Preparation Of Tungsten-based Chalcogenides And Their Sodium Storage Performance

Renewable clean energy such as wind energy and solar energy cannot provide stable energy due to environmental factors,while lithium-ion batteries have the advantages of high energy density and excellent cycle performance,which have penetrated into all aspects of our lives.However,lithium resources continue to be depleted and unevenly distributed around the world,making it increasingly expensive and unable to meet the huge market demand.Therefore,sodium,which is in the same main group as lithium,have similar physical and chemical properties,are abundant in reserves and lower in price,and have aroused widespread attention.Therefore,the sodium-ion batteries are expected to be promising energy device.However,sodium ion exhibits large radius,slow ion transport,poor electronic conductivity,and slow kinetics.Recently,two-dimensional（2D）crystalline materials,especially transition metal dichalcogenides（WS₂,WSe₂,etc.）have been widely explored in the field of sodium storage.In this dissertation,the tungsten sulfide and tungsten selenide composites are rational designed and fabricated,as well as the optimized performances are obtained.Tungsten selenide and tungsten selenide have shortcomings such as large volume expansion,poor cycle performance,and rapid capacity decay.Common modification strategies include carbon confinement,doping and defect functionalization,and structure and morphology design.We plan to adopt a combination of multiple modification strategies such as carbon coating,nano confinement,defective enginnering and anion doping,etc.,to obtain the tungsten-based chalcogenide electrodes for high-performance of sodium-ion batteries.Therefore,this dissertation is divided into the next three chapters,and the results are as follows:Firstly,chlorella is used as adsorbent and reactor to absorb phosphotungstic acid,and calcined by vapor deposition method.Then tungsten sulfide nanocrystals confined in N,P co-doped carbon（WS₂/NPC）was obtained during sulphurization process.Acting as an anode of sodium-ion batteries,the WS₂/NPC-2 delivers a reversible specific capacity of302 m A h g^-1 at 1 A g^-1 for 2800 cycles.It also exhibits a reversible specific capacity of436 m A h g^-1 at 0.1 A g^-1 for 100 cycles.Secondly,sulfurized polyacrylonitrile（SPAN）is used as the matrix of nano-confinement and defective enginnering,to enhance the performance of tungsten sulfide for sodium storage.The WS₂ nanoparticles embedded in SPAN nanofibers（WS₂-SPAN）were initially synthesized through electrospinning route and employed as anodes of SIBs and PIBs for the first time.Among the three samples,the WS₂/SPAN-2sample showed the best excellent cycling stability and rate performance.When used as an anode of SIBs,the WS₂-SPAN-2 exhibits a reversible specific capacity of 370 m A h g^-1after 1500 cycles at a high current density of 2 A g^-1.Furthermore,it also shows a stable reversible capacity of 129 m A h g^-1 at a high current density of 10 A g^-1 up to 18000cycles,which is one of the best sodium storage properties of tungsten sulfide reported so far.When used as an anode of potassium-ion batteries,it also shows excellent performance.After 1100 cycles at a high current density of 1 A g^-1,the WS₂-SPAN-2 also delivers a reversible capacity of 300 m A h g^-1.Finally,the WS_xSe_2-x nanoparticles embedded in selenized polyacrylonitrile nanofibers(WS_xSe_2-x-Se PAN)were initially synthesized through electrospinning route.We use the Se PAN as the matrix of nano-confinement,defective enginnering and anion doping to fabricate a series of sulfur-doped tungsten selenide-Se PAN composites.Acting as the negative electrode of a sodium-ion batteries,the WS_xSe_2-x-Se PAN-2 maintains a reversible specific capacity of 477 m A h g^-1 after 700 cycles at a high current density of 2A g^-1.Furthermore,it also displays excellent rate performance.