Preparation Of Mo/Zn Selenide Based Nanomaterials And Their Sodium Storage Properities

Energy shortage and environmental pollution have been the major problems for the nowadays society.As a new and efficient energy storage device,the batteries with high specific capacity and outstanding cycling performance have attracted much attention of the researchers.As known to all,lithium ion battery has undergone several generations of innovation and has penetrated into our daily life.However,as a national strategic resource,lithium metal has a high price and limited storage capacity,which limits the application prospect of lithium ion battery.In contrast,sodium ion battery has become a promising new energy device,owning to its abundant resources,environmental benigness,low redox potential and so on.But,the research of sodium ion battery is still in the primary stage.How to improve the electrochemical performances(long cycle life,high energy/power density,excellent rate performance and stable high/low temperature performance,etc.)and technological application(optimization of battery assembly technology and testing method)of sodium ion battery are urgent tasks for researchers.In the anode materials of sodium ion battery,alloying-type and conversion-type materials have higher theoretical specific capacity and better safety than the intercalation-type materials.But alloying-type anode materials are faced with their own defects,such as the large volume expansion during cycling,resulting in the pulverization of the electrode materials and the short circuit of the battery.This thesis focuses on the conversion-type metal selenide and their electrochemical performance improvement through structure modification,combining with carbon and optimization of electrolytes as well as the voltage windows.The micro-nanostructures can shorten the diffusion path of sodium ion and increase the diffusion rate of sodium ion.The carbon composite can enhance the electronic conductivity and improve the cycling stability of the electrode material.In addition,compared with carbonate-based electrolytes,ether-based electrolytes possess lower electrochemical reaction energy barrier and higher solvent-salt stability.Especially for metal sulfides and selenides,ether-based electrolytes are difficult to react with the intermediate products.And the higher discharge terminal voltage can effectively enhance the cycling performance.The main research contents are as follows:(1)Layered MoSe2 grown on N,P co-doped rGO as an advanced anode material for sodium ion batteries.The two-dimensional layered MoSe2 has expanded interlayer space which can be easily employed for reversible storage of sodium ions.But,the further practical application of MoSe2 has been hamstrung by its drawbacks such as large volume variation and relatively low electronic conductivity.While composited with the heteroatom doped graphene could dramatically improve the electronic conductivity and increase reversible storage sites.Therefore,MoSe2/N,P-rGO was synthesized by a solvothermal reaction followed with a high-temperature calcination.This material exhibits high specific capacity(387 mA h g-1 after 1000 cycles at 0.5 A g-1),remarkable cycling stability(capacity retention of 87%at the second cycle)and superior high-rate capability(the capacity at 15 A g-1 equals?55%of that at 0.5 A g-1),which is much better than MoSe2/rGO and pure MoSe2.In order to further investigate the practical application of MoSe2/N,P-rGO,the full cell was assembled and characterized,demonstrating that MOSe2/N,P-rGO is a superior electrode material for high-rate sodium ion batteries.Furthermore,ex situ Raman spectra and high-resolution transmission electron microscopy images were carried to confirm the electrochemical mechanism of MoSe2.(2)ZnSe nanoparticles anchored on amorphous carbon-coated multi-walled carbon nanotubes as an anode material for sodium ion batteries.MWCNTs?a-C@ZnSe composites were obtained by hydrothermal process.The MWCNTs@a-C as carbon substrate not only provides defect sites,but also enhances the electronic conductivity.As a result,MWCNTs@a-C@ZnSe shows superior electrochemical properties.The reversible capacity of MWCNTs@a-C@ZnSe is remained at 251 mA h g-1 after 5000 cycles at a current density of 2 A g-1.For comparison,the reversible capacity of MWCNTs@ZnSe and pure ZnSe electrode exhibit a relatively rapid degradation with a left capacity of 176 and 92 mAh g-1 after 2800 cycles,respectively.The better cycling performance of MWCNTs@a-C?ZnSe can be associated with the following reasons.First,the amorphous carbon coated with MWCNTs can improve the load of ZnSe nanoparticles and thus owning a higher capacity.Sencond,the carbon substrate can effectively buffer the volume expand upon cycling and also enhance binding energy between intermediate products and current collector.Finally,MWCNTs@a-C have an extremely high specific surface area and high electrical conductivity.These characteristics are the main reasons for the excellent rate performance of MWCNTs@a-C@ZnSe.In addition to the structural modification,the choice of electrolyte and voltage window is also greatly important to the electrochemical performance.Compared with the carbonate-based electrolyte,the ether-based electrolyte is more stable that can effectively avoid the dissolving of intermediate products during cycling.Moreover,the higher discharge terminal voltage can reduce the.discharge depth and improve the cycling stability.(3)ZnSe composite with polymer or carbon-based materials as anode materials for sodium ion batteries.ZnSe as a potential anode material for sodium ion battery has some defects such as low electronic conductivity,huge volume changes and big polarization.Based on our previous work and other literatures,it has been proved that the combination of ZnSe and carbon materials can effectively improve the battery performance.Besides MWCNTs composites,the electrochemical properties of ZnSe-PPY,ZnSe-rGO and PHCS@ZnSe were also investigated.Compared with pure ZnSe,the experimental results show that the properties of all composites were improved distinctly.ZnSe-PPY exhibits good cycling stability(240 mAh g-1 after 1000 cycles at the current density of 0.5 A g-1)and rate capability(140 mAh g-1 at the current density of 10 A g-1);The reversible capacity of ZnSe-rGO retained at 326 mAh g-1 after 800 cycles at 0.5 A g-1 and the rate capability still retained 168 mAh g-1 at 10 A g-1;The reversible capacity of PHCS@ZnSe kept at 198 mAh g-1 after 600 cycles at 0.5 Ag-1.The introduction of conductive polymers,graphene and porous hollow carbon spheres can improve the electrical conductivity and inhibit volume effect of electrode.