| Supercapacitors,as one energy storage technology with ultra-long cycle life,high power density and low cost,play important roles of instantaneous high-power output,rapid energy recovery and load regulation in the systems such as power grid,automobile power,rail transit and so on.However,there is a technical bottleneck of low energy density for supercapacitors,which limits their further development and application.The key reason is that the electrode/electrolyte interface reaction is limited to the surface of the electrode material,which only provides a limited double-layer capacitance or pseudocapacitance and unable to achieve its intrinsic high theoretical specific capacity.Hence,the effective strategies such as increasing the electrode/electrolyte contact area,enriching the redox active sites and improving the electrode electronic conductance and ion diffusion coefficient could construct supercapacitor materials with higher energy density.In addition,the two-dimensional materials with the characteristics of large specific surface area and high carrier migration rate exhibit excellent performance in the application of energy storage electrode materials.Therefore,the specific capacity of the electrode materials in practical applications could be further improved by adjusting transition metal compounds into two-dimensional materials,which combined the special properties of two-dimensional materials and the advantage of transition metal with unfilled variable valence d orbital.This paper is devoted to the two-dimensional structure design and composite modification of titanium,cobalt oxide,carbide,phosphide,which are applied in water and non-water system supercapacitors.The crystal growth regulation mechanism,the microstructure and improving mechanism of electrochemical energy storage by optimizing the microstructure and electronic structure of composite electrode and other aspects were deeply studied.Finally,the supercapacitors with high energy density were constructed.The main research results are as follows:(1)The 2D-2D hirarchical cobalt-based layered double hydroxides(LDH)nanosheets and MXene composite materials(Co LDH-MXene)were prepared by one-step hydrothermal method.We obtained Co LDH-MXene(Co-MX10),Co-Fe LDH-MXene(Co Fe-MX10),Co Mn LDH-MXene(Co Mn-MX10)and other composite materials by adjusting the content of MXene,transition metal ion species and other components in this process.The results showed that the addition of Fe and Mn ions can effectively regulate the microstructure of Co LDH nanosheets.The capacitance performance of Co LDH-MXene in KOH aqueous solution had been significantly improved owing to the ultrathin and defective LDH nanosheets with the addition of Fe and Mn,and the excellent carrier mobility of MXene.For example,Co Fe-MX10 composites exhibited high specific capacity of 808 F g-1 at 0.5 A g-1 and 340 F g-1 at 10 A g-1.The specific capacities of Co Mn-MX10 were 562 F g-1 and 309 F g-1 at current density of 0.5 A g-1 and 10 A g-1,respectively.The supercapacitor(Co Fe-MX10//AC)was assembled with Co Fe-MX10 and active carbon AC,which delivered a maximum energy density of 20 Wh kg-1.(2)Multi-metal composites have more redox active sites compared with mono-metal or bimetallic composites,which can effectively improve the electrochemical performance.Therefore,the Co-Mn-Fe ternary phosphide(CFMP),Co-Mn bimetallic phosphide(CMP),single metal cobalt phosphide(Co P),carbon doped Co-Mn-Fe ternary phosphide(C-doped CFMP)and other composite materials were synthesized by the hydrothermal method and subsequent high temperature phosphating method.In the hydrothermal process,the formation of hexagonal prism nanoarray hierarchical structure was attributed to the co-regulation of ammonium fluoride and hexamethylene tetramine,the anisotropic growth of Co Mn oxide with Fe2+ion induced.The ultrathin nanosheets(about 15nm in thickness)were intersected through competitive growth to form hexagonal prism nanoarray.The surface defects of the nanosheets resulted in the presence of plentiful nanoparticles and pores with a size of about 10 nm during the high temperature phosphating process.This unique structure could significantly increase the electrode/electrolyte contact area,enrich the redox active sites and avoid the presence of"dead volume".Therefore,the specific capacity of C-doped CFMP electrode material reached to 4.36 F cm-2 at 2 m A cm-2,much higher than that of bimetallic phosphides(CMP)and monmetallic cobalt phosphides(Co P).In addition,the carbon doping substantially improved the cycling stability of CFMP electrode material because it effectively buffers the strain/volume expansion during fast redox reactions.Especially even at a high current density of 20 m A cm-2,100%capacitance retention of C-doped CFMP was achieved after 5000 cycles,while CFMP can only remains67%.By optimizing the electrochemical properties of the electrode materials,the maximum energy density of C-doped CFMP//NG could be increased to 53.2 Wh kg-1.(3)The energy density of capacitor is affected by the specific capacitance of the electrode material and the working voltage window.Therefore,based on the optimization of structure and performance of the electrode material,the energy density of the supercapacitor can be further improved by expanding the voltage window.The Ti O2/MXene composite with2D-2D layered hierarchical structure was prepared by molten-salt method.In the molten-salt method process,plentiful ultrathin Ti O2 nanosheets can be rapidly grown vertically on the surface of MXene based on the strong oxidation of nitrate and the strong reducibility of MXene.The resulting Ti O2nanosheets of 4-5 nm thick inherited specific layered atomic structures of 2D MXene,and thus were composed of few layers with?0.8 nm layer spacing.In addition,Ti O2/MXene composite possessed the large specific surface area and pore structure,and the resulting Ti O2 nanosheets could avoid the restacking of MXene flakes that can accommodate electrolyte ion insertion,which facilitated the diffusion and migration of electrolyte ions.What's more,the improved charge transfer rate between Ti O2 and MXene interface was achieved by interface modification,which led to the high specific capacity and rate performance of Ti O2/MXene composites.For example,the Ti O2/MXene composite exhibited a specific capacity of 130 m Ah g-1 at 50 m A g-1 in nonaqueous electrolyte.The lithium ion capacitor assembled with Ti O2/MXene and AC as positive electrode delivered a maximum energy density of 59.66 Wh kg-1,(4)To further improve the energy storage performance of the capacitor based on the research in the previous chapter.In this chapter,the Ti O2/MXene electrode material was further optimized by solvothermal method,and Lithium-titanium phosphate(LTP)and Ti O2/MXene composite(LTP-Ti O2/MXene)were prepared.In the solvothermal reaction process,partial Ti O2 was transformed into lithium titanium phosphate(LTP)nanocrystals,which uniformly grew on the Ti O2/MXene interlayer and surface by solvothermal method.That resulted in the reduction of the length of Ti O2 nanosheets,effectively reducing the migration path improving the migration rate of lithium ions.The 2D-2D hierarchical structure was maintained during the process further improving the specific capacity of the composites.Therefore,the specific capacity of LTP-Ti O2/MXene electrode material can be increased to204 m A g-1 at 50 m A g-1.The lithium ion capacitor was assembled with LTP-Ti O2/MXene as the negative electrode,AC as positive electrode,and non-aqueous electrolyte.It delivered a maximum energy density of the capacitor was up to 151.2 Wh kg-1,which was much better than the energy density of the previously water supercapacitors owing to the expand the voltage range with non-aqueous electrolyte. |
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