Viable Synthesis of Porous MnCo2 O4 /Graphene Composite by Sonochemical Grafting: A High-Rate-Capable Oxygen Cathode for Li-O2 Batteries.

With an anticipation of their use in electric vehicles, Li-O2 batteries are found to be attractive despite their complex chemistry and drawbacks. To be successful, cathode materials that are robust enough to overcome the sluggish kinetics of the charge-discharge reactions are essential. This work reports sonochemically synthesized porous MnCo2 O4 /graphene (MCO/G) as a hybrid cathode material in nonaqueous Li-O2 batteries. The MCO/G hybrid is synthesized in less than four hours and offers a strong synergistic coupling between the MnCo2 O4 nanospheres and graphene sheets. It catalyzes the oxygen reduction through a three-electron-transfer process and initiates the oxygen evolution at 1.55 V vs. RHE in basic medium. A small charge-discharge voltage hysteresis of 0.8 V and a cycle life of 250 cycles at a limited capacity of 1000 mAh g-1 in a tetraglyme-based nonaqueous Li-O2 battery is demonstrated. The porous channels created on the sonochemically synthesized cathode facilitates easy oxygen adsorption onto the active sites to accommodate more discharge products following its decomposition. It exhibits a better rate capability in comparison to the widely used Vulcan carbon and benchmark Pt/C catalysts. The excellent cyclability, rate capability, and low overpotential demonstrates MnCo2 O4 /graphene composite as a promising cathode candidate for Li-O2 batteries. The porous nanosphere architecture with internal oxygen diffusion pathways and peripheral conductive graphene extensions fulfils the requirements that a robust cathode is expected to have to overcome the harsh Li-O2 battery conditions and to serve as a high-rate-capable cathode for Li-O2 batteries.