An efficient, bifunctional catalyst for lithium-oxygen batteries obtained through tuning the exterior Co²⁺/Co³⁺ ratio of CoO_:X on N-doped carbon nanofibers

The design and fabrication of efficient catalysts are urgently desired for Li-O₂ batteries to prompt both the oxygen reduction reaction and oxygen evolution reaction. Morphology/surface optimization can be intensively applied to enhance the catalytic activity of specific materials. Herein, we propose an electrospinning method followed by heat treatment to fabricate CoO_x nanoparticles on N-doped carbon nanofibers (CoO_x@NCF). Significantly, the electrocatalytic activity can be largely improved through modulating the exterior Co²⁺/Co³⁺ ratio by controlling the heat-treatment process. The prepared CoO_x nanoparticles possess high uniformity together with good dispersion, which integrates the advantages of morphology and surface properties, making them an effective active catalyst for Li-O₂ batteries. The cell with the CoO_x@NCF catalyst shows impressively good performance towards the ORR and OER with a high initial discharge capacity (7763.7 mA h g^-1), enhanced cycling stability, desirable rate capability and relatively low overpotential. Moreover, the result of LSV indicates that CoO_x@NCF enables the decomposition of the side product LiOH efficiently. In addition, the CoO_x@NCF catalyst is capable of forming nanosheet-like Li₂O₂, which is much easier to decompose, compared with toroidal-like Li₂O₂. The boosted cycling life can be attributed to the synergistic effect of architecture design and surface engineering.