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

Cobalt oxides (denoted as CoO_x) 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 CoO_x 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 CoO_x 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 CoO_x 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 CoO_x//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 CoO_x mesoporous microspheres with outstanding electrochemical properties, which can be used in supercapacitors and other energy storage devices.