodes for high performance rechargeable batteries. Herein, MnCo₂S₄ nanosheets are grown on carbon paper (MCS/CP) via facile electrodeposition followed by low temperature vulcanization for application in Li-O₂ batteries for the first time. The electrochemical performance of freestanding, binder-free MCS/CP oxygen electrodes is compared with those prepared from MnCo₂O₄ nanosheets on CP (MCO/CP). The MCS/CP electrode delivers an extremely high initial specific capacity of 10760 mA h g^-1, twice that of MCO/CP. The former electrode sustains 96 cycles at an upper limit capacity of 500 mA h g^-1 at 200 mA g^-1, whereas the latter counterpart survives only a few cycles with a poor round trip efficiency. The superior performance of MCS/CP is in part proven by the four times higher electrical conductivity and 250% higher lithium diffusion coefficient than MCO/CP. In addition, the 3D interconnected web of 2D MCS nanosheets offers a few micrometer open voids to accommodate discharge products and a large surface area with internal mesopores providing abundant active sites. The density functional theory calculations reveal a lower adsorption energy for LiO₂ on the surface of MCS than on MCO, which is responsible for the lower OER overpotential and the higher catalytic ability of MCS/CP. The predicted density of states signifies metallic properties of MCS in agreement with the high electrical conductivity of MCS/CP.