Hierarchical hollow microspheres grafted with Co nanoparticle-embedded bamboo-like N-doped carbon nanotube bundles as ultrahigh rate and long-life cathodes for rechargeable lithium-oxygen batteries

Rational design of efficient, affordable, and durable electrocatalysts for the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) is essential for rechargeable lithium-oxygen (Li–O₂) batteries. We present for the first time hierarchical hollow microspheres grafted with metallic Co-embedded bamboo-like N-doped carbon nanotube bundles (Co-b-NCNTs hollow microspheres) as oxygen electrodes for Li-air batteries. Hierarchical composite microspheres are prepared via a facile two-step process involving synthesis of Co₃O₄-MgO hollow microspheres by spray pyrolysis, followed by internal and external growth of bamboo-like NCNTs in the shells. During post-treatment, metallic Co and MgO nanoparticles play key respective roles in catalyzing in-situ growth of NCNTs and maintaining structural integrity of the composites. The hierarchical composite structure with Co and N doping not only provides ample active sites for the OER and ORR, but also sufficient space for storing produced Li₂O₂. Thus, Co-b-NCNTs hollow microspheres exhibit high initial round-trip efficiency, long-term cycling and ultrahigh rate performances when applied as oxygen electrodes for Li–O₂ batteries. The initial discharge capacity and round-trip efficiency at a current density of 200 mA g⁻¹ are 28,968 mA h g⁻¹ and 78.2%, respectively. Specific capacities at cutoff capacities of 500 and 1000 mA h g⁻¹ are stable for 201 and 157 cycles, respectively.