Study On Electrochemical Properties Of Nickel Based Nanocomposites For Energy-storage Device

With the rapid development of the social economy and the improvement of the people’s living standard,many social problems such as population explosion,environmental pollution and the excessive consumption of fossil fuels are also affecting people’s quality of life.In order to achieve sustainable development and reasonable utilization of energy,the development of new energy nanomaterials and energy storage device has aroused people’s attention.At present,many new energy storage devices including lithium-ion batteries,sodium batteries,super capacitors,lithium-sulfur batteries and some alkaline batteries have been studing.Compared with other energy storage devices,supercapacitor has many advantages including high power density,good cycle performance,fast charge discharge capability,low cost and environmental friendly etc.However,improving energy density of supercapacitors remains a major challenge.A supercapacitor consists of two electrodes,electrolyte and a separator.The electrochemical activities and reaction power of the electrode material could influence the electrochemical properties of supercapacitor signficantly.Enhancing the transmission power in the electrode structure could increase the power density of a supercapacitor.In addition,designing reasonable electrode and device structure according to the energy storage principle can improve the energy density of the supercapacitor.Following the lead-acid batteries,nickel-hydroxide batteries,nickel-cadmium batteries and lithium-ion batteries,nickel-zinc secondary batteries were praised as the real "green battery”.Compared with other batteries,nickel-zinc secondary batteries has many merits such as a high operating voltage,energy density,wide operating temperature（-20-500℃）,no memory effect and environmental friendly etc.However,nickel-zinc batteries have the disadvantages of poor cyclic stability and discharge performance because the zinc anode could be dissolved into the strong alkaline electrolyte and deformation of zinc dendrites could occur during the charge and discharge process.In the meantime,nickel hydroxide is generally used as the cathode material for nickel-zinc secondary batteries.Structure expanision of nickel hydroxide electrode could occur during the charge and discharge process.And the energy density of a nickel hydroxide-based nickel-zinc battery only is～70 Wh kg^-1.Thus,new cathode material also needs to be studied and applied to alkaline zinc batteries.Since the 1970s,nanotechnology has been greatly developed and nanomaterials are widely applied in many fields including medicine,environmental science,electronics and aerospace etc.Due to the quantum size effect,the small size effect and the macroscopic quantum tunneling effect etc.,nanomaterials could be applied in electrochemical energy storage device and exhibit excellent electrochemical performance.This paper mainly aims to improve the electrochemical performance of supercapacitors and zinc batteries by the inroduction of new nanomaterials and the optimization of electrode and device structure.（1）In this work,Ni₃S₂ nanorod arrays were grown on a nickel wire by a facile hydrothermal metrhod.Ni₃S₂ nanorod arrays grown on Ni wire could be used as positive electrode for supercapacitor.A flexible coaxial-type fiber solid-state symmetrical supercapacitor（SSC）based on Ni₃S₂ nanorod array electrode was successfully fabricated.The SSC device exhibits a voltage window of 0.8 V and a capacitance of 9.0 F g^-1 at a scan rate of 10 mV s-1.In order to improve the electrochemical properties of energy-store device,a flexible coaxial-type fiber solid-state asymmetrical supercapacitor（ASC）based on Ni₃S₂ nanorod array electrode and carbon nanoparticle electrode was also successfully assembled.Compared with the SSC device,the ASC device presents a higher voltage window of 1.4 V and behaves a high specific capacitance of 34.9 F g^-1 at a scan rate of 10 mV s-1,an increased energy density of 8.2 Wh kg^-1.In addition,the ASC device also exhibits excellent electrochemical stability with 93.1%of initial specific capacitance after 3000 consecutive cycles and good mechanical stability.These encouraging results present its great potential in flexible solid-state energy storage devices.（2）NiCo hydroxide nanosheets were directly grown on Ni foam by an electrodeposition method at room temperature.The capacitance of NiCo hydroxide nanosheets in alkaline electrolyte is 1155 F g^-1 at a scan rate of 2 mV s-1,which is far lower than their theorticl capacitance.In order to improve the electrochemical properties of electrode structure,three-dimensional hybrid structure with Ni₃S₂ nanorod arrays as core material and NiCo hydroxide nanosheets as shell material is fabricated directly on flexible Ni foam.This hierarchical hybrid structure without any additives and binders behaves good electrochemical properties including a high specific capacitance of 1800 F g^-1,good rate capability and great cyclic stability.Moreover,an asymmetric supercapacitor（ASC）based on NiCo hydroxide@Ni₃S₂ hybrid structure on Ni foam as positive electrode and activated carbon as negative electrode is successfully assembled to further evaluate the electrochemical properties of the hybrid structure in a full-cell configuration.The ASC device also performs excellent electrochemical properties including a large potential window of 0-1.8 V,a high specific capacitance of 215.8 F g^-1 at a current density of 1 A g^-1 and long-term cycling performance.In the meantime,the ASC device also delivers a high energy density of 92.8 Wh kg^-1 at a power density of 1.0 KW kgg^-1.These results suggest that 3D NiCo hydroxide@Ni₃S₂ hybrid structure has promising potential for use in high-performance supercapacitor.（3）Transition metal phosphides are a sort of submetallic alloy that consists of metallic element and phosphorus.Compared with transition metal hydroxide,transition metal phosphides have has higher conductivity.Among these transition metal phosphides,cobalt phosphides,as a sort of promising electrode material,hold two kinds of combination ways of Co element.The Co combined by elements bonds can improve the conductivity of the phosphide and provide free electrons.And Co2+combined by covalent bonds could store charges and provide higher capacitance characteristics by the faraday reaction.In this work,three-dimensional Cop@Ni（OH）2 hybrid structure is synthesized successfully and applied in supercapacitor.CoP is prepared by phosphating Co（OH）2 nanosheet arrays that were directly grow on a carbon cloth by a electrodeposition pross.The three-dimensional composite structure is synthesized by electrodepositing Ni（OH）2 nanosheet structure on synthesized CoP nanosheet array.This composite structure also exhibits excellent electrochemical properties such as 1989 F g^-1 at a current density of 2 A g^-1 and good rate capacity with 1840 F g^-1 at the current density of 30 A g’1.In addition,an asymmetric supercapcitor based on the hybrid structure and activated carbon electrode has been synthesized successfully.The asymmetric supercapacitor exhibits excellent electrochemical properties including a voltage window of 1.8 V,a specific capacity of 224 F g’1 and an energy density of 95.2 Wh kg^-1.（4）Ni₂P nanosheet arrays were directly grown on a carbon cloth by a two-step method.First,the nickel hydroxide nanosheet array is grown on a carbon cloth by hydrothermal method.Ni₂P nanosheet arrays were grown on a carbon cloth by phosphating Ni（OH）2 nanosheet arrays.Ni₂P nanosheet array electrode shows good electrochemical performance including a high specific capacity of 242 mAh g^-1 at a current densityof 1 A g^-1,rate capacity with a specific capacity of 185 mAh g^-1 at the current density of 20 A g^-1,and good cyclic performance with 14%capacity loss after 5000 continuous charge-discharge procedures.In addition,the Ni₂P nanosheet array electrode and metal zinc electrodeposited on a carbon cloth was assembled to a Ni₂P//Zn battery.The battery also exhibits exhibits electrochemical properties such as a voltage platform of 1.78 V,a specific capacity of 238 mAh g^-1 at a current density of 1 A g^-1,energy densities of 328 Wh kg^-1 and good cyclic performance...