| Supercapacitor is an electrochemical device with good rate capability and cycling stability.It behaves much higher energy density than that of traditional capacitor,but is inferior to lithium battery.According to the formula of E=CV2/2,to get higher energy density,we can choose to increase the specific capacitance or the working voltage range.Asymmetric capacitors have varied positive and negative materials,which can make full and flexible use of their specific capacitance and working voltage range,optimizing the energy storage performance of the whole device.It is an inevitable choice to develop excellent electrode materials,study its design scheme,preparation process,and energy storage mechanism to effectively improve the energy density of asymmetric supercapacitors.The range of manganese-containing oxide electrode materials is wide,the preparation methods are relatively simple,the valence states of manganese are diverse,the structures are changeable,and the theoretical specific capacitance is high.These features are in line with the design requirements of active materials for supercapacitors.However,the current research on manganese-containing oxide supercapacitor cathode materials mainly has these following weak points.Firstly,the poor conductivity of the manganese-containing electrode materials is an important factor that limits its specific capacitance and rate performance;some composite materials have monotonous design routes that are relatively high cost,environmentally unfriendly,and limited in comprehensive energy storage performances.Secondly,some materials have unstable structure and poor chemical stability,resulting in poor cycling stability.Thirdly,due to the obvious difference of the energy storage mechanisms between porous activated carbon materials and manganese-containing electrode materials,only increasing the specific surface area may not improve the specific capacitance and rate performance,while the attention should be paid to the pore structures.Fourthly,the lack of analysis on the energy storage mechanism of manganese-containing electrode materials is not conducive to the in-depth understanding of its energy storage mechanism.Based on the existing studies,this experiment took manganese-containing silicate materials as the starting point,followed different routes to explore and optimize the process,and developed several manganese-containing oxide materials with good performance for supercapacitors,and their physical and chemical properties and electrical properties were characterized,analyzed and discussed.In this experiment,these manganese-containing oxides were used as cathode materials to prepare asymmetric supercapacitor devices,which exhibited energy storage advantages at different current densities.The details are as follows:Firstly,the cycle stability of MnSiO3 is improved by carbon coating with pyrrole as the carbon source and subsequent CO2 heat treatment.The obtained MnSiO3/C composite material has a hierarchical pore structure,a thinner and more ordered carbon coating layer,higher electrical conductivity,and uniform and mixed manganese valence states.In 1 M Na2SO4 electrolyte,the CV curve is quasi-rectangular,and MnSiO3/C exhibits continuous and reversible pseudocapacitance energy storage properties,and excellent cycling stabilities.By testing the materials after 1000 cycles of charge and discharge,the morphologies,chemical states,and electrochemical properties of the MnSiO3/C composite material change very little,which proves that MnSiO3/C has a better structure and chemical stability in the rapid energy storage process,which is consistent with its excellent cycling stability.By observing the MnSiO3 material after heat treatment without adding pyrrole,it is found that the addition of pyrrole not only provides a thin carbon coating layer but also prevents the excessive growth of the particle size of the material.Through the assembly of asymmetric supercapacitors,it is found that the corresponding device has an energy density of 25.8 Wh kg-1 at the power density of 1 kW kg-1.After the 10000 cycles,it behaves 95.5%retention.Secondly,Mn3O4 nanoparticles with high crystallinity,hierarchical pore size distribution,mixed manganese valence states,and rich oxygen defects were obtained by hydrothermal treatment of MnSiO3 precursor with 4 M NaOH.In the electrolyte of 1 M Na2SO4,Mn3O4 exhibits continuous and reversible pseudocapacitive energy storage properties,which is affected by both capacitive and diffusion-controlled storage mechanisms.It has good specific capacitance of 260 F g-1 at 1 1 A g-1 and good cycling performance.The results show that the assembled asymmetric supercapacitor has an energy density of 40.2 Wh kg-1 at a power density of 500 W kg-1,and the retention could reach 96.9%after 5000 cyclesThirdly,MnO2/NH4MnF3 core-shell nanorods were synthesized by hydrothermal treatment of MnSiO3 precursor with 1.6 M NH4F solution and compared with the pure NH4MnF3 particles and ?-MnO2 nanorods.The MnO2/NH4MnF3 composite material has two phases that are closely combined and doped with each other,more Mn(IV),more oxygen defects,and a hierarchical pore structure.In 1 M Na2SO4,this material has a capacitive and diffusion-controlled hybrid energy storage mechanism,with good rate and cycling performances.The asymmetric device has an energy density of 5.2 Wh kg-1 at the power density of 17 kW kg-1,and the retention is 93.6%after 10000 cycles.In 1 M KOH,the energy storage mechanism of MnO2/NH4MnF3 is also hybrid,which is closer to pure MnO2 but is also affected by pure NH4MnF3.The rate capability is excellent at 100 A g-1,which is 50.8%of 1 A g-1.Its asymmetric device has an energy density of 4.6 Wh kg-1 at the power density of 64 kW kg-1,and the retention is 96.1%after 10000 cycles.Finally,through the process optimization,the MnSiO3/CuSiO3 precursor was treated with 4 M NaOH at 180? for 5 h.The amorphous precursor transforms into a branch-like CuMnO2 with nanosheets gathered near a nanorod,with mixed-valence states of manganese and copper and a continuously distributed hierarchical pore structure.In 1 M KOH,the redox reaction of CuMnO2 at the electrode/electrolyte interface is mainly controlled by the diffusion process,but is also affected by the capacitive behavior.Compared with the precursor and 1 h,24 h hydrothermal samples,the 5 h sample has more capacitive energy storage.This material has good specific capacitance,rate capability,and cycling stability.The rate capability at 30 A g-1 is 39.1%of that at 1 A g-1.Its asymmetric device has an energy density of 7.2 Wh kg-1 at the power density of 32 kW kg-1,and the retention is 90.1%after 15000 cycles.The manganese silicate precursors studied in this dissertation have the advantages of abundance,simple preparation method,small pore size structures and large specific surface area.Four kinds of manganese-containing oxide supercapacitor electrode materials prepared by carbon coating or hydrothermal etching process have excellent performance,thus expanding the selection range of manganese-containing oxide supercapacitor cathode materials.These materials have unique morphology,mixed manganese valence states,hierarchical pore structure,high surface activity,low resistance,and the hybrid energy storage mechanism.They show good electrochemical performance and have important research significance as well as application value for the research and development of new manganese oxide supercapacitors. |
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