Microwave-assisted Synthesis Of A New Carbon-based Material Modified Lithium-sulfur Separator And Its Electrochemical Properties

With the rapid development of human society and the great improvement of people’s life,the demand of both the quantity and quality for electronic equipments is growing rapidly.However,the commercial lithium-ion batteries（LIBs）have almost reached the function limit due to their low theoretical capacity and energy density.Recently,lithium-sulfur batteries（LSBs）have attracted much attention of researchers owing to their advantages of high theoretical capacity(1675 m Ah g^-1)and energy density(2600 Wh kg^-1),together with the low cost and the environmental friendliness.They are considered as one of the new generation of energy storage devices for replacing LIBs.However,in the long period exploration of LSBs commercial production,there are still some key technical problems need to be solved.These problems include the insulation of the active substance,the final product lithium sulfide（Li₂S）in the discharge process,the irreversible structural damage caused by the volume expansion of about 80%during the reaction process,the severe shuttle behavior caused by the dissolution of lithium polysulfide（Li PSs）resulting in low active substance utilization,and the poor battery life.Those technical obstacles inevitably lead to low sulfur utilization,slow reaction kinetics,rapid capacity decay and severe negative electrode corrosion of LSBs.This thesis focuses on the above problems and carries out the following two aspects of researching work by combining the synthetic technology of carbon materials and the modification design of separator.（1）Hollow carbon nanospheres（HGS）were successfully synthesized by microwave-assisted reduction of cheap organic carbon sources like methanol and ethanol.The mesoporous structure and high specific surface area of the HGS material could improve the wettability of the separator materials as the electrolyte and also reduce the loss of active substances.In addition,the HGS has its own unique hollow nanosphere structure.As an excellent conductive framework,HGS reduces the internal resistance of the battery and facilitates the transmission of electrons and ions.Meanwhile,it also displays a physical limitation effect in polysulfide（Li PSs）,which could reduce the"shuttle effect"and thus improve the battery life.This method is simple,rapid,cost-low and operation-easy.Then,the nitrogen-doped hollow carbon nanospheres（NHGS）and sulfur-doped hollow carbon nanospheres（SHGS）were synthesized by microwave-assisted one-step method.According to the electrochemical performance test and DFT theoretical calculation,the hetero-atomic doping increases the number of active sites and the polarity of the materials,which promotes the chemical adsorption and catalytic transformation of Li PSs,together with reducing the"shuttle effect".In general,this synthesis method is worthy of further research and development.The as-synthesized materials display good potential in application after optimization.（2）A CoS₂/NHGS composite material was prepared,followed by applying in the separator modification,which could further reduce the"shuttle effect".The CoS₂displayed very strong polarity and its morphology could be easily regulated.By changing the cobalt source amount to adjust the desired CoS₂ content and nanoparticle size,the optimal catalyst was obtained.The binding energy of CoS₂ and Li PSs is very high and close to the highest vanadium sulfide（VS）.However,it is superior over the VS since it is environmentally friendly.Thus,it is more suitable for commercial application.Comparing with the pure NHGS,the CoS₂/NHGS（NHGS-100）modified separator-based lithium-sulfur batteries containing 100 mg cobalt exhibited much better electrochemical characteristics.The maximum discharge specific capacity of1024.3 m Ah g^-1 can be maintained after 100 cycles at 0.5 C current density.However,the NHGS displayed only 891.9 m Ah g^-1.