Functionalized Carbon-based Composites:Design,Preparation And Their Application In Lithium-Sulfur Batteries

Lithium-sulfur(Li-S)batteries with extremely high theoretical capacity(1675 mAh g-1)and high theoretical energy density(2600 Wh kg-1)have attracted tremendous attention in the past few years and have been widely researched considering its promising potential to satisfy the increasing demand for portable devices,electric vehicles and grid energy storage systems.Sulfur as the cathode active material has the characteristics such as rich reserves,cost-effectiveness,and environmental friendliness.However,a variety of shortcomings,such as the electronic insulation of elemental sulfur and its discharge products(Li2S and Li2S2),the huge volume changes during charge and discharge,the serious shuttle effect,and the pollution and corrosion of lithium anodes,result in the low sulfur utilization(<80%),the rapid capacity decay and serious self-discharge,which seriously restrict the practical application and commercialization of lithium-sulfur batteries.In order to solve the issues faced by lithium-sulfur batteries,an effective way to improve the electrochemical performance is to design sulfur hosts or interlayer materials that can effectively confine and avoid polysulfides diffusing into electrolyte.Based on the rational design and elegant optimization for lithium-sulfur batteries,a variety of functionalized carbon-based composites were prepared as sulfur host or interlayer materials.And their structures,compositions and electrochemical performances were measured in detailed.The main contents of the study are as follows:(1)A sulfur-hydrazine hydrate solution had been employed to chemically prepare the composite of S@N-rGO with 76%sulfur laoding to directly sereve as the cathode material.After refluxing,sulfur crystal was homogeneously deposited within N-rGO matrix from the sulfur-hydrazine hydrate solution,wherein the rGO was reduced and N-doped by the hydrazine hydrate.Hence,the hydrazine hydrate in this work played as a dual role,which acted as not only a solvent for sulfur but also a reducing and doping agent for graphene oxide.The S@N-rGO composite was characterised by SEM and TEM,and no large aggregated sulfur was observed,demonstrating the superiority of chemical sulfur loading by hydrazine hydrate.Residual oxygen atoms and doped N atoms in the multifunctional carbon matrix of N-rGO acted as chemisorption sites,ensuring effective binding of polysulfides and intimate contact of sulfur with the conductive matrix.Meanwhile,the porous network framework can satisfy the huge volume change of sulfur during the charge-discharge reaction process.Therefore,the prepared S@N-rGO composite has good cycle stability and high rate performance as cathode material of lithium batteries.(2)Biomass jellyfish umbrellas were used to prepare N,P-codoped activated carbon(denoted as NPAC)through soaking in NH4H2PO4 solution and activation process by potassium hydroxide and applied as the holder of the electrochemical active sulfur for Li-S batteries.Benefiting from the three-dimensional interconnected porous architecture,the high surface area,the large pore volume and the dual-doping of nitrogen and phosphorus,the as-obtained NPAC facilitated a high utilization of sulfur and enabled good capacity retention.Even loading with 72%sulfur,NPAC-S72 delivered a discharge capacity of 517 mAh g-1 after 300 cycles at 1 C with coulombic efficiency close to 100%.When the sulfur content was as high as 82%,the NPAC-S82 sample showed the comparable cycling capability to the NPAC-S72.This work confirmed the unique property of the biochar materials,which proposed the promising route for the synthesis of the sulfur matrix.(3)A multifunctional and hierarchical porous PCIIFe3O4@rGO||graphite hybrid interlayer was explored for lithium-sulfur batteries,which had been ingeniously synthesized with filter paper as backbone through capillary,pencil-drawing and calcination method.The hybrid interlayer showed the merits of high specific surface area,3D interwoven fiberous network,hierarchically porous structure,enhanced polarity,and superior conductivity,which could efficiently trap polysulfides,channel the electrolyte,and be beneficial to the fast transportation of Li ions and electrons,coinciding with the results of in situ UV-vis absorption spectra and kinetics analysis.The application of PCIIFe3O4@rGO||graphite as interlayer enabled the sulfur cathode without complicated synthesis or surface modification to display high-performance,including greatly-enhanced capacity,long cycling stability,and ultrahigh rate capability.It delivered a reversible capacity of 653,660,590,and 418 mA h g-1 at 1 C,2 C,4 C,and 10 C for 500 cycles,respectivley.Even when the sulfur increase to 8.05 mg cm-2,the capacity can still be as high as 518 mAh g-1(4.17 mAh cm-2)at 0.1 C after 100 cycles,corresponding to 76.3%of the fifth cycle discharge capacity.The investigative results demonstrated that the strategy proposed here could be helpful for exploring and applying new multifunctional interlayer and makes a step towards the application of advanced Li-S batteries(4)The paper-based GPCIIFe@Fe2O3||Fe1-xS@Fe2O3 composite was prepared by a simple and environmentally friendly route of "stepwise immersion impregnation and carbonization".It was confirmed by FESEM,HAADF-HRTEM and XRD that GPCIIFe@Fe2O3||Fe1-xS@Fe2O3 consisted of core-shell Fe@Fe2O3 and FeS@Fe2O3 nano-particles uniformly embedded in reduced graphene coated paper carbon.The self-supporting sandwich-structured lithium-sulfur batteries were prepared by using GPCIIFe@Fe2O3||Fei-xS@Fe2O3 simultineously as the carrier of sulfur and functionalized interlayer.Due to the favorable features of high specific surface area,3D fiber interlacing,hierarchical porous structure and strong polarity,the GPC||Fe@Fe2O3||Fe1-xS@Fe2o3-1/S elecrode displayed better capacity retention,cycling stability,and rate capability than the GPC one.In the rate performance test,the GPC||Fe@Fe2O3||Fe1-xS@Fe2O3-1/S electrode could retain a discharge capacity of 714 mAh g-1 when the current density was increased to 5 C.The discharge capacities were maintained at 486,421 and 356 mAh g-1 after testing 1000 cycles at current densities of 1 C,2 C,and 5 C,respectively.Even at an areal sulfur loading up to 10.46 mg cm-2,the reversible capacity remained at 638 mAh g-1 after 60 cycles at the current density of 0.1 C.In summary,the method of preparing GPC||Fe@Fe2O3||Fe1-xS@Fe2O3 composite proposed in this paper had certain enlightening effect on the exploration of lithium-sulfur batteries with high energy density.