Highly efficient hierarchical multiroom-structured molybdenum carbide/carbon composite microspheres grafted with nickel-nanoparticle-embedded nitrogen-doped carbon nanotubes as air electrode for lithium-oxygen batteries

Designing an efficient and effective air electrode catalyst material is the most intrinsic requisite for rechargeable lithium-oxygen (Li-O₂) batteries showing long cycling lives and high rate capacities. Here, we present for the first time, hierarchical multiroom-structured molybdenum carbide/carbon composite microspheres grafted with Ni nanoparticle-embedded N-doped carbon nanotubes (NCNTs) (i.e., mNi-NCNT-MoC-C), which are prepared by pilot-scale spray drying and subsequent surface growth of NCNT bundles. The designed mNi-NCNT-MoC-C microspheres show efficient bifunctional catalytic activities toward both oxygen reduction and evolution. In addition, the hierarchical multiroom structure of mNi-NCNT-MoC-C microspheres increases the discharge capacity by providing sufficient space to accommodate Li₂O₂, which forms during discharging. A Li-O₂ battery prepared using mNi-NCNT-MoC-C microspheres as an air electrode exhibits excellent electrochemical performances including a long cycle life (199 cycles) and low overpotentials of 0.20 and 0.21 V for charging and discharging, respectively. The synergetic effects of efficient morphology (i.e., the multiroom structure), highly electrocatalytically active materials (i.e., NCNT, Ni, and MoC), and high electrical conductivity (imparted by the NCNTs) is responsible for the superior performance of mNi-NCNT-MoC-C microspheres as an air cathode material for Li-O₂ batteries.