Electronic Structure And Conductivity Modifications Driven By Transition Metal Doping Of Ni_xS_y Nickel-based Sulfides

With the characteristics of fast charging and discharging capability,long lifetime and high power density,supercapacitors have now become an ideal choice for efficient energy storage devices in social life,but their deficiency in energy density has seriously limited its development.Electrode material is the core component of supercapacitor,so the research related to electrode material is an important work to advance the practical application of supercapacitor.Transition metal sulfides（TMSs）have a wide source distribution,low cost,better thermal stability,high theoretical capacity,and unique advantages over other kinds of electrode materials,but they also suffer from defects such as insufficient ionic/electronic conductivity.In recent years,many works on the modification of transition metal sulfides have revolved around morphology and nanostructure design,compositional modulation and interfacial modulation,etc.Its lower conductivity as a semiconductor material is difficult to regulate directly,in contrast,the doping of specific atoms into the main lattice of the semiconductor allows effective modulation of the electronic structure and related property functions of the material,but the relevant work using doping modification methods However,most of the works using doping modification methods also do not provide theoretical support for the effect of doping on the intrinsic conductivity of the material.Therefore,the changes in the electronic structure of doped materials need to be further investigated.In this thesis,Ni-based sulfides Nix Sy with different doping concentrations and doping elements（Co,Mn and Ag）were synthesized by hydrothermal method,and X-ray diffraction（XRD）,X-ray photoelectron spectroscopy（XPS）,scanning electron microscopy（SEM）,energy dispersive X-ray spectroscopy（EDX）and transmission electron microscopy（TEM）were used to characterize the composition,crystal structure and The electrochemical properties of the doped materials were tested using cyclic voltammetry（CV）,constant current charge/discharge（GCD）,alternating current impedance（EIS）and cyclic stability tests to investigate the effects of doping on the electrochemical properties of the materials.The main works are as follows.1.Nix Sy nickel-based sulfide nanospheres doped with transition metals Co,Mn and Ag were synthesized by a simple one-step hydrothermal method to obtain undoped samples NS（nickel sulfide）,Co NS（Co-doped nickel sulfide）,Mn NS（Mn-doped nickel sulfide）,Mn NS（Mn-doped nickel sulfide）and Ag NS（Ag-doped nickel sulfide）,and the synthesized Nix Sy nickel-based sulfide nanomaterials were structurally characterized by XRD,Raman,TEM,EDS,XPS and other testing methods to demonstrate the successful doping of the transition metal and the successful doping of the transition metal.The successful doping of transition metals and the relative consistency of the composition of Nix Sy nickel-based sulfide nanospheres before and after doping were demonstrated.Based on the first nature principle,computational models were developed for the doped Ni S2（space group:Pa-3）and Ni3S4（space group:Fd-3m）to study the changes of energy bands and density of states before and after doping with metal ions.The calculated results show that the band gap widths of Ni S2 and Ni3S4 decrease by 0.177e V and 0.081 e V,respectively,after Co doping.The band gap widths of Ni S2 and Ni3S4decrease by 0.838 e V and 0.546 e V,respectively,after Mn doping.The band gap widths of Ni S2 and Ni3S4 decrease by 0.306 e V and 0.048 e V,respectively,after Ag doping.For Co and Ag doping,the doping elements do not have a large impact on the electron density distribution of the main states,and the slight change in the electron distribution in the conduction and valence bands reduces the band gap width to a lesser extent;for Mn doping,the doping elements are the main constituents of the impurity energy level,which reduces the band gap width to a greater extent and should have better conductivity and performance.2.Electrochemical tests were performed on the synthesized materials to compare the magnitude of specific capacitance values of the samples under different doping concentration gradients,and the best doping concentration samples were selected as 2%Co NS,0.5%Mn NS and 0.5%Ag NS;the mass specific capacitance value of NS samples at current density of 1 A·g^-1 was 2221.35 F·g^-1,compared with 2%Co NS,0.5%Mn NS and 0.5%Ag NS,Co NS,0.5%Mn NS and 0.5%Ag NS had mass specific capacitance values of 2874.59,2612.51 and 2587.30 F·g^-1,respectively,indicating that the electrochemical properties of the doped Nix Sy nickel-based sulfides were all improved,and analysis of the electrochemical impedance spectra of the samples showed that the 0.5%Mn NS sample had the smallest charge transfer This is consistent with the theoretical calculations showing that Mn NS has the relatively smallest band gap width.The prepared samples were assembled into an asymmetric supercapacitor device to investigate its practical application capability,and NS//r GO has an energy density of27.44 Wh·kg^-1 and 4.44 Wh·kg^-1 at 800 W·kg^-1 and 8000 W·kg^-1,respectively,while 2%Co NS//r GO,0.5%Mn NS//r GO and 0.5%Ag NS//r GO have the lowest charge transfer resistance at 800 W·kg^-1 and 8000 W·kg^-1,respectively.Ag NS//r GO at 800 W·kg^-1 and8000 W·kg^-1 were 36.62,10.00,36.09,9.11,35.98,and 8.66 Wh·kg^-1,respectively;NS//r GO,2%Co NS//r GO,0.5%Mn NS//r GO,and 0.5%Ag NS//r GO showed specific capacitance values of 69.1%,76.1%,82.6%and 79.4%relative to the initial specific capacitance after 10,000 cycles,respectively,and electrochemical tests on the materials proved that the power and energy density as well as cycling performance of the assembled asymmetric supercapacitor devices were improved to some extent compared to those before doping.