Improved electrocatalytic activity of δ-MnO2@MWCNTs by inducing the oriented growth of oxygen reduction products in Li-O2 batteries

The carbon supported composite catalysts for Li-O2 batteries with optimized structure seem attractive in terms of their advantages of electronic conductivity and light weight. In this work, a facial solvothermal method was employed to prepare the composite with 34 wt% δ-MnO2 wrapped on the surface of MWCNTs in form of monomolecular layer by the chemical binding (δ-MnO2@MWCNTs). The experimental results indicate that δ-MnO2@MWCNTs retains the high specific surface area and abundant mesoporous structure of MWCNTs. Moreover, the dominated (002) facet and abundant oxygen vacancies of the δ-MnO2 layer accelerate the adsorption of O2 and elongate the O-O bond, which promote the oxygen reduction reaction activity. In particular, the oriented growth of discharge products (Li2O2) forming flake- and flower-like depositions with an increase of DOD ensures the stable reaction zone in the cathode with sufficient charge transfer and oxygen diffusion channel. Therefore, the Li-O2 batteries with δ-MnO2@MWCNTs as cathode catalysts demonstrate the high discharge capacity of 28517 mA h g−1 with the voltage platform of 2.82 V in the initial discharge (100 mA g−1) and the excellent rate performance with 75% capacity retention at current density of 1000 mA g−1. Meanwhile, the δ-MnO2 @MWCNTs catalysts are of benefit to the excellent electrochemical reversibility with 190 cycles (100 mA g−1) and 20 cycles (1000 mA g−1) under a limited discharge depth of 1000 and even 4000 mA h g−1, respectively.