The Confined Sulfur Spcies And Electrochemical Performance Of Carbon And Sulfur-Based Electrode Materials

Lithium-sulfur?Li-S?batteries with high theoretical energy density(?1675 mA h g^-1,or 2600 Wh Kg^-1)and cost effectiveness have attracted tremendous interest.However,the sulfur cathode in Li-S batteries have been faced with three main problems:i)the conductivity of sulfur and lithium sulfide is very poor;ii)the soluble lithium polysulfide would lead to unavoidable shuttle effect in the process of charging and discharging,resulting in lower coulomb efficiency;iii)the volume change after the lithiation of sulfur would lead to the collapse of the electrode.In this paper,carbon or sulfide was used in Li-S battery cathode materials to ease shuttle effect caused by the dissolution of lithium polysulfide,which could enhance the Li-S battery coulomb efficiency and cycle stability.This thesis mainly studies the three aspects of content:1)Nitrogen-doped porous double-layer hollow carbon nanospheres were studied to fix the polysulfide in Li-S battery and the corresponding lithium/sodium storage properties of the porous double-layer hollow carbon materials.2)As a cathode for Li-S batteries,GO-coated,PANI-wrapped bipyramidal sulfur particles?S@PANI/GO?were prepared to enhance the capability and cycle stability.3)The mental sulfides with different electrochemical platform as additives of polysulfide in Li-S batteries were studied systematically.In addition,the lithium storage properties of the as-prepared Fe7S8/C composites were studied due to the special platform??1.5 V?.The main research work of the paper is summarized as follows:1.Uniform nitrogen-doped double-shelled hollow carbon spheres?N-DHCSs?were prepared by Fe₃O₄ template and carbon source of glucose with different acids.Then we coated nitrogen-doped porous double-hollow carbon nanospheres on the polymer membrane through the water-based binder,and the study shows that the coating can effectively inhibit the shuttle effect,improving the specific capacity and cycle stability with 508 mA h g^-1 after 300 cycles at 2 C even after a different rate cycling.On this basis,XRD,XPS,EA and FT-IR,HRTEM were analyzed to reveal the mechanism process during cycling process in lithium ion battery and sodium ion battery.All of the DHCSs materials by different acid treatment had good cycling stability.While,as anode materials in LIBs,the N-DHCSs?HNO3 treated?show higher capacity of 512mAhg^-1(only 300 mA hg^-1 with DHCSs)after 500 cycles at 1.5 C(558 mA g^-1)comparing with DHCSs.Moreover,the N-DHCSs used as anode materials in sodium ion battery also exhibit reversible capacity of 120 mA h g^-1 even at 0.2 A g^-1 after 100 cycles.2.At the low temperature,we successfully prepared the S@PANI/GO composite.As a carbon source,the functional groups on the GO surface can alleviate the dissolution of polysulfides by chemical bonding even though sacrificing partial conductivity of graphene.This composite exhibits better electrochemical performance(specific capacity of 875 mA h g^-1 after 100 cycles at 0.2 C)and high-rate capability(4 C,466 mA h g^-1)than those of S,S/GO,S@PANI.Even at 1 C,a specific capacity of 640 mA h g^-1 after 300 cycles can still be obtained.This improved electrochemical performance is mainly caused by the core-shell structure in composite and the synergistic effect of graphene oxide and polyaniline.3.The roles of transition metal sulfides with different electrochemical platforms such as zinc sulfide?<1.0 V?,iron sulfide??1.5 V?and niobium sulfide??2.3 V?on the performance of lithium-sulfur batteries were investigated.The results show that as additives,these transition metal sulfides have not improved the energy density of whole lithium-sulfur batteries,but get a certain effect on polysulfides adsorption,which can improve the cycle stability in lithium-sulfur batteries.Such as,the batteries could retain 600 mA h g^-1?NbS2 as additive?,493 mA h g^-1?Fe7S8 as additive?and 440 mA h g^-1?ZnS as additive?,while the bare electrode?without additive?only got 300 mA h g^-1 after 200 cycles.Fe7S8 and carbon coated Fe7S8 nanoparticles were used as additives to characterize the stability of Li-S battery.The results showed that uncoated Fe7S8 nanoparticles could improve the cycling stability of lithium-sulfur battery.Last but not the least,scalable and uniform Fe7S8@C nanospheres were synthesized by a simple solid-solid reaction.The as prepared samples exhibited an excellent cycle performance.The core-shell Fe7S8@C composites showed a very high capacity of 695 mA h g^-1 at current rate of 0.1 A g^-1 after 50 cycles between 3 and 0.01 V.The Fe7S8@C composites also exhibited a discharge plateau at about 1.5 V,and illustrated a capacity of 250 mA h g^-1 even at 1 A g^-1 between 1.20 and 2.50 V.