Research On Biochar-based Composite Materials As Lithium-Sulfur Battery Separator

The rapid growth of global energy demand has promoted the development of next-generation high-energy-density rechargeable batteries.Lithium-sulfur（Li-S）battery is considered to be a new generation of secondary battery with great development prospects due to its high specific capacity and high theoretical energy density.However,Li-S batteries have some problems,such as poor conductivity,shuttle of intermediate products（Li₂S₆ and Li₂S₄）,and volume expansion of sulfur during charging and discharging,resulting in low sulfur utilization,serious self-discharge,and poor cycle stability.In recent years,researchers have discovered that separator modification is an effective strategy to inhibit the shuttle effect.Separator modification can provide a fast lithium ion transmission route,and at the same time,it can also block the diffusion of lithium polysulfide（LiPSs）,inhibit the shuttle effect and improve the cycle stability of the battery.In particular,the unique pore structure and rich porosity of carbon materials can capture LiPSs through physical adsorption,effectively preventing the transfer of soluble LiPSs to the negative electrode,and improving the reaction kinetics of Li-S batteries.Among them,the biochar material has a hierarchical structure from macropores to micropores.Macropores can provide a fast channel for electrolyte transfer,while micropores and mesopores can physically adsorb soluble LiPSs.However,the van der Waals interaction between non-polar biochar materials and polar LiPSs is relatively weak,and it is still difficult to avoid serious capacity degradation during long-cycle cycles.There are research reports,there is a strong Lewis base interaction between polar metal-based compounds and polar LiPSs,which can provide more chemisorption sites.Therefore,the composite modified separator of metal compounds and biochar materials is used in Li-S batteries,and the synergistic effects of biochar physical adsorption and chemical adsorption（catalytic conversion）of metal compounds are used to inhibit the shuttle effect of LiPSs and improve battery performance.Therefore,in view of the shortcomings of Li-S batteries,this paper has carried out the following work:In this research,biomass-derived carbon was selected as the base material for research.Biomass-derived carbon materials can maintain the porous structure of the original materials,and their low price and abundant resources are conducive to large-scale applications.The research content of this paper mainly includes the following two aspects:（1）EFC@TiO₂ composite material was obtained by loading polar metal oxide TiO₂on fungus residue slag-derived carbon（EFC）.By modifying EFC@TiO₂ on the surface side of the commercial PP separator,a modified separator was obtained.Among them,TiO₂ has obvious advantages,such as an environmental friendliness and high stability.In the synergistic effect of the physical adsorption of EFC and the chemical adsorption of TiO₂,the modified separator greatly blocked the diffusion of intermediate products（Li₂S₆ and Li₂S₄）during the charge and discharge process,and inhibited the shuttle effect of LiPSs.The Li-S battery assembled with EFC@TiO₂ modified separator has a capacity of 748.2mAh g^-1 after 200 cycles at a current density of 0.2 C and a capacity decay rate of 0.203%per cycle.Even at a high current density of 2 C,the initial specific capacity is still as high as 866.3 mAh g^-1,and good electrochemical performance is obtained.（2）The biochar compound zinc sulfide（SC@ZnS）transformed from silkworm excrement was synthesized by hydrothermal reaction,and it was modified on one side of the separator to obtain the SC@ZnS modified separator.Among them,the biochar material with high specific surface and rich porosity can be used as a physical barrier to intercept the shuttle of LiPSs,and ZnS can promote the redox reaction of LiPSs through catalytic conversion.At the same time,the ZnS surface has a suitable potential barrier,which is conducive to the diffusion of lithium ions and improves the reaction kinetics of the battery.The Li-S battery assembled with SC@ZnS modified separator,when the sulfur loading is2.0-2.3 mg·cm^-2 and the current density is 0.2 C,the first-cycle specific capacity of the SC@ZnS modified separator battery is 1603.0 mAh g^-1,the remaining specific capacity after 100 cycles is 1156.4 mAh g^-1,showing good lcycle stability.