Design And Preparation Of Nanometer Manganese-based Materials And Their Applications In Cathodes Of Lithium-sulfur Batteries

Lithium-sulfurbatteries are considered to be one of the most promising battery systemsdue to their high theoretical specific capacity(1675 mAh·g^-1),high energy density(2600 Wh·kg^-1),non-pollution and low cost.However,the electronic and ionic insulation characteristics of the elemental sulfur（S）and lithium disulfide,volume expansion during the reaction,the dissolution in the electrolyte and shuttle effect of lithium polysulfide（LiPS）which formed during the charging and discharging process of the sulfur cathode,leading to the extremely low utilization ratio of the active substances and the poor cycle life of Li-S batterries.These problemsmake them impossible to commercialize.Therefore,there is an urgent need to develop cathode materials of Li-S battery which have good conductivity,large space to relieve volume expansion,and can suppress the shuttle effect.This thesis mainly studies the design and synthesis of cathode materials.The main contents are as follows:1.The precursor MnO（OH）₂ NSs were synthesized by using PAA-NH₄nanospheres（NSs）as template.Then,the multi-shell Mn₂O₃ NSs with uniform particle size and good dispersion were formed by high temperature calcination.Subsequently,the Mn₂O₃-S composites with the S content was 69.8%were prepared by fusion method.Under 0.2 C rate,the capacity retention rate reaches as high as 73.9%after200 cycles.This material combines the physical limitations of the multi-shell with the chemical adsorption of metal oxides to LiPS,making it an ideal host material for Li-S batteries.2.The MnS/PAA NSs were synthesized using Na₂S₂O₃ and MnCl₂·4H₂O as main raw materials by hydrothermal method and then calcined at high temperature to obtain MnS/C NSs.The small particles of MnS were uniformly dispersed on the surface of the sphere,which facilitated the timely capture of LiPS by MnS.The MnS/C-S composite with the S content of 71%was obtained.When tested at 1 C,the electrode exhibits the initial discharge capacity of 1191.5 mAh·g^-1 and maintains at 463.3mAh·g^-1 after 400 cycles with a capacity retention rate of 53.1%and a high coulombic efficiency.The material fully integrates the advantages of small particle packing structure and nanosized metal sulfide,leading to the good cyclic performance as a cathode material of Li-S battery.3.The MnS/Co₉S₈/PAA NSs precursor was firstly synthesized based onPAA-NH₄NSs as template by using MnCl₂·4H₂O and CoCl₂·6H₂O as main raw materials.Then,the MnS/Co₉S₈/PAA NSs were crystallized into MnS/Co₉S₈/C NSs at high temperature.Subsequently,the MnS/Co9S8/C-S composite was prepared by melt-fusion method.The analysis results show that the small particles of MnS and Co9S8 are uniformly dispersed on the surface of the sphere,so they can capture LiPS in a timely and effective manner.The S content in the MnS/Co₉S₈/C-S composite is calculated to be 76%.The MnS/Co9S8/C-S composite shows a relatively high capacity with the initial discharge capacity of 1132 mAh·g^-1 at 1 C,and after 500 cycles,the capacity can still maintain at 709 mAh·g^-1 with the corresponding capacity retention rate of62.6%.The material shows an excellent cycle stabilitywith a high coulombic efficiency mainly due to the formation of the MnS-Co9S8 heterostructure,which promotes charge transport and enhances surface reaction kinetics.