| Owing to the high energy density(2600 Wh kg-1)and superior theoretical specific capacity(1675 mAh g-1)of sulfur cathode,rechargeable lithium-sulfur(Li-S)batteries have been recognized as a promising candidate for next-generation energy storage systems.Nowadays,the commercialization of Li-S batteries is mainly hindered by the low sulfur utilization and the rapid capacity fade,owing to the poor electrical conductivity(5 × 10-30 S cm-1)and large volumetric expansion(?80%)of elemental sulfur,the high solubility of intermediate polysulfides(Li2Sn,4 ? n?8)formed during galvanostatic charge-discharge processes and the back and forward shuttling effect of these polysulfides between cathode and anode.As previously reported,incorporation of sulfur into a porous conductive matrix could improve the conductivity of elemental sulfur and effectively buffer the volumechange during cycling.And also,the surface pendent functional groups of porous conductive framework together with the confined effect of pores could retard the shuttling effect of intermediate polysulfides and then improve the utilization of elemental sulfur.Therefore,to resolve the above mentioned problems,this dissertation focuses on the design and preparation of low-cost porous carbon as the sulfur-loading hosts at first,then on the structural and electrochemical characterizations of these carbon-sulfur composites,and then on the relationship between the structure of porous carbon and the electrochemical properties of corresponding sulfur-loading composites.The research works in this dissertation can be simply divided into 3 parts,shown as the following:(1)A framework of macro-/micro-porous carbon derived from commercial lignin is prepared by one-step carbonization/activation method and then utilized as sulfur-loading matrix to assay the effect of sulfur-loading time on the structural and electrochemical properties of carbon-sulfur composite(C-S-t,t defined as sulfur-loading time).As-prepared porous carbon possesses a high specific surface area of 1211.6 m2 g-1 and a pore volume of 0.59 cm3 g-1,acquires oxygen-containing functional groups on the surface of framework and functionalizes for the chemical adsorption of elemental sulfur.Under N2 atmosphere(flow rate?60 mL min-1)the longer is the sulfur-loading time,the lower value is the total sulfur content of carbon-sulfur composite and the higher percentage of sulfur embedded within the micropores.At a sulfur-loading time of?10 h,the resulting C-S-10 composites have a total sulfur content as low as 50.0 wt%and 44.8%in micropores,while C-S-6 composites formed at 6 h have a total sulfur content of?58.8 wt%and 29.4%in micropores.When applied as Li-S battery cathodes,C-S-10 composites deliver a high discharge capacity of 1241.0 mAh g-1 in 2nd cycle and maintain a specific value of 791.6 mAh g-1 in the 100th cycle at 0.5 C,much higher than C-S-6 composites(the 2nd capacity?920.4 mAh g-1;the 100th value?674.9 mAh g-1;0.5 C).Therefore,the controlling encapsulation of sulfur into the micropores of lignin-derived macro-/micro-porous carbon endows the resulting C-S-t composite with high electrochemical performance for its potential application as Li-S battery cathode.(2)Cross-linked non-porous carbon nanofibers(CNF)and hydroxylated mesoporous carbon nanofibers(m-HCNF)are prepared and comparatively utilized as sulfur-loading matrices to improve the lithiation-delithiation reversibility between molecular S8 and metallic lithium.As-prepared m-HCNF nanofibers gives a specific surface area of 1808.9 m2 g-1 and a pore volume of 1.77 cm3 g-1,larger than those of CNF(specific surface area?192.5 m2 g-1 and pore volume?0.42 cm3 g-1).After sulfur-loading,the sulfur contents of S/CNF and S/m-HCNF are 47.0 and 62.0 wt%,respectively.When model cells are assembled,the reversible capacity of S/m-HCNF electrode reaches a high residual value of 884.1 mAh g-1 after 150th cycles at the current rate of 0.5 C,higher than that of S/CNF(599.3 mAh g-1)under the same electrochemical state.According to the concentration of S8 molecule,at open-circuit voltage,the estimated lithium-ion diffusion coefficient(DLi)value of for S/CNF or S/m-HCNF composite is 3.51 × 10-15 or 4.21 × 10-5 cm2 s-1.By comparison,the DLi value in S/m-HCNF electrode is?1.2 times as big as that in bulk S/CNF,which can be reasonably attributed to the shorter Li+-ion diffusion path for mesoporous structure of S/m-HCNF.From the 10th cycle to the 100th cycle,the estimated DLi values of S/CNF and S/m-HCNF composite electrodes decrease by 64.3%and 12.8%in sequence.These further indicate a superior lithiation-delithiation reversibility of elemental surlfur in composite S/m-HCNF.In a word,all these results indicate an enhanced lithiation-delithiation reversibility of composite S/m-HCNF for high-performance Li-S batteries.(3)Cobalt carbonate hydroxide was firstly prepared using CO(NH2)2 and Co(NO3)2·6H2O and then reacted with 2-Methylimidazole,the resulting product was porous Co-containing MOF.The MOF-derived mesoporous carbon(i.e.C-pristine)was prepared by carbonization of porous Co-containing MOF and the modified sample of mesoporous carbon(i.e.C-modified)was prepared by acid etching of C-prisitne.The C-pristine and C-modified were used as conductive matrices to encapsulate elemental sulfur,then the structural and electrochemical properties of two sulfur-containing composites(i.e.composite-1 and composite-2)were studied to clarify the effect of acid etching.As-prepared C-pristine acquires a specific surface area of 206.5 m2 g-1 and a pore volume of 0.29 cm3 g-1,and the loading amount of elemental sulfur is less than 45.2 wt%for the corresponding composite-1.After etching the modified mesoporous carbon(C-modified)exhibits an improvement in porosity(i.e.,the area?650.2 m2 g-1 and the volume?0.77 cm3 g-1),and the corresponding composite-2(S-content?50.0 wt%)is endued with the quite different structural properties.Also,at 0.5 C the composite-2 cathode delivers a discharge capacity of 925.1 mAh g-1 in 2nd cycle and maintains a high value of 781.1 mAh g-1 in the 140th cycle,much higher than those of the composite-1 cathode(the second?690.9 mAh g-1 and the 140th?338.2 mAh g-1).Both of the composite electrodes display a slight increase of electrolyte-solution resistance Re or surface-film resistance Rf and an obvious decrease of charge-transfer resistance Rct.At the constant dosage of electrolyte(30 mL)per gram of sulfur,the Re,Rf and Rct values of composite-1 electrode are always bigger than those of composite-2 cathode under each electrochemical state and the enhanced trends of the composite-2 may relate to its superior electrochemical performance comparatively.All these results indicate the wet-chemical modification of Co-containing MOF-derived mesoporous carbon is of crucial importance for its potential application as a sulfur-loading host in rechargeable Li-S batteries. |
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