| Li-S batteries have attracted intensive research attentions own to its high theoretical specific capacity density and high theoretical specific energy density.However,Li-S batteries have its'bottlenecks,such as the low electric conductivity of sulfur,the shuttle phenomenon of polysulfide,the volume effect of active material during cycling and so on,all of which make Li-S batteries with low active utilizaition,poor cycle performance and poor rate performance,and cannot used in commericial.Selenium,owing to its similar electrochemical properties with sulfur,has attracted intensive research attentions as the most promising electrode material for Li-Se battery.Li-Se batteries have high volumetric capacity(3253 AhL-1)which is similar to that of Li-S batteries,besides the electronic conductivity of Se(1 × 10-3 Sm-1)is much higher than that of S and the shuttle phenomenon of polysulfide is much lower than that of polyselenide.However,the Se cathode also has its' issues,like low utilization of bulk Se and the shuttle phenomenon of polyselnide.Nowadays,method to overcome the issues of Li-S batteries(Li-Se batteries)is mainly focused in combining sulfur(selenium)with conductive material,then through traditional doctoc-blading process to fabricate electrode film,thus to improve the electrochemical performance of Li-S batteries(Li-Se batteries).However,one on aspect,this method didn't effctively inhibit the shuttle phenomenon of polysulfide(polyselinide).One the other aspect,traditional doctoc-blading process introduces non-active material(like PVDF,acetylene black)inevitablely,which greatly reduce the volume energy density of batteries.In addition,as the limited abundance of Li in the Earth's crust,room temperature(RT)Na-S batteries and Na-Se batteries also exhibit promising future.This thesis fabricates porous carbon nanofiber films(copper-immobilized sulfur-porous carbon nanofibers films)through electrospining technique,then infiltrates sulfur(selenium)into the fabricated porous films by co-heating process,finally obtained porous carbon nanofiber(copper-immobilized sulfur-porous carbon nanofibers)-sulfur(selenium)composite films.The free-standing films were directly used as the working electrode to perform batteries with 2032 coin cells.Li/Na metal was used as counter and reference electrodes.Electrochemcial performance of the batteries are further tested.Main results of this thesis are summarized as follows:In chapter 1,the author gives a detailed discussion about the working mechanism,the research progress of electrode materials and electrolyte,and the obstacles of Li-S batteries.After that,the research progress of Li-Se batteries,room temperature Na-S batteries and electrospinning technique are introduced.Based on these discussions,the author presents the research background and research content of this thesis.In chapter 2,the author introduces the reagents,chemicals,synthesis strategies and testing methods involved in this thesis.Among them,methods to prepare traditional electrode films and self-surpported electrode films with excellent flexibility are emphatically introduced.In addition,the author gives a briefly introduction about the assembly processing of half-battery and the electrochmical testing.In chapter 3,we infiltrate sulfur molecules into porous carbon nano fiber films that werefabricated by electrospining technique.The obtainedflexible and free-standing sulfur/porous carbon nanofibers cathode electrode was used for Li-S batteries.After introduction of optimized CNT amount into the porous carbon nanofibers,electron conductivity of the electrode material was further improved.Compared with the sample without CNT,the sample with CNT added exhibits a much better lithium storage performance.When cycled at 0.05Ag-1 for 100 cycles,a reversible capacity of 637mAhg-1 is still maintained.Even at lAg-1,it could still deliver a high capacity of 437mAhg-1.The excellent electrochemical performance of this material is ascribed to the introductionof CNT and the uniform distribution of micropores in the porous carbon nano fibers.In chapter 4,a proper content of copper salt is added into the electrospinning precursor solution.Through electrospinning technique,Cu embedded porous carbon nanofiber films wassuccesfully prepared.After impreganating sulfur into the films,a high performance electrode material for Li-S batteries wasobtained.The Cu-S bonding in the electrode material can efficiently inhibit the shuttle phenomena of polysulfide.Compared with the electrode