Controllable Preparation And Sodium Storage Property Of M_xSe（M=Cu,Fe）

In recent years,sodium ion batteries（SIBs）have become promising candidates for large-scale energy storage systems due to their low cost,wide range of sodium resources and considerable energy density,attracting extensive attention from the scientific research and industry.However,the larger radius of sodium ions and slower ionic diffusion dynamics lead to a big gap between the performance of SIBs and practical.As for sodium ion battery,anode material has a significant influence on its overall performance,so it is extremely urgent to develop high-performance anode material.Among them,transition metal selenides have attracted much attention because of their excellent electronic conductivity,high theoretical capacity and low cost,making them the most competitive candidate materials for electrochemical storage of alkali metal ions.However,it has been found that the volume expansion effect in the process of repeated charge and discharge tends to cause the material to fall off and the structure to powdery,which leads to the low rate capacity and poor cycle stability.So far,the design of micro/nano structure and carbon coating is an effective strategy to enhance the structural stability of the electrode and promote the transport of sodium ions,thus improving the cycle stability and rate performance.In this thesis,a series of research work was carried out on transition metal selenides as high-performance anode materials for sodium ion batteries,including sample preparation,characterization,electrochemical performance testing and analysis,and sodium ion storage mechanism.The research contents are as follows:（1）A one-step solid-phase method was proposed to prepare Cu₂Se.This method has simple raw materials and can not require complex operations and instruments.Based on this method,the effects of sintering time on the structure,morphology and electrochemical properties of the materials were investigated.At the same time,the influence of electrolyte on the performance of Cu₂Se electrode material was discussed.The obtained Cu₂Se has excellent cycling performance and rate performance in sodium ion half cell.The electrode provides reversible capacities of 264 and 241 m Ah g^-1 at 0.1 and 10 A g^-1.At 1 A g^-1,the capacity retention rate is about 100%over 1000 cycles.The full battery composed of an anode of Na₃V₂（PO₄）₂F₃@r GO（NVPF@r GO）and Cu₂Se anode also has excellent energy storage performance.The storage mechanism of Cu₂Se was further revealed by ex-situ X-ray diffraction（XRD）,ex-situ transmission electron microscope（TEM）and first-principles calculation.（2）Firstly,a core-shell structure of nitrogen-doped carbon-coated ferrous selenide（Fe Se@NC）composite was synthesized using dopamine coating Fe OOH precursor and subsequent selenization process.It is found that the nitrogen-doped carbon coating can not only effectively regulate the volume expansion during Fe Se conversion reaction,but also improve the electronic conductivity,thus obtaining excellent electrochemical performance.Meanwhile,the effects of selenization time and temperature on the morphology,crystal structure and electrochemical properties of the composites were investigated.The results showed that Fe Se@NC under optimized conditions showed excellent rate performance of 364m Ah g^-1 at 10 A g^-1,and the full battery matching the cathode of NVPF@r GO maintained approximately 100%capacity and stable long cycle performance of more than 2000 cycles at1 A g^-1.The storage mechanism of FeSe was further revealed by ex-situ XRD,ex-situ TEM and first-principles calculation.（3）FeSe₂ material was prepared by mixing FeOOH and selenium powder in different proportions and sintering them in a mixture of argon and hydrogen,in order to improve its sodium storage performance.Fe Se₂ was optimized by recombination with carbon.The effects of adding dopamine on the crystal structure and electrochemical properties of the materials were further investigated.The study showed that the addition of dopamine in the system made the size of Fe Se₂@NC composite more uniform to a certain extent.At the same time,the addition of dopamine improved the conductivity of Fe Se₂ material,which greatly improved the cycling performance of the material.At 5 A g^-1,the specific capacity of Fe Se₂@NC composite remaineds at 443 m Ah g^-1 after 1000 cycles,and the Coulombic efficiency remains at～100%.The storage mechanism of Fe Se₂ was further revealed by ex-situ XRD,X-ray photoelectron spectroscopy（XPS）and TEM.