Design of Separator Modifier and Cathode Host Materials in Lithium Sulfur Batteries for Enhanced Performance
Author:Tian Wen Zhi
Supervisor:xiong sheng lin
As a clean and available energy,electricity is getting more and more wildly used With the development of the society and the advances in technology,the public pay more and more attention on the environmental issues.The application of electricity is not only limited in the large-scale equipment and construction projects,but also the demands in the smaller electronic devices and vehicles are growing.Secondary batteries attract worldwide attentions because of their convenient and efficient in electricity storage and release.As a first generation secondary battery,the consumption of lead-acid cell decreases year by year because of its lower specific capacity and environment harmful heavy metal.Lithium ion batteries,as alternative secondary batteries,whose usage is increasing progressively year after year.But there are also some defects that astrict the further application of commercialized lithium ion batteries,such as the moderate capacity,the higher price and the lower stocks of cobalt.Lithium sulfur battery have become hot areas for a period because of the abundant reserves for the raw materials,the cheaper prices and their environment-friendly character.Using sulfur as cathode,lithium sulfur battery has higher specific capacity(1675 mAh g-1)and high energy density(2500 Wh Kg).As a result of this,lithium sulfur battery was considered to be one of the most promising secondary battery systems.However,there still some obstacles that hinder the commercialization of the lithium sulfur battery,such as,the electrical conductivity of sulfur and lithium sulfide which as the discharge final product are lower(<10-17 S m-1)against the electrochemical process.As well as the higher volume expand effect during the charge-discharge process lead to the pulverization and peeling of the cathode material such to influence the life of the battery.Furthermore,the intermediate products of long-chain polysulfide would diffuse to the anode side and deposition on their surface,result in the lower Coulombic efficiency,poor cycling stability and worse performance,which is called the shuttle effect.In respect of the lithium sulfur issues above,our attention is focused on the design of the modified layer coated on the separator and the synthesis of cathode sulfur carrier material.Moreover,the optimization of the material is accomplish after testing the electrochemical performance and analyzing the result.Our main works are summarized as the following:(1)The mixture of multiwall carbon nanotube,commercial acetylene black and polyvinylidene fluoride mixed with a certain mass ratio,then coated on the separator of Celgard 2400 which is used as separator in lithium sulfur battery.The cathode is a mixture of sulfur and acetylene black spreading on the aluminum foil without thermal melting at 155℃.Through changing the thickness of the coated layers,the relationship between the performance and the thickness of the coated layers is discussed.When the coated layer is 300μm,the lithium sulfur battery showed the best cycling stability and rate performance.But if we consider the overall weight of the whole modified separators,the separator which thickness is 200 μm possess the best performance.Furthermore,through the comparison of two assembling methods,we demonstrate that only if the conductive modifier cling to the cathode,the coated layer can be a second electron collector.Meanwhile,the modifier can be a sulfur reservoir during the charge-discharge process to prevent the loss of sulfur.(2)The calcium citrate is used as precursor,after calcination process and washing with hydrochloric acid,the three-dimensional porous carbon whose specific surface area is as high as 1335 m2 g-1,composed of carbon nano-sheets is obtained.The electrical conductivity,degree of graphitization as well as the specific surface area of the porous carbon regulated can be regulated through controlling the calcination temperature.We take this porous carbon to modified the separators using in lithium sulfur batteries.It is confirmed that the degree of graphitization will increase with the temperature raising.Moreover,the specific surface area and average pores size also increase with the higher temperature.Furthermore,as a result of the smaller resistance,the lithium sulfur batteries assembled with separator modified using porous carbon obtained at 1300℃ show best rate performance(3)Monodispersed VO ultrafine nanocrystals immobilized on nitrogen-doped graphene(VO NC@NG)was obtained through reflux method with mixture of GO dispersion solution,melamine and sodium metavanadate.Furthermore,after selenylation with selenium in the tube furnace,the monodispersed VO particles transferred to be VSe2 nanoparticles,which are still uniformly dispersed(VSe2 NC@NG).Nextly,the nitrogen doped-reduced graphene oxide(NG),VO NC@NG and VSe2 NC@NG are used to modify the separator used in lithium sulfur batteries.Through the comparison,it is clear that VSe2 NC@NG is the most suitable material for lithium sulfur battery with best cycling stability(971 mAh g-1 after 200 cycles at 1 C,760 mAh g-1 after 550 cycles at 2 C)and excellent rate performance(850 mAh g-1 at 2 C).In order to explore the deep reason of the excellent performance,electrochemical tests like EIS,CV,Tafel and GITT curves are carried and we find that the battery with the VSe2 NC@NG modified separator shows the minimum charge-transfer resistance as well as maximum lithium diffusion coefficient,implying the more rapid electrochemical reaction rate.Furthermore,through the Li2S deposition test and the adsorption test,it is confirmed that the VSe2 NC@NG can promote the uniform nucleation process of Li2S as well as immobilize lithium polysulfide,which are benefit for the cycling stability.(4)Few layers MoSe2 modified reduced graphene oxide(FM-rGO)is acquired through one-pot hydrothermal reaction.Through the XPS and HRTEM we confirm that it is composed of two phases,one is the 1-T MoSe2 and the other is 2-H MoSe2.When used as substrate for sulfur,there generate Mo-S bond between the MoSe2 and sulfur,such that sulfur is immobilized on the surface of FM-rGO.After assembled as cathode in lithium sulfur battery,it shows high capacity of 1086 mAh g-1 after 240 cycles at a current of 0.25 C(1 C=1675 mA g-1).The adsorption test results and the SEM images of the separator from unpacked after-cycled cell,give visual evidence of the strong affinity between lithium polysulfide and FM-rGO.The EIS indicate that the overall resistance of material increased after the introduction of MoSe2,while the diffusion coefficient increased slightly instead of decrease.Meanwhile,when the mass loading of sulfur raised to 4.6 mg cm-2,the capacity of the battery can still keep at 870 mAh g-1 after 120 cycles at 0.3 C.