material without Cu added,the electrode material with Cu added exhibits a much better electrochemical performance.When cycled at 0.05Ag-1 for 100 cycles,a reversible capacity of 680mAhg-1 is still maintained.Even at 1Ag-1,it could still deliver a high capacity of 415mAhg-1 with colombic efficiency almost reaching 100%.The improved electrochemical performance of electrode material is ascribed to the chemical stability effect from Cu-S bonding and physical stability effect from micropores of porous carbon nanofibers.In chapter 5,as selenium has similar electrochemical performance to that of sulfur,and the electrochemical performance of battery with selenium as cathode material is more stable than that of sulfur,the authors focus their research attention on the battery with selenium as cathode material.We designed larger pores in carbon matrix for loading seleniumbecause of the larger diameter of selenium molecule.We introduced F-127 which acts as soft template into the electrospinning precursor solution.Through electrospining technique and activation processing,porous carbon nanofiber films with abundant mesopores is prepared.We further load the selenium molecules into the porous nanofiber films by co-heating processing.Compared with the sample without F-127 added,the sample with F-127 added exhibits a much more superior lithium/sodium storage peformance.When cycled at 0.5Ag-1 for 900 cycles,a reversible lithium storage capacity of 516mAhg-1 is still maintained.Even at 4Ag-1,it could still deliver a high capacity of 306mAhg-1.For Na-Se bateery,when cycled at O.05Ag-1 for 80 cycles,a reversible lithium storage capacity of 520mAhg 1 is still maintained.Such a wonderful electrochemical performance of Li-Se batteries and Na-Se are seldom seen in the previous reports,and our work also gives a new method to prepare mesoporous carbon material.In chapter 6,the authors give a much easier procedure to prepare flexible electrode material for Li-Se and Na-Se batteries.Through co-heating the Se powder and electrospun derived polymer nanofibers,flexible electrode material with structure of Se distributed uniformly in carbon nanofiber matrix is obtained.Such a co-heating processing is easy and convenient.Beside,such a co-heating processing can lead a chemical bonding between carbon nanofiber matrix and Se molecules.A proper amount of CNT is further added into the carbon nanofibers to improve the electron conductivity of electrode material.The results of electrochemical tests show that the sample owns excellent electrochemical performance when used as cathode material for both Li-Se and Na-Se batteries.In chapter 7,as we know the electrochemical performance of battery with selenium as cathode material is more stable than that of sulfur,we design a procedure to introduce a small percentage of Se atoms into S molecules.After that,the modified sulfur is further loaded into the pores of porous carbon nanofiber films prepared in our previous work.The obtained electrode exhibits a superior lithium/sodium storage performance.Even at 20Ag-1,it can deliver a reversible lithium capacity of 181 mAhg-1 And it can also deliver a reversible capacity as high as 190 mAhg-1 for sodium storage when cycled at 2Ag-1.Such a superior electrochemical performance of electrode material is ascribed to the Se-S bonding that effectively inhibits the shuttle phenomena of polysulfide.Besides,Se doping also greatly improves the conductivity of electrode material.In chapter 8,the authors focus their research attention on the carbon-based anode material for Na ion batteries.First,through electrospining technique and aqueous polymerization processing,hollow polypyrrole nanofibers were fabricated.Second,after activation at high temperature,N doped hollow porous carbon nanofibers with abundant micropores in the carbon nanofibers is succesfully prepared.N doping can improve the electron conductivity of carbon material and the hollow carbon nanofiber structure is in favor of the soaking of electrolyte.Besides,the micropores in the carbon nanofibers also provide abundant sitesfor sodium storage.Compared with the hollow carbon nanofibers without activation processing,the hollow carbon nanofibers with activation processing exhibits a better electrochemical performance.After 100 cycles at 0.05Ag-1,a reversible capacity as high as 160mAhg-1 can still be delivered.In chapter 9,the authors give a summary of achivements and an outlookof this thesis in the end of thesis. |
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