mater.scichina.com link.springer.com Published online 24 June 2020 | https://doi.org/10.1007/s40843-020-1357-9 Sci China Mater 2020, 63(10): 2013–2027 Oxygen vacancies boosting ultra-stability of mesoporous ZnO-CoO@N-doped carbon microspheres for asymmetric supercapacitors Di Yao 1 , Fulei Wang 1 , Wu Lei 1* , Yan Hua 1 , Xifeng Xia 1 , Jinping Liu 2 and Qingli Hao 1* ABSTRACT Long-term cycling stability of pseudocapacitive materials is pursued for high-energy supercapacitors. Herein, the mesoporous zinc-cobalt oxide heterostructure@nitrogen- doped carbon (ZnO-CoO@NC) microspheres with abundant oxygen vacancies are self-assembled through a hydrothermal method combined with an annealing post-treatment. The multifunctional polyvinyl pyrrolidone (PVP) is used as a structure-directing agent, the precursor of NC and the in- itiator of abundant oxygen vacancies in zinc-cobalt oxide microspheres. XPS demonstrates the generation of surface oxygen vacancies resulted from the reduction effect of con- ductive NC, and further confirms the weaker interaction be- tween the metal ions and oxygen atoms. As a result, the electrode based on ZnO-CoO@NC in 2 mol L -1 KOH shows enhanced capacitive performance with an excellent cycle sta- bility of 92% retention of the initial capacitance after 40,000 charge-discharge cycles at 2 A g -1 , keeping the morphology unchanged. The assembled asymmetric supercapacitor, gra- phene//ZnO-CoO@NC, also performs good cyclic stability with 94% capacitance retention after 10,000 cycles at 2 A g -1 . The remarkable electrochemical performance of the self-as- sembled ZnO-CoO@NC composite is attributed to the meso- porous architecture, abundant oxygen vacancies, conductive ZnO scaffold for CoO crystals forming heterostructure of ZnO-CoO and the high conductive NC layer covering outside of the multi-metal oxide nanoparticles. Hence, the ZnO- CoO@NC holds great promise for high-performance energy storage applications. Keywords: supercapacitor, zinc oxide, cobaltous oxide, doped carbon, cycling stability, heterostructure INTRODUCTION Supercapacitors (SCs) are attractive alternatives to bat- teries with a great application potential and attracting ever-growing attention recently, due to their intriguing features such as high power capabilities, long cycle life and rapid charge/discharge processes [1–5]. But the low energy density of SCs, especially the electric double layer capacitors (EDLCs) based on carbonous materials, must be boosted further for the demand of next generation electronic devices. Pseudocapacitors as another kind of SCs, based on pseudocapacitive materials storing charges through Faradaic reactions, may reach several times higher specific capacitance and energy density than nor- mal EDLCs. With unique characteristics like environ- mental benignity and high specific capacitance, transition metal oxides are intensively explored as redox oxides with high pseudocapacitance for SCs. Cobalt oxides, like Co 3 O 4 [6,7] and CoO, also have high theoretical specific capacitance [8]. However, they suffer from the inferior rate capability and cycling stability because of their poor electrical conductivity and volume change during the charge/discharge process [9]. Therefore, combining elec- trically conductive components is considered to accel- erate the electron transfer rate of cobalt-based composites. For instance, CoO@NiO nano-architectures on the activated carbon textile [10], CoO@CNT-x nano- composite [11], Co 3 O 4 @PDA-rGO [12], and gold nano- particles-decorated rGO-ZnCo 2 O 4 nanocomposite [13] were successfully prepared for SCs and exhibited en- hanced electrochemical performances. With the ad- vantages of mechanical flexibility, excellent conductivity and easiness of combining with other materials, ZnO 1 Key Laboratory for Soft Chemistry and Functional Materials, Ministry of Education, School of Chemical Engineering, Nanjing University of Science and Technology, Nanjing 210094, China 2 Chemical Engineering and Life Science and State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, China * Corresponding authors (emails: leiwuhao@njust.edu.cn (Lei W); qinglihao@njust.edu.cn (Hao Q)) SCIENCE CHINA Materials ................................ ARTICLES October 2020 | Vol. 63 No.10 2013 © Science China Press and Springer-Verlag GmbH Germany, part of Springer Nature 2020