Controllable Preparation And Sodium Storage Properties Of Cobalt Selenide Based Composite Nanomaterials

In order to meet the need for the sustainable development and utilization of energy,and better realize the "cutting peak and filling valley" and effective utilization of electric energy,it is very important to develop low-cost and high-performance energy storage system.In the earth's crust,sodium resources are very abundant.And the energy storage mechanism of sodium ion batteries(SIBs)are similar with the commercial lithium ion batteries(LIBs).As one of the potential large-scale energy storage systems,SIBs have many advantages,such as compatible with the current lithium-ion battery production equipment,low cost and so on.The properties of electrode materials seriously affect the performance of energy storage devices.At present,the research of SIBs anode materials still faces the key problems of relatively low capacity,limited cycle life and low initial coulomb efficiency.In this thesis,a series of cobalt selenide based composite structures have been designed and synthesized,and their composition and morphology have been adjusted to improve their sodium storage performance.(1)Ultra thin 2D Fe_xCo_1-xSe₂ nanosheets(about 3.05 nm)were successfully synthesized by one-step solvothermal method,and the sodium storage performance of Fe_xCo_1-xSe₂ nanosheets can be effectively improved by appropriate amount of iron ion doping.When X = 0.08,that is,Fe_0.08Co_0.92Se₂ nanosheets show excellent comprehensive electrochemical performance.At a current density of 1 A g^-1,the initial discharge capacity reaches 679.8 mAh g^-1,the coulomb efficiency is 85.5 %,and the reversible specific capacity after long cycle is 510.4 mAh g^-1.The excellent sodium storage performance of Fe_0.08Co_0.92Se₂ nanosheets is probably related to the abundant active sites and shortened diffusion distance of sodium ions due to the ultrathin 2D structure,and the synergistic effect of bimetallic ions.Therefore,this study provides a simple method to prepare ultrathin 2D selenide nanosheets and an effective low-cost control strategy to improve the storage performance of sodium ions.(2)In this work,the Co-PAN nanowires with uniform distribution of Co were prepared by electrospinning.After different post-treatment process,CoSe₂@C?CoSe₂@C@MnSe₂ and CoSe₂@C@MnO₂ materials with rich phase interface structure have been prepared.When used as the anode material for SIBs,CoSe₂@C@MnSe₂ possess stable structure,and can display the synergistic effect of different components,and show excellent electrochemical performance.After results analysis,the one-dimensional structure of CoSe₂@C@MnSe₂including carbon can effectively alleviate the material volume expansion caused by sodium ion Insertion / detachment during the charge discharge process of SIBs,and further improve the cycle stability of electrode materials.The specific capacity of CoSe₂@C@MnSe₂ as SIB anode reaches 356.6 mAh g^-1at 1 A g^-1current.After 1600 cycles of charging and discharging,the specific capacity almost remains unchanged.(3)CoS_2-xSe_x nanowires with different morphologies have been synthesized by solvothermal method in one step.The coil like CoS_2-xSe_x assembly of nanowires are prepared by optimizing the composition ratio.The prepared materials are characterized systematically(XRD,SEM and EDS),and the morphology and chemical composition of CoS_2-xSe_x nanowires are deeply understood.The results show that the morphology and sodium storage properties of CoS_2-xSe_x can be effectively by adjusting the Se/S ratio.CoS_2-xSe_x nanomaterials show the best comprehensive sodium storage performance at X = 1.The reversible specific capacity of CoSSe nanowires is 440.0 mAh g^-1after 600 cycles at current density of 1 A g^-1.The excellent sodium storage performance of CoSSe nanowires is possible related to the abundant active sites and unique sodium ion transport channels in 1D structure.This work provides a simple synthesis idea for high performance selenide-based sodium storage electrode materials.