Preparation,Device Assembly And Electrochemical Performance Of Cathode Materials For Lithium-sulfur Batteries Based On Porous Biomass Carbon

With the large consumption of fossil energy and the increasing environmental pollution,the application of renewable energy has gradually occupied a large share,and a new type of efficient and clean energy storage system is urgently needed.Lithium-sulfur（Li-S）batteries have recently been considered as a promising electrochemical device for energy conversion and storage applications because of their high theoretical specific capacity(1675 m Ah·g^-1),high energy density(2600 Wh·kg^‐1),low-cost,and environmental friendliness.Research on lithium-sulfur batteries is continuing and great progress has been made.However,the practical application of lithium-sulfur batteries faces many obstacles,such as the insulation of sulfur and its discharge products,large volumetric expansion/shrinkage（80%）during discharge/charge process,and the problem of polysulfide shuttling back and forth between the cathode and anode.One of the effective solutions to solve these problems is penetrating sulfur into the porous carbon which has conductive frameworks and can confine the dissolved polysulfides.The diverse textures and tunable surface properties of abundant bioresources offer great opportunities to utilize biochar materials as sulfur hosts for naturally boosting the electrochemical performances of Li-S batteries.In this work,we used boat-fruited sterculia seed as the carbon source to prepare the porous carbon by chemical activation with KOH and modified the surface of porous carbon by heteroatom doping.The porous carbon before and after heteroatom doping is combined with sulfur to prepare a sulfur-carbon composite.The morphology and structure of these porous carbons and carbon sulfur composites were characterized,and the electrochemical performance of carbon sulfur composites was studied.The main research results are as follows:（1）Three types of porous carbons were prepared by chemical activation at three different activation temperatures using boat-fruited sterculia seed as the carbon source and KOH as the activator.The three kinds of porous carbon were further omposited with sulfur by melt-diffusing to obtain sulfur-carbon composites.The structure and morphology of the porous carbons and the carbon sulfur composites were characterized and the electrochemical properties were tested.The results indicate that 800℃is the best activation temperature.The porous carbon activated under 800℃（denoted as PBC-800）has the largest specific surface area(2045.3 m²·g^-1)and pore volume(0.965cm³·g^-1).The porous carbon obtained has micro-mesoporous structure with a main pore size distribution of 1-3 nm.The PBC-800 can physically confine the dissolved polysulfides within the pore structure,therefore S/PBC-800 cathode exhibits great cycling performance.At 0.5 C,the initial discharge specific capacity is 968 m Ah·g^-1,the specific capacity retains 363 m Ah·g^-1after 400 cycles,and the specific capacity retention remains to be 37.5%.（2）Nitrogen-doped porous carbon（donoted as N-PBC）was prepared by treating PBC-800 at high pyrolysis temperature using urea as nitrogen doping source,and the S-infiltrated N-PBC composite（denoted as S/N-PBC）was further fabricated using thermal diffusion strategy.The results show that the electrochemical performance of the S/N-PBC electrode is better than that of the S/PBC-800 electrode.It has the initial discharge capacity of 1046 m Ah·g^-1,retains a capacity of 527 m Ah·g^-1after 400 cycles at 0.5 C,and the capacity retention remains to be 50.4%within 400 cycles.The nitrogen doping of N-PBC not only improves the conductivity of the carbon material,but also chemically adsorbs the polysulfide,which greatly improves the cycling performance.（3）Nitrogen and sulfur co-doped porous carbon（donoted as NS-PBC）was prepared by treating PBC-800 at high pyrolysis temperature using thiourea as N,S doping source,and the S-infiltrated NS-PBC composite（denoted as S/NS-PBC）was further fabricated using thermal diffusion strategy.The results show that benefit from the synergistic effect of nitrogen and sulfur sites in chemically trapping the polysulfides,the S/NS-PBC could further suppress the shuttle effect of polysulfides.Therefore,the electrochemical performance of S/NS-PBC has further improvement compared to the S/N-PBC cathode.It has the initial discharge capacity of 1089 m Ah·g^-1,retains a capacity of 654 m Ah·g^-1after 400 cycles at 0.5 C,and the capacity retention remains to be 60.0%within 400cycles.