Synthesis Of Nickel Sulfide/selenide With Tailored Muti-morphologies And Their Lithium Storage Properties

In recent years,the excessive utilization of fossil energy makes human beings facing serious problems of energy shortage and environmental pollution.In order to alleviate these difficulties,it is urgent to develop renewable and environment-friendly energy storage technologies.Lithium-ion batteries are widely used as a green energy in portable electronic devices because of its high energy density,high power density and long cycle life.Among all kinds of anode materials for lithium-ion batteries,the transition metal sulfide/selenide are considered as promising electrode materials due to their higher theoretical capacity and good conductivity than the corresponding oxide.However,the poor cycle stability and low rate property in the charging/discharging process and lower conductivity?as compared with the carbon material or alloy material?of transition metal sulfide/selenide have hindered their applications in the anode materials for lithium-ion batteries.There are two common ways to overcome these shortcomings,including designing a variety of nanostructures such as nanorods,nanowires and nanosheets,and direct synthesis of electrode materials on conductive substrates for good contact between the active material and the conductive substrate?current collector?for better interface electron transport.Due to the great effects of the structures and morphology of the materials on the properties of the composites,it is highly demand to investigate the morphology effect on the electrochemical behaviors in a battery.In this work,synthesis methods are developed to make unique nanomaterials with various morphologies and further investigate their effects on the battery performance.The work can provide thoughtful information to improve the electrochemical behaviours of a battery for practical application.This thesis mainly includes the following five parts:?1?The advantages,structures,working principles and research progress of lithium-ion batteries are reviewed,and some typical anode materials are compared.?2?The synthesis method and characterization of the electrode materials are described in detail.?3?Ni₃S₂ with a rhombohedral structure is suitable for the large-scale production of high-rate devices because of its high theoretical capacity and unique crystal structure.However,the lower conductivity,poor cycle stability and other drawbacks of Ni₃S₂ greatly limit its practical application in lithium-ion batteries.Therefore,it is highly desirable to synthesize Ni₃S₂ nanostructures with good conductivity,high specific capacity and good cycling performance for the anode material.To achieve this,the nanostructured nickel sulfides with different morphologies were synthesized on nickel foam by hydrothermal method.The influences of the reaction time and temperatures on their morphological and crystal structures were studied.When the reaction temperature was set at 130°C,the respective nanosheet in a small amount,nanosheet array and nanostructured embossment of nickel sulfides were formed with the reaction time of 1,2 and 4 h.When the reaction time was set for 2 h,nanosheet in a small amount,nanosheet array and short rod-like nanostructure of nickel sulfide were formed at the reaction temperature of 80,130 and 180°C,respectively.The Ni₃S₂nanosheet array,nanostructure embossment and the rod-like nanostructure were then studied as the anode material for lithium-ion batteries.The results show that the initial discharge capacities of the Ni₃S₂ nanosheet array,nanostructure embossment and the rod-like nanostructure electrode are 1020.6,619.6 and 447.3 m Ah g^-1,respectively.After 60 cycles,the respective discharge capacities of the Ni₃S₂ nanosheet array,nanostructure embossment and the rod-like nanostructure electrode reduce to 84.5%,81.6%and 56.8%,relative to their second discharge capacities.In comparison with the other two Ni₃S₂ nanostructures and others reported in the literatures,the Ni₃S₂nanosheet array exhibits the best electrochemical performances.Moreover,the electrochemical impedance measurements of these three electrodes show that the kinetics of the electrode materials are controlled by charge transfer rather than diffusion process.For the first time,we studied the charge transfer resistance and the chemical diffusion coefficient of the Ni₃S₂ nanosheet array as the anode material at different discharge depths and current densities by electrochemical impedance spectroscopy.The results show that both charge transfer and Li⁺diffusion are fastest at the charge/discharge plateau region in the process of constant current(100 m A g^-1)charging and discharging.At the charge/discharge plateau region,the charge transfer?4?Among the anode materials for lithium-ion batteries,most of them are based on the transition metal oxides and sulfides,whereas the study of the transition metal selenides as anode electrode materials is rarely reported.The conductivity of metal selenides is generally higher than that of metal sulfides or oxides,enabling them to be a potential electrode material for high power lithium-ion batteries.Therefore,it is interesting to synthesize the nickel selenides with different morphologies and to explore their possibilities as anode materials for lithium-ion batteries.The nickel selenides with different morphologies were synthesized by one-step solvothermal method on the foamed nickel conductive substrate.The effects of reaction time,temperatures and solvents on their morphological and crystal structures were investigated.When the reaction time was set for 24 h and the reaction temperature was set at 140°C,the flower-like Ni Se₂ nanostructure,Ni Se multistage microntube and Ni₃Se₂ and Ni Se₂ mixtures of nanosheet,micron flake structure and bulk nanostructures were obtained by changing the volume ratio of ethylenediamine and ethylene glycol.When the reaction time was set for 24 h and the volume ratio of ethylenediamine and ethylene glycol was set to be 1:2,the Ni Se and Ni Se₂ mixtures of packed micro-rod structure and pine dendrity-like Ni Se₂ and Ni₃Se₂ mixtures were synthesized at different temperatures.The growth of Ni Se multistage microntube was further monitored.In the beginning of the reaction,the nickel selenide did not have specific morphology,and gradually grew into a porous cone,and finally became a multistage microtube structure.The Ni Se multistage microtube exhibits higher specific capacity and better rate capability as the anode electrode material for lithium-ion batteries,and their initial charge and discharge capacity are 663.3 and 905.0 m Ah g^-1,respectively.?5?Conclusions are summarized and future works are also forecasted.