Nowadays, the lithium-oxygen battery has captured world-wide attention recently because of its extremely high theoretical energy density. A typical nonaqueous Li-O₂ battery consists of a lithium−metal anode, organic electrolyte, and a porous air cathode exposed to gaseous O₂ during cell operation. The air electrode is crucial to improve the electrochemical performance for rechargeable nonaqueous lithium–air batteries. Among the components of the air electrode, the catalyst is important in that it can enhance the charge reaction by reducing the voltage required to dissociate the reaction products (such as Li₂O₂) into lithium metal and oxygen. The electrocatalyst is currently being investigated with special attention being paid to solve the sluggish kinetics related to the main electrochemical reactions, as well as the instability of the discharge products. Recently, a variety of electrocatalysts, including noble metals, transition metal oxides and carbon-based materials, have been explored in Li-O₂ cells with a high reversible capacity and a lower charge potential for oxygen evolution reaction (OER) than bare carbon. A problem for the cathode lies in that the solid discharge products are insoluble and thus precipitate in the pores of cathode, which gradually block the catalytic sites as well as the diffusion pathways of electrolyte and oxygen, especially for the inner part next to the separator, and eventually degrade the performance of Li-O₂ batteries. Considering this point, how to improve the pore utilization and promote the O₂ transport in the electrode plays an important role in determining the battery performance. Therefore, it is challenging and highly desirable to develop optimum electrocatalysts to efficiently catalyze Li-O₂ reactions while simultaneously facilitate rapid oxygen and electrolyte diffusion.

In this work, we present a facile solvothermal synthesis of 3D hierarchical spinel core-shell microspheres with a porous structure and applied as efficient 3D electrocatalysts in nonaqueous Li-O₂ batteries. Such unique structure can improve the availability of the catalytic sites and facilitate the diffusion of electrons and reactants. Due to the synergistic effect of high catalytic activity of the NiCo₂O₄ and unique porous core-shell, the 3D hiercarchical NiCo₂O₄ catalyst shows superior specific capacity, rate capability, and cycle stability in Li-O₂ batteries. Furthermore, other spinel oxides (e.g. MnCo₂O₄ and ZnCo₂O₄) are also have been synthesized through similar procedures, suggesting the generality and feasibility of this facile strategy.

The superior catalytic performance of NiCo₂O₄ was further examined in Li-O₂ batteries. It is found that the Li–O₂ battery with the NiCo₂O₄ electrode exhibits rather stable specific capacities above 7000 mAh g^-1_carbon for five cycles and can keep specific capacities to be 6100 mAh g^-1_carbon after 10 cycles at a current density of 100 mA g^-1_carbon. Such performance can make NiCo₂O₄ core-shell microspheres as one of the best mixed metal oxides catalysts in Li-O₂ batteries.

Remarkably, it is found that our strategy could be extended to fabricate other spinel catalyst electrodes, such as MnCo₂O₄ and ZnCo₂O₄, suggesting the generality and feasibility of this facile strategy.

In summary, we have demonstrated controlled synthesis of hierarchical spinel core-shell microspheres by a facile solvothermal method. The spinel microspheres constructed by porous nanoplates can act as high-performance catalysts in Li-O₂ batteries. By taking advantage of the superior electrocatalytic activity and porous unique features, the as-prepared catalyst electrode exhibited low overpotentials, high rate capacity as well as excellent long-term cyclability. Significantly, this work offers exciting possibilities for the development of new functional materials in high-density storage devices

Acknowledgements

The authors are thankful for the support by funding from the Singapore National Research Foundation (NRF-CRP10-2012-06).