Construction And Electrochemical Properties Research Of Manganese Oxides-based Electrode Materials For Supercapacitors

With the increasing depletion of fossil energy and the rapid development of new energy systems,the requirements for energy storage devices in various fields of society are gradually increasing.Supercapacitors have been widely used in new energy vehicles,portable electronic products and smart power systems due to their high power density and ultra-long cycle life.As an electrode material for high-performance supercapacitors,manganese oxides have attracted extensive attention from researchers at home and abroad due to their rich content,variable structure,and high theoretical specific capacitance.Although the specific capacitance value of manganese oxide electrode materials can be improved through carbon material composite and nanometerization,the actual capacitance is far from the theoretical specific capacitance value.According to the calculation formula of E=CV²/2,we can start from two aspects in order to improve the energy density:increasing the specific capacitance value and broadening the working voltage range.The key to increase the energy density is to improve the charge storage performance of the electrode active material and widen the working voltage range of the device.This paper systematically reviews the research progress of supercapacitors and the research status of manganese oxides in supercapacitors.With the goal of constructing manganese oxide-based transition metal oxide nanoelectrode materials with excellent electrochemical performance and assembling supercapacitors with excellent energy-storage performance,in-depth research has been carried out on the preparation and structural regulation of manganese oxide nanostructures.High-performance manganese oxide-based electrode systems and asymmetric supercapacitor devices were constructed.The specific work is as follows:（1）In order to improve the electrochemical and cycling performance of transition metal manganese oxides,carbon fibers were prepared by electrospinning,pre-oxidation and carbonization in this study,andα-K_0.2Mn Ti_0.07O_0.8 hollow fiber electrode was prepared by hydrothermal reaction in neutral solution.The hollow nanotubes with high K contents are beneficial to the migration of ions and the stabilization of the structure,which can effectively promote the charge conduction of the electrodes.Directly used as an electrode material for supercapacitors,it exhibits excellent cycling stability(60000 cycles at 4 A g^-1,with a capacitance retention of 88%)and capacitance performance(334.5 F g^-1).The study found that K in theα-K_0.2Mn Ti_0.07O_0.8 hollow fiber is easily replaced by Na⁺ions in the electrolyte,and the constructedα-K_0.2Mn Ti_0.07O_0.8//Na ion supercapacitor has a maximum output energy density of 25.16 Wh kg^-1,and has good rate capability.（2）In order to reveal the growth process of different morphologies of the manganese oxide-based matrix and increase the interfacial bonding degree of the composite electrode,the method of electrochemical in-situ induction of MnO₂/PANC（polyacrylonitrile carbon）nanostructures in Na₂SO₄ electrolyte was used to obtain polymorphic manganese oxide-based nanostructured electrodes.By adjusting the reaction time and the number of electrochemical cycles,the morphology,structure and electrochemical performance of the nanoelectrodes were optimized.The evolution process of composite nanostructures was studied by scanning electron microscopy and transmission electron microscopy.Combined with other physicochemical characterization methods and electrochemical techniques,the phase transition of MnO₂nanosheets to nanowire porous structures during the enlargement of PANC nanolayers was confirmed.The manganese oxide-based electrode with the obtained porous core-shell structure was tuned to exhibit outstanding electrochemical performance(capacitance value of 632.1 F g^-1 at 1 A g^-1,and the capacitance retention rate is 96.4%after 20,000 cycles at 4 A g^-1),which can be attributed to the interaction of Na⁺with MnO₂/PANC to form a porousα-Na_0.28MnO₂/PANC_0.1 layered structure with obvious defects and increased ion adsorption/desorption flux during the electrochemical induction process.The constructed MnO₂/PANC//Fiber/PANC asymmetric supercapacitor has a maximum energy density of 49.58W h kg^-1.（3）For achieving high-performance electrode materials with highly studied value,the MnO₂/Mn₃O₄ composite nanospheres were synthesized in situ in the original MnO₂ sheet,and the MnO₂/Mn₃O₄ composite nanospheres were synthesized in a neutral electrolyte in this study.Chemical induction in situ was employed to the formation of porous manganese oxide-based nanosheet structures.It was found that the nanosheet structure and electrode electrochemical performance tended to be stable after four electrochemical cycles at 1 A g^-1.The formation process of the nanosheet structure was studied by scanning electron microscopy,and the phase transition from MnO₂/Mn₃O₄ nanospheres to layered MnO₂ was confirmed by combining with other physicochemical characterization methods.Based on the concentration diffusion of Na⁺and a small amount of SO₄^2-at the top and bottom of the nanosheets and the pre-insertion of Na⁺in the interlayer gap during the electrochemical process,the phase-transformed nanosheet-structured electrodes exhibited excellent specific capacitance(1 A g^-1,the capacitance value is1709 F g^-1)and electrochemical cycling stability(capacitance retention rate of 71.7%after20,000 cycles at 10 A g^-1).The constructed OCFC-MnO₂/Mn₃O₄//SFCF asymmetric supercapacitor exhibits excellent electrochemical capacitive performance,an energy density as high as 234 W h kg^-1 at a power density of 724.9 W kg^-1,and high rate capability.（4）By pre-introducing Ag quantum dots into the manganese oxide nanostructure,the voltage window,capacitance performance and cycle stability performance of the manganese oxide nanosheet electrode are greatly improved.Phase-change nanostructures modified by Ag-QDs with graded insertion of MnO_x nanosheets during electrochemical effect.Pre-modified silver quantum dots have been proved to be able to stabilize physical and chemical properties,regulate manganese valence states and enhance the electroactive transfer of manganese oxide based electrodes.meanwhile,Ag-QDs are evenly embedded in the nano-slice and rearranged to produce surface transmission areas.Cyclic MnO_x-Ag-QDs electrode exhibits excellent electrochemical performance due to its outstanding characteristics including unique nanostructure,introduced defects and effective ways of ion diffusion and electron transport in electrode materials.More importantly,the constructed MnO_x-Ag-QDs//COCFC asymmetric supercapacitors exhibit superior energy density storage performance and application value.