| With the rapid development of electronic equipment and new energy,energy storage devices with high capacity,high charge and discharge rate,and long service life are in demanded.Not only in the civilian fields such as wind energy and solar energy,but also in the military fields such as laser weapons and electromagnetic ejections,the energy storage systems inside for energy supply require high energy density and high current charging/discharging ability.Existing conventional capacitors and batteries are difficult to satisfy both requirement at the same time.Therefore,supercapacitors（SC）with high power density and stability have succeeded competition in the industry and industry.Therefore,supercapacitors（SCs）with high power density and long cycle capability have attracted the attention of academics and industry,which have been greatly developed.However,due to thermodynamic limitations,electrochemical double layer capacitors which had been commercialized still have the disadvantage of lower energy density,which limits their further application.Nanostructured transition metallic sulfides have attracted much attention and interest of many researchers for the high pseudocapacitance specific capacitance,better electron conductivity compare with oxides,and achieved better electrochemical performance.Copper sulfide could be excellent pseudocapacitance materials and applied in asymmetric capacitors for the good electron conductivity,wide voltage range,high specific capacitance and abundant metal reserves,but still confront some challenges.Firstly,the specific capacitance of nanostructured sulfides is susceptible to morphology and particle size,so lower loading limits their packing density.Secondly,the preparation process and method of nanomaterials suffered from the poor consistency and difficulty in mass production,which affects its further commercialization and engineering.This research is mainly based on copper sulfide materials,via the solvothermal method for large-scale production,systematically studied the preparation process of solvothermal method and its application in aqueous hybrid capacitor electrode materials and asymmetric capacitor devices.The specific capacitance,rate characteristics,cycle life and mass loading are enhanced through the morphology control of the electrode material,secondary structure design and the composite with carbon material,which contributes for further engineering.The main research contents are as follows:1.The three-dimensional copper sulfide materials with different morphologies and crystal phase are prepared by controlling the reaction conditions of solvothermal reaction.The results demonstrate that the copper source species plays a decisive role in the formation of secondary structures from nanoparticles,resulting in the formation of microspheres,nanoclusters,submicron rods and other topography.The addition of surfactants plays a vital role in the formation of the hierarchical structure,resulting in the formation of nanosheets on the surface of the microspheres.Among them,the spheroidal Spionkopite cooper sulfide exhibits the highest specific surface area of 30 m-2 g-1,and it also exhibits the best electrochemical properties.In the 0.5 M NaOH aqueous electrolyte,the specific capacitance could achieve 320 F g-1(576 C g-1)at a current density of 2 A g-1,which is better than the submicro-rod with a specific surface area of only 6 m-2 g-1 and a specific capacitance of 30 F g-1(54 C g-1).The electrochemical energy storage process is based on the extrinsic pseudocapacitance process.The excellent electrochemical properties are mainly derived from its rationally designed three-dimensional hierarchical structure.The"petal"gap of the flower ball facile the diffusion of electrolyte ions,and the flower sphere formed by the assembly of nanosheets becomes a channel for carrier transport,effectively reducing the resistance,while the three-dimensional structure can prevent the stacking of materials and increase the loading mass(5 mg cm-2).However,the uniformity and cycle stability of the flower structure product need to be further improved.2.Based of the works in previous chapter,the reaction process and assembly mechanism of three-dimensional multi-level flower-like copper sulfide microspheres are investigated,which make the morphology could be controlled.The electrochemical properties are also investigated.The mechanism of growth is analyzed by controlling the reaction time.The reaction time and temperature hugely influence the density and thickness of the surface nanoplate of the microsphere.The mol ratio of the sulfur source to the copper source is the most critical factor which affecting the crystal phase and uniformity of the product.Adjusting the surface nanosheet thickness by doping cobalt nitrate hexahydrate during solvothermal process,the spherical copper-blue phase copper sulfide with a 10 nm thick nanoplate could be synthesized.The specific capacitance reaches 580 F g-1(812 C g-1)at a current density of 0.25 A g-1 and has been applied to a asymmetric capacitor,resulting in an high energy density and power density of up to43.37 Wh kg-1(53 Wh kg-1)and 1877 W kg-1(12.64 Wh kg-1).3.The morphology and structure of copper sulfides collapse due to repeated deposition of side reaction products during the reaction,resulting in poor cyclic stability.Therefore,cooper sulfides are composited to carbon layer with good composite stability via surface-modified in the form of carbon coating and carbon loading and then assembled to three-dimensional structure.The carbon-coated cuprous sulfide core-shell structure material are prepared by anionic-cationic surfactant-assisted hydrothermal reaction and surfactants act as both morphology control agent and carbon source.Three-dimensional hierarchical bead network structure could be assembled by properly adjusting the concentration of the surfactant,which achieving a specific capacitance of 162 F g-1(243C g-1).The results prove that the carbonization only occurs on the surface of copper sulfide and the thickness of the carbon layer is independent to the carbon source concentration,which means the copper sulfide plays a key role for the carbon layer.Based on the anchoring and confinement of ultra-thin carbon layers,the cyclic stability of the electrode material is improved(only 20%attenuation for 1,000 times at 5 A g-1).In addition,by replacing the copper source,it is assembled to form an opened three-dimensional flower-like structure,which further improves the specific capacitance and cycle stability.The specific capacity reached 360 F g-1(545 C g-1)at 1 A g-1,and the specific capacitance could remain 60%after 3000 cycle under the high current density of10 A g-1.It was finally applied to fabricate an asymmetric capacitor with a voltage of 1.8V and possess a maximum energy density and power density of 46.8 Wh kg-1(330 W kg-1 Wh kg-1)and 2700 W kg-1(19.5 Wh kg-1).4.To fabricate an electrode,extra conductive agent and binder are needed,which reduces active material utilization and increases internal resistance.Consequently,the commercial carbon cloth with good flexibility and electrical conductivity has been surface functionalized by electrochemical activation,and then deposited the copper sulfide on the surface in-situ by solvothermal method.The hierarchical structure assembly process has been researched by adjusting the reaction time.Due to the fast carrier transfer channel constructed by its copper sulfide nanosheet-microsphere-carbon cloth hierarchical structure,the as-prepared electrode exhibit an excellent electrochemical properties with a capacitance of 485 F g-1(727.5 C g-1)at the current density of 0.25 A g-1 and maintain 60%of initial specific capacitance under a larger current density.Besides,owing to the strong adhesive force of cooper sulfides to functional surface,the electrodes achieve a good flexibility（97%capacity retention after 160°bended）and long cyclic life（80%retention after 1000 cycles）.The assembled hybrid capacitor device has an energy density of up to 55 Wh kg-1(900 W kg-1).5.Due to the presence of the flexible substrate,the actual packing density of the electrode material is relatively low.The copper sulfide and carbon nanotubes were composited by a two-step method to prepare a flexible self-supporting pseudocapacitance electrode material with high area mass loading.The carbon nanotube aerogel was prepared by CVD method,and the copper sulfide material is synthesised by solvothermal method to construct a three-dimensional hierarchical structure with the carbon nanotube skeleton.The loading amount can be adjusted by the reaction time.The self-supporting electrode material with an mass loading density of 6 mg cm-2 reacted for 2h has an excellent specific capacitance of 561.73 F g-1(1010 C g-1),which based on the mass of the whole electrode,exhibiting excellentpseudocapacitance performance.Besides,the cyclic stability was significantly improved based on the stability of the CNTs skeleton and loose structure of whole 3D electrodes,which could maintain the origin specific capcitance of 95%after 3000 cycles. |
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