Synthesis Of Copper Chalcogenides And Tlheir Lithium(Sodium) Storage Properties

"Energy" and "Environment" have become the most critical social problems and have drawn much attention in recent decades.Developing electric vehicals and smart grids to make better use of renewable energy can relieve these crises effectively.Due to the advantages of high energy density,long cycle life and low cost,lithium ion battery and sodium ion battery have become the most competitive candidates in electric vehicles and large-scale energy storage system.However,carbon based materials such as graphite and hard carbon which have low energy density are still used as anode materials in commercial lithium/sodium ion batteries.Owing to the advantages of high energy density,long cyclic stability,metal chalcogenides can be next generation anode materials for lithium/sodium ion batteries.In this dissertation,CuS spheres and CuS-Graphene composite(CuS-G)are synthesized by a facile microwave-assisted method and Cu2-xSe nanorods are prepared by a simple water evaporation induced self-assembly approach.Lithium/sodium storage properties and electrochemical reaction mechanisms of CuS and Cu2-xSe are investigated.The main results and originalities of this dissertation are summarized as follows:(1)CuS spheres are synthesized by a facile microwave-assisted method in Chapter 3.As-prepard CuS spheres show good lithium storage—properties.After 100 cycles,the reversible capacity of CuS stands at 405 mAh g-1,showing good cyclic stability.Ex-situ XRD test reveals that electrochemical reactions of CuS/Li batteries includes irreversible conversion from CuS to Cu2-xS and reversible conversion between Cu2-xS and Cu,Na2S.(2)In orde to improve the long cyclic stability of CuS,CuS-G is prepared by one-step microwave synthesis in Chapter 4.CuS-G shows better lithium storage properties compared with CuS.The reversible capacity of CuS-G can be as high as 497 mAh g-1,and the capacity can stand at 348 mAh g-1 even after 1000 cycles at high current density.EIS,CV and GITT tests indicate that the CuS-G electrode has lower resistance and higher Li+ ion diffusion coefficient compared with bare CuS electrode,which can be attributed to the conductive network constructed by graphene sheets.(3)In Chapter 5,CuS microspheres prepared by microwave synthesis are used as anode materials for sodium ion batteries.As-prepared CuS microspheres display steady reversibility by adopting TEGDME as the electrolyte solvent and adjusting cut-off voltage to 0.6-3.0 V.The discharge capacity stands at 162 mAh g-1 after 200 cycles with the capacity retention of 95.8%.Superior electrochemical kinetics of CuS electrodes is revealed by EIS,CV and GITT tests.Beyond that,electrochemical reactions of CuS electrodes are explored using ex-situ XRD.(4)In Chapter 6,Cu2-xSe nanorods are synthesized by a facile water evaporation process.The electrochemical reaction mechanism is investigated by ex-situ XRD.By adopting ether-based electrolyte instead of carbonate-based electrolyte,the electrochemical performance of Cu2-xSe electrodes improved significantly.After 1000 cycles,160 mAh g-1 can be maintained with a retention of 80.3%.The 4-probe conductivity measurements along with EIS and CV tests illustrate that Cu2-xSe electrodes display high specific conductivity and impressive Li+ ion diffusion rate,which makes Cu2-xSe a promising anode material for lithium ion batteries.(5)Sodium storage properties of Cu2-xSe nanorods are explored in Chapter 7.By limiting the discharge voltage to 1.0 V,Cu2-xSe electrodes express outstanding electrochemical properties.The initial discharge capacity is 149.3 mAh g-1 at the current density of 100 mA g-1,and the discharge capacity can maintain at 106.2 mAh g-1 after 400 cycles.Owing to the excellent electronic conductivity and one-dimensional structure of Cu2-xSe,Cu2-xSe electrodes manifest fast Na+ ion diffusion rate.Moreover,detailed Na+ insertion/extraction mechanism is further investigated by ex-situ XRD,XPS,Raman,TEM tests and theoretical calculation.