| Among many clean energy technologies as an alternative for fossil resource,Lithium-sulfur（Li-S）batteries have been one of domestic and abroad research focus because it owns advantages such as high energy density（2600 Wh/kg）,portability and reuse.However,as for Li-S batteries,there are still certain challenges as follow:Firstly,sulfur and discharge products Li2S2/Li2S present disadvantages of the poor electronic/ionic conductivity,which directly causes electrode impedance of assembled Li-S batteries to increase and simultaneously utilization of active substance to lower;Secondly,long-chain polysulfide is easily soluble and diffuses in the electrolyte,then act as a trigger for shuttle effect,which directly causes battery capacity of Li-S batteries to fade rapidly;Thirdly,density differences between elemental sulfur and Li2S cause to the volume expansion/contraction of sulfur electrode happened in charge-discharge cycle of Li-S batteries,which poses a severe danger to stability of electrode structure.The aforementioned hot potato not only has impeded Li-S batteries into commercial applications,but also has lowered practical energy density of the batteries and has weakened rate capability and has shortened cycle life.To overcome the mentioned problems,a composite method of sulfur and carbon-based materials owning excellent electrical conductivity has been a prime choice for enhancing the electrochemical performance of Li-S batteries.As one of biomass carbon-based materials owning super quality and competitive price,yeast not only can provide suitable hierarchical porous structure for Li-S batteries,but also are rich in N and P heteroatoms and better compatible with metal ions,which make it have a potential application prospect in improving electrochemical performance of Li-S batteries.In this paper,yeast was selected as both template and carbon source,Co-embedded N&P-codoped hierarchical porous yeast-based carbon（Co-YC）was successfully fabricated,electrochemical performance of Li-S batteries assembled with the yeast-based carbon materials had been also investigated.The main work and conclusions of this thesis is as follows:（1）For a difficult problem against the deformation and pore-forming of yeast-based carbons under conditions of high temperature pyrolysis,a coupled method of vacuum freeze-drying and pyrolysis has been proposed,Co-embedded N&P-codoped hierarchical porous yeast-based carbon（Co-YC）microspheres had been successfully prepared with the help of the pore-forming synergism of sodium dodecyl sulfate（SDS）and Co2+,the morphologies and microstructure of these Co-YC carbon microspheres had been investigated,electrochemical performances of Co-YC/S electrodes had also been explored,the rearrangement of sulfur particles on the surface of Co-YC carbon microspheres had been analyzed with HR-SEM-EDS,the effect of cobalt crystalline particles on the enhanced nucleation of Li2S on the surface of the carbons had been verified by the experiment.Li-S batteries assembled with Co-YC/S composites had an initial discharge capacity 1462 m Ah/g at 0.1 C;after 300 cycles,the capacity maintained 1010.8 m Ah/g at 0.2 C（the retention was77%,the decay rate was 0.076%per cycle）;after 200 cycles,the capacity still kept at 727.5 m Ah/g at 1.0 C.（2）For the different microstructure of yeast between the cellular wall and the cytoplasm,through the observations of the growth difference of cobalt crystalline particles in both the cellular wall and the cytoplasm,the formation mechanism of ordered hierarchical porous structure distributed in yeast-based carbon microspheres had been investigated,the spatial confinement of the cellular wall of yeast against the insertion of Co ions had also been investigated.The influence of particle size of cobalt crystallites on the electrochemical performance of the assembled batteries had been explored,the intrinsic mechanism between cobalt crystalline particles and enhancement of utilization of active substance sulfur has concurrently been revealed.The initial discharge capacity of the Co-YC-SDS-700/S electrode was 1375.4 m Ah/g at 0.1 C,after 200 cycles,the capacity maintained 986.3 m Ah/g at 0.2 C and the retention was 80.2%;when the areal mass loading of sulfur increased to 3 mg/cm2,the electrolyte-to-sulfur（E/S）ratio decreased to 7μL/mg,areal capacity arrived at2.92 m Ah/cm2,after 100 cycles,the areal capacity slightly reduced to 2.62 m Ah/cm2;the smaller the crystalline particles of cobalt embedded in yeast-based carbons,the more uniform the distributions,the better for the promotion in the utilization of elemental sulfur and further the enhancement of electrochemical performances of Li-S batteries.（3）For scarcity and environment unfriendly property of cobalt,through the exploration against the influence of various concentrations of cobalt chlorides（Co Cl2）on the morphologies and structure of yeast-based carbons,the aim was to decrease the amounts of cobalt chlorides used in the impregnation method.Direct vacuum freeze-drying not only made Co ions be totally utilized,but also guaranteed the obtained yeast-based carbons had higher loading of cobalt,which had achieved the win-win in environmental protection and optimizing carbon-based materials.The synthesis strategy was designed to fabricate a porous yeast-based carbon material（P-Co-YC-0.3M）with more suitable specific surface area（246.6 m2/g）and higher loading of cobalt（66 wt%）.The inner of the gained P-Co-YC-0.3M carbon microspheres had macroporous structure,but the shell of outer cellular wall presented microporous structure,the average particle diameter of cobalt crystalline particles embedded in these carbon microspheres was 30.6 nm.The initial discharge capacity of the assembled Li-S batteries was 1215.7 m Ah/g at 0.2 C.When rate capability increased to 4.0 C,the capacity still maintained 562.8 m Ah/g;after 498cycles,the capacity kept at 771.8 m Ah/g at 1.0 C（the corresponding decay rate was0.0317%per cycle）.After the areal mass loading of sulfur increased to 2.50 mg/cm2and the E/S ration decreased to 14μL/mg,the initial areal capacity of the P-Co-YC-0.3M/S electrode was 2.87 m Ah/cm2 at 0.1 C,and the areal capacity maintained 2.32 m Ah/cm2 after 100 cycles;after putting the electrode there for a year,through the first several cycles,the areal capacity would return to about 2.50m Ah/cm2.Comparing with Density Functional Theory calculations（DFT）and the experimental data of adsorption,it gets the conclusion that cobalt crystalline particles not only effectively reinforced the adsorption capacity of the carbon microspheres against polysulfides,but also play an important role in keeping the cycling stability of sulfur electrode. |
