Core@shell CoO@Co3O4 nanocrystals assembling mesoporous microspheres for high performance asymmetric supercapacitors

•Core@shell CoO@Co3O4 nanocrystals assembling mesoporous microspheres are prepared.•The CoOx sample shows a high specific capacitance of 3377.8 F g−1 at 2 A g−1.•The CoOx//graphene ASC exhibits excellent electrochemical performance.

Cobalt oxides (denoted as CoOx) mesoporous microspheres assembled by core@shell CoO@Co3O4 nanocrystals are successfully prepared via a solvothermal method followed by a special post annealing treatment. Benefiting from its unique structure, high porosity, and the synergistic effect between CoO and Co3O4, when employed as electrode materials for supercapacitors, the CoOx microspheres exhibit a high specific capacitance of 3377.8 F g−1 at a current density of 2 A g−1, which are far superior to those of individual CoO or Co3O4. In addition, the CoOx electrode shows an excellent rate property of 66.5% capacity retention at 20 A g−1 and can still remain a high specific capacitance of 1331.1 F g−1 after 4000 cycles at a high current density of 10 A g−1. Besides, an asymmetric supercapacitor (ASC) is constructed by using the CoOx microspheres as the positive electrode and graphene as the negative electrode. The ASC delivers a high specific capacitance of 123.9 F g−1 at 1 A g−1 and an energy density of 44.06 Wh kg−1 at a power density of 800 W kg−1. Furthermore, the CoOx//graphene ASC shows a good cycling stability with 74.9% specific capacitance retention after 6000 cycles at a high current density of 3 A g−1. This work demonstrates a facile synthesis strategy of CoOx mesoporous microspheres with outstanding electrochemical properties, which can be used in supercapacitors and other energy storage devices.

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