Research And Application Of Electrochemical Performance Of Transition Metal Compound Flexible Supercapacitors

Supercapacitors,as important energy storage devices,with s fast charge/discharge rate and long cycle life,have attracted significant attention and been widely applied in portable and wearable electronic equipment.However,supercapacitors show a slightly lower energy density than other energy storage devices.At present,electrodes always use metal-based current collectors with heavy mass to transfer current and to load active materials,which have no contribution to capacity.To resolve this problem and improve the energy density,flexible self-standing positive and negative materials without the use of metal current collectors have been designed and studied in this thesis.Asymmetrical all-solid flexible supercapacitors with enhanced working voltage and energy density have been built based on the materials obtained.The main work and research results are as follows:Firstly,in order to explore negative materials with high-performance for supercapacitors.A simple method was proposed to deal with a kind of common carbon cloth,through which high-performance flexible self-standing negative electrode was fabricated.The active carbon cloth exhibits a wide electrochemical potential window,high capacitance capacity and excellent cycle performance in the Na₂SO₄ solution.At the same time,the flexible carbon cloth fiber also shows excellent performance in 2M KOH solution.Secondly,MnO₂ grown on acid-treated CNT surface have been successfully prepared by a water bath method.Then,self-standing,flexible layered films for pseudocapacitor electrodes were fabricated via stacking MnO₂@CNTs composite layers and pure CNTs layers alternately.The layered film displayed an excellent volumetric capacitance of 177.5 F cm^-3 at 0.2 A g^-1.It also exhibited excellent rate capability(only about 15%fading for the capacitance when the discharge current increased from 0.2 A g^-1 to 5 A g^-1),outstanding cycling stability and high flexible ability.Moreover,a symmetric flexible cell was also assembled,and it exhibited a good conductivity,an excellent ion conductance,and an outstanding electrochemical performance.Thirdly,NiCo₂O₄@CNT was fabricated by a coprecipitation method followed by annealing in air,which displays an excellent specific capacitance of 1590 F g^-1 at 0.5 A g^-1.To further explore the practical application of NiCo₂O₄@CNT Air,lightweight,flexible and self-standing film electrodes were fabricated via stacking NiCo₂O₄@CNT and CNT alternately through vacuum filtration.Finally,an asymmetric all-solid-state supercapacitor was assembled using the optimized NiCo₂O₄@CNT/CNT film electrode as the positive electrode and a treated carbon cloth as the negative electrode,which has high energy density(²7.6 Wh kg^-1),good capacity retention?retaining⁹5%of the initial value after 5,000 cycles?and excellent flexibility,demonstrating excellent potential for flexible supercapacitor.Fourthly,NiCo₂S₄@CNT composite with low crystallinity was obtained through co-precipitation,ion exchange reaction and low temperature annealing.Then the composite and CNT was stacked alternately through vacuum filtration to obtain flexible and self-supporting NiCo₂S₄@CNT film with high conductivity and high porosity.The film electrode not only exhibits a high volumetric capacity of 443 F cm^-3 but also an excellent rate performance(only3.8%fading,from 0.2 A g^-1 to 5 A g^-1).Furthermore,a flexible asymmetric supercapacitor device was prepared,which exhibits outstanding high energy density and power density,indicating high commercial prospect and value.In this paper,a flexible,self-standing carbon fiber for supercapacitors negative electrode materialandthreecathodefilmsMnO₂@CNT/CNT,NiCo₂O₄@CNT/CNT,NiCo₂S₄@CNT/CNT were prepared separately by low-cost,environmentally-friendly,simple methods.They were assembled to be a series of all-solid flexible asymmetric supercapacitors and their electrochemical performance were studied systematically.These studies are of great significance for the design of all-solid-state flexible supercapacitors and commercialization.