Structure Design And Electrochemical Study Of Transition Metal Sulfides/Oxides Based Pseudocapacitor Materials

Supercapacitors（SCs）are deemed as a new type of efficient electrochemical energy storage device,because of their high power density and ultralong cycle life,contributing to solving the global energy storage issue.Transition metal sulfides/oxides have attracted numerous attentions owing to high theoretical specific capacity and electrochemical activity.However,hindered by the poor electrochemical kinetics of the transition metal sulfides/oxides electrode reported at present,the SCs based on transition metal sulfides/oxides inevitably suffer from the problem of insufficient energy density,so that they can hardly fulfill the diverse requirements of future electronic equipment for high-efficiency energy storage devices.In order to solve this problem,we design and develop several new transition metal sulfides/oxides materials for efficient SCs.Through reasonable structural design,the kinetics behavior between electrode and electrolyte is regulated,the electrochemical performance is improved and the energy density of SCs is greatly promoted.This paper mainly focuses on the structure design and electrochemical study of transition metal sulfides/oxides electrodes in SCs.From increasing the specific capacitances and expanding the working voltage window of SCs,the problem of insufficient energy density faced by supercapacitors in practical application is effectively solved.On the one hand,through the scientific and reasonable design of the transition metal sulfides/oxides,we try to improve the specific capacitances of the pseudocapacitor electrodes,especially to ensure that the electrodes can still maintain high specific capacitances under large current densities and after repeated cycles.On the other hand,based on the features of electrolytes,pseudocapacitor electrodes with matching structure are designed.By optimizing the interface structure between electrode and electrolyte,electrochemical performance of transition metal sulfides/oxides is further improved,which greatly increases the energy density of SCs and expands their practical applications.The main research contents and obtained progresses are as follows:1.A three-dimensional microcapsules encapsulated nickel sulfide electrode has been prepared,which promotes the electrical conductivity and overcomes the volume expansion of nickel sulfide during repeated charge-discharge cycles.The microcapsule electrode delivers a specific capacitance of 1600 F g^-1,as well as high rate performance and long cycle life in aqueous electrolyte.On the basis of 3D nickel sulfide microcapsule electrode,as-fabricated SCs show high energy density(48.02 Wh kg^-1)and ultrahigh power density.2.Aiming at the shortcoming of low tap density of the microcapsule electrode,we focused on the consideration of high tap density and rapid electrochemical reaction of nickel sulfide electrode.We pioneered a three-dimensional in-situ growth strategy to prepare a novel pomegranate-like dual nickel sulfides electrode in order to improve energy density of SCs,possessing higher tap density,electrical conductivity and structural stability.The pomegranate-like electrode not only achieves a high specific capacitance of 2331.3 F g^-1,but also exhibits ultralong cycling life.Furthermore,the assembled supercapacitors achieve an energy density up to88 Wh kg^-1.3.To further boost the energy density of SCs,we started from the perspective of expanding the operating voltage window of SCs.A mesoporous iron oxide electrode（P-Fe₂O₃/G）showing typical pseudocapacitance in ionic liquid with a wide working voltage window was prepared.Benefiting from the appropriate structure,P-Fe₂O₃/G electrode shows good electrochemical behavior in ionic liquid electrolyte.The supercapacitors based on P-Fe₂O₃/G can still show good capacitance behavior in a wide working voltage range of 0～3.5 V,achieving a very high energy density(98.75 Wh kg^-1).4.In order to improve the pseudocapacitive reaction of metal oxides in ionic liquid,a 3D bimetal oxide（Mn Fe₂O₄）electrode with higher electrochemical activity and specific capacitance was innovatively designed.The interface structure between electrode and ionic liquid was studied using differential capacitance.It was proved that there was a characteristic adsorption between the electrode and the cation of the ionic liquid electrolyte,which significantly enhanced the pseudocapacitance reaction of Mn Fe₂O₄ electrode,leading to better electrochemical performance.Furthermore,the assembled flexible SCs display an attractive energy storage capacity(68μWh cm^-2)and exhibit long cycle life in various states.