Construction Of Bio-carbon-based Composite Structures And Performance Of Lithium-sulfur Batteries

Lithium-sulfur batteries are widely studied due to advantages of sulfur materials.Sulfur possesses characteristics of abundant raw materials,low cost,eco-friendly and high theoretical specific capacity(1675 mAh g-1).Shuttle effects of polysulfides in lithium-sulfur batteries lead to large capacity losses during cycling process.A variety of methods have been adopted to alleviate shuttle effects,including preparing functionalized cathodes,modificating separator,optimizing electrolyte and designing composite structures.Designing composite structures for loading sulfur and polysulfides is the most effective way to alleviate shuttle effects of polysulfides.The composite structure captures polysulfides by spatial and chemical constraints,which can effectively enhance cycling stabilities of lithium-sulfur batteries.Regulation of carbon-based composite structures can be applied by changing factors such as hirarchical pore structures,surface functionalization,dimension and electrical conductivities,which significantly affect electrochemical performance of lithium-sulfur batteries.Limited sulfur-loading abilities,complex preparation methods and low volume capacity of traditional carbon materials is not benefit for their developments.Abundant raw materials,simple preparation methods and abundant oxygen-containing functional groups on the surface of bio-carbon materials are beneficial to loading of sulfur.Metal oxides(such as titanium dioxides and tin dioxides)have high volume capacity and strong surface polarity,which can enhance abilities to capture polysulfides.Loading titanium dioxides and tin dioxides on the surface of bio-carbon can combine strong conductivities of carbon materials and strong polarity of oxide surfaces,inhibit shuttle effects of polysulfides and accelerate their conversion,which improve performance of lithium-sulfur batteries.Dandelion(fiber plant)is mainly used as raw materials and their surface structures are regulated as basic skeletons in this paper.Cattail(fiber plant),cobalt-doped modified titanium dioxides,tin dioxides/stannous sulfides and other materials are loaded on the surface of dandelion to obtain composite structures,which are suitable for high sulfur-loading.The performance of lithium-sulfur batteries with multiple composite structures is tested and the relationship between structure and electrochemical performance is analyzed.Besides,the electrochemical reaction mechanism is analyzed.Research results are concluded below:(1)By controlling carbonization atmosphere,carbonization hours and air atmosphere,defects of bio-carbons are adjusted to obtain intrinsic defects,oxygen-containing defects and both intrinsic defects and oxygen-containing defects,respectively.As for carbon materials with both intrinsic defects and oxygen-containing defects,when the electrode surface contains high intrinsic defects and low oxygen-containing defects,excellent rate performance can be achieved under high sulfur-loading conditions(80 wt%)(20C 299.4 mAh·g-1).Intrinsic defects possess stronger polysulfide conversion abilities than oxygen-containing defects.(2)Taraxacum and dandelion are used as bio-carbon sources to prepare granular structure(PC),tubular structure(PGY)and composite-structure of bio-carbon materials.When the mass of PC is 3 times than that of PGY,two structures are more closely combined,which is more conducive to uniform loading of sulfur inside the carbon material,thereby improving cycling stabilities of lithium-sulfur batteries.Under high sulfur-loading conditions(80 wt%)and at current density of 1C,composite-structure electrodes possess capacities of 496.7 mAh·g-1 after 500 cycles.At the same time,composite-structure electrodes also possess good rate performance.The capacity can reach 524.9 mAh·g-1 at a current density of 5C.Two kinds of C-S bonds are produced in composite structure electrodes.The C-S bond formed by sulfur and PC has a strong ability to convert polysulfides and the C-S bond formed by sulfur and PGY can promote diffusion of lithium ions during the reaction process.(3)Titanium oxides are loaded on the surface of bio-carbon and differernt titanium oxides/bio-carbon composite structures are obtained by adjusting load position,load thickness and surface activities of titanium oxides.When the mass ratio of surfactant and bio-carbon is 1:1 and the molar ratio of Co and Ti is 0.05:1,composite structures can achieve good rate performance(20C 500.9 mAh·g-1)while ensuring their cycling stabilities.The reversible capacity is 375.0 mAh.g-1 after 500 cycles at 1C.The introduction of cobalt sources has two effects.On the one hand,more Ti-O defects are produced during the nucleation of titanium oxides and surface activities of titanium oxides are enhanced;On the other hand,oxygen atoms on the surface of bio-carbons are replaced and crystallization of bio-carbons is catalyzed,which improve conductivities of composite materials.Good surface activities and conductivities both promote reversible adsorption of poly sulfides on the surface of composite structures.(4)Regulating pyrolysis process of bio-carbon is used to obtain composite structures of bio-carbon/tin oxides with different graphitization degrees and different surface pre-carbonization degrees.Furtherly,on the basis of works above,tin oxides are vulcanized to directly obtain C/SnO2/SnS/S composite structures.Partially vulcanized C/SnO2/SnS/S composite electrode materials have excellent cycling stabilities and rate performance.The reversible capacity is 531.4 mAh·g-1 at 1C for 500 cycles.The capacity remains 552.1 mAh·g-1 at 20C.All sulfur atoms in C/SnO2/SnS/S composite electrodes participate in the electrochemical reaction process.Sn-S bonds lay the foundation for rapid lithiation of sulfur,C-S bonds ensure structural stabilities and C-O-Sn bonds make rapid capture of polysulfides possible.Under synergistic effects of these chemical bonds,high capacity(189.2 mAh.g-1 20C)of P1 part is realized.