Design And Flexibility Of High Performance Asymmetric Supercapacitors

Supercapacitors, as an important energy storage device, with superior power density, fast charge/discharge rates and long cycle life, have attracted significant attention and been widely applied in electrical/hybrid automobiles and portable electronics. However, Low energy density is the shortcoming of supercapacitors, which greatly limits their practical application. How to enhance the energy density without sacrificing their power density remains a great challenge for supercapacitors. Increasing the operating voltage is an efficient method to improve the energy density. Asymmetric supercapacitors (ASCs) are designed and arranged to extend the operating voltage by making use of the different potential window of the two electrodes (Positive electrode has more positive potential window compared with the negative electrode). Despite significant progress has been made in ASCs, many works have been focused on exploring positive electrode materials, while negative electrode materials drew much less attention. Recently, the development of flexible, wearable and miniature electronic devices require more for structures and performances of the energy storage devices. Based on the above consideration, we have tried to improve energy density by searching and fabricating the negative electrode with higher performance, designing the electrode structures with novel and enhancing the operating voltage. Moreover, we have constructed flexible electrode and designed different device architectures to meet the requirement for supercapacitors. The main contents and results are described as follows:(1) Three-dimensional hierarchical CoSe2 nanostructures were successfully grown on conductive carbon cloth by a facile two-step hydrothermal method. In the three-electrode system, the CoSe2/carbon cloth electrode exhibited good electrochemical performance. Impressively, the CV curves of the CoSe2/carbon cloth electrode ranged from -0.8-0 V vs SCE, implying three-dimensional hierarchical CoSe2 nanostructures can be potential candidate for negative electrode materials. ASCs were fabricated using CoSe2/carbon cloth, MnO2/carbon cloth and PVA/LiCl as negative electrode, positive electrode and solid electrolyte, respectively. The prepared ASCs showing high maximum energy density of 0.588 mWh/cm3, superior electrochemical stability and excellent flexibility and mechanical stability.(2) ASCs were fabricated with voltage window between 0-1.7 V by using MnO@C nanosheets as negative electrode and Co3O4 nanowires as positive electrode, which were directly grown on conductive carbon cloth with simply hydrothermal method. Compared with working in the voltage window of 0-0.6 V, the energy density of the ASC was enhanced by 2560%, and maximum energy density of 52.98 Wh/kg was achieved when the working voltage was extended to 1.7 V.(3) We demonstrated direct growth of ultrathin MnO2 nanosheet arrays on conductive carbon fibers with simply hydrothermal method, which exhibited a high specific capacitance of 634.5 F/g at a current density of 2.5 A/g and possessed superior cycle stability. Fiber-shaped all-solid-state ASCs were constructed by assembling MnO2/carbon fiber electrodes (positive electrode), graphene/carbon fiber electrodes (negative electrode), and PVA-LiCl gel electrolytes (both the ionic electrolyte and separator). The prepared fiber-shaped all-solid-state ASCs exhibited excellent flexibility, electrochemical stability and even can be woven into fabric. In addition, as-prepared fiber-shaped ASCs could successfully power a photodetector based on CdS nanowires without applying any external bias voltage.(4) We have researched high performance and stretchable fiber-shaped ASCs in this work. MnO2 nanosheets and RGO nanosheets were grown on the highly conductive 316 L Stainless steel (SW). Then, a stretchable ASC was successfully fabricated by winding aligned MnO2/SW and RGO/SW as positive and negative electrode, respectively, PVA/LiCl gel electrolytes as both ionic electrolyte and separator. The maximum operating voltage of the stretchable ASC reached 1.6 V and electrochemical performance remained almost unchanged even when the ASC was stretched up to 400%.