Study On The Structure Regulation Of Copper Sulfides And Their Performance For Supercapacitor

Faced with the environmental problems and energy crisis caused by the use of fossil fuels,renewable energy should be vigorously developed,such as wind,solar and tidal energy.However,these energy sources are transient and unstable,and need to be converted into electrical energy for storage.Therefore,energy storage devices are significant,and supercapacitors,which have both advantages of capacitors and batteries,are one of the important category.However,the performance of negative electrode materials is generally poor,resulting in low energy density of supercapacitive devices and limited practical applications.Therefore,it is urgent to improve the performance of negative electrode materials.Copper sulfides are a new type of electrode material for supercapacitors,which is limited in pratical applications by poor electrical conductivity and low specific capacitance.In this thesis,by improving specific surface area and constructing defect engineering（heterostructure,element doping and anion vacancies,etc.）,from macro-control to micro-modification,the interface and electronic structure of copper sulfides were regulated,achiving a series of negative electrode materials for supercapacitors with excellent performance.1.Polygonal Cu S prisms with unique structure were obtained by solid-state grinding method.To increase the specific surface area of Cu S,a porous copper Prussian blue analogue（Cu Fe-PBA）was used as a precursor,thereby increasing the effective contact between the electrode material and the electrolyte,which enhanced the electrochemical performance.Moreover,the employment of grinding method not only endowed Cu S a prismatic structure,but also promoted the exposure of active sites on its surface,which improved the specific capacitance.The ex-situ X-ray photoelectron spectroscopy（XPS）and Raman spectra showed that both Cu and S are active sites involved in the electrochemical reaction during the electrochemical test,endowing Cu S with excellent electrochemical performance.Cu S was used as negative electrode material and exhibited a specific capacitance of 1850 F g^-1 at a current density of 1 A g^-1.After being assembled with Cu Fe-PBA（a positive electrode material）as an asymmetric supercapacitor,the energy density was 56.0 Wh kg^-1 at a power density of 250 W kg^-1.After 5000 cycle tests,83.3%of the initial capacity was maintained.Although increasing the specific surface area can improve the specific capacity of supercapacitor electrode materials,the capacity enhancement is limited.Therefore,while using a suitable precursor as a hard template to provide a large specific surface,micro-modification methods represented by defect engineering（element doping,anion vacancies and lattice distortion,etc.）were introduced into copper sulfides to regulate their electronic structures,from the inside of the bulk phase,improve the electrochemical performance of electrode materials,and obtain copper sulfides as negative electrode materials for supercapacitors with excellent performance.Heterostructures are accompanied by defects such as lattice distortion,and solid-state grinding is often used to prepare defected structures.In addition,different types of defects can control the electrochemical properties of electrode materials differently,such as improving charge separation efficiency,enriching active sites,and improving electrical conductivity.Therefore,by combining different types of defects,such as lattice distortion and vacancy defects,the electrochemical process can be synergistically promoted.2.Using tantalum-doped Cu₂O as the precursor,tantalum-doped Cu₇S₄hollow spheres（Ta-Cu₇S₄）were fabricated besed on the kirkendall effect by a simple hard template method.The hollow-sphere structure fully increased the specific surface area of the electrode material and facilitated the exposure of active sites.In addition,the experimental results and density functional theory（DFT）calculations showed that the doping of Ta element changed the surface morphology of Cu₇S₄,increased the amount of active sites,optimized the electronic structure,enhanced the electron migration rate,and improved the conductivity.Furthermore,the increased wettability promoted the contact between the electrode material and the electrolyte,which accelerated the migration rate of electrolyte ion and reduced the adsorption energy of OH^-.Due to the lower electronegativity,Ta preferentially obtained the electrons of OH^-and transfered them to Cu,which promoted the electrochemical reaction of Cu.At a current density of 1 A g^-1,Ta-Cu₇S₄ exhibited a specific capacitance of 675 F g^-1.After assembling with Ni-Co hydroxide/Cu（OH）₂/CF into a solid-state asymmetric supercapacitor device,an energy density of 111Wh kg^-1 was achieved at a power density of 800 W kg^-1.Benefiting from the solid-state electrolyte effectively suppressing the loss of active material,the retention rate of specific capacitance was 83.3%after 5000 cycles.3.Cu S/Fe₂O₃ nano-heterostructures with O and S vacancies were prepared by calcination and selective sulfidation process using Cu Fe-PBA as the precursors.The combination of Cu and Fe enriched the electrochemical reaction of the electrode material and improved the specific capacitance.Moreover,the addition of Fe element broadened the voltage window of Cu S and further improved the energy density of the supercapacitor.Experiments and DFT simulations showed that the construction of the heterostructure and the introduction of anion vacancies optimized the electronic structure of the material,improved the conductivity of the electrode material,facilitated the electron/ion transfer efficiency,and exposed more electrochemical active sites.As a negative electrode material for supercapacitors,the maximum specific capacitance of Cu S/Fe₂O₃ is 921 F g^-1 at a current density of 1 A g^-1.After assembling with Mn O₂ into a solid-state supercapacitor device,an energy density of 56.6 Wh kg^-1 was obtained at a power density of 900 W kg^-1 in PVA/KOH gel electrolyte.When a flexible solid-state supercapacitor device was assembled in PVA/SA/KOH gel electrolyte,an energy density of 27.8 Wh kg^-1 was achieved at a power density of 900 W kg^-1.Furthermore,the device can maintain 87.5%of the initial capacity after 5000 cycles.4.Cu S/Mn₃O₄ nanosheets were prepared by grinding method using Prussian blue analogues（PBA）as the precursor.Na BH₄ solution was used to treat Cu S/Mn₃O₄,so that part of S was replaced by O,and the oxygen-modified Cu S/Mn₃O₄ was obtained.The unique nanosheet structure increased the contact between the electrolyte and the active material.DFT simulation and experimental results showed that the defect structure casued by the solid-state grinding method increased the amount of electrochemical active sites.The construction of heterostructures optimized the electronic structure,accelerated the electron transfer rate,and enhanced the electrical conductivity.Oxygen modification improved the wettability of the surface of the electrode material and reduced its adsorption energy of OH^-.At a current density of 1 A g^-1,the oxygen-modified Cu S/Mn₃O₄ as negative electrode material exhibited a specific capacity of 1307 F g^-1,which was nearly 5 times higher than that of the pristine Cu S.At a current density of 10 A g^-1,the capacity retention was65.0%,showing excellent rate performance.In addition,an asymmetric supercapacitor device assembled with Mn O₂ achieved an energy density of34.4 Wh kg^-1 at a power density of 800 W kg^-1,and maintained 85.7%of the initial capacity after 5000 cycles,showing outstanding cyclic stability.