Research On Silkworm Sand Biochar-based Composite Materials Layer-modified Separtor For Lithium-sulfur Batteries

Li-ion batteries with lower theoretical specific are unable to reach the high specific energy requirements of high-end storage devices.Lithium-sulfur batteries have abundant sulfur resources and high theoretical energy density,which become one of the most promising new energy storage systems.However,the poor conductivity of sulfur and Li₂S₂or Li₂S,serious shuttle effect of Li PSs（Li₂S_n,4≤n≤8）and volume change of sulfur during cycling lead to poor cycling stability of the battery.The construction of functional modified separator is a valid strategy to settle the above problem of lithium sulfur.The modified separator can not only intercept the migrating polysulfides,it also serve as second current collector to realize the reuse of the trapped active materials.Starting from the mechanism of interaction with Li PSs,the strategies of separator modification can be divided into physical adsorption,chemical adsorption and catalytic conversion.Carbon materials with large specific surface area physically adsorb polysulfides through its pore structure.Which become the most common coating materials for separator modification.Biomass carbon materials have the advantages of extensive sources,sustainability and hierarchical porous structure,the large pore structure of biomass carbon materials is beneficial to improve ion mobility and electrolyte permeability,the microporous/mesoporous structure is not only beneficial to the diffusion and and transport of Li⁺,it also acts as a physical barrier to block the shuttle of Li PSs.However,the force between carbon materials and polysulfides is too weak,non-polar carbon materials cannot effectively alleviate the shuttle of polysulfides.This research selects the biomass derived carbon as basal material,chemical adsorption or catalytic conversion properties of metal compounds and physical adsorption of biomass-derived carbon synergistically inhibit the shuttle of polysulfides.A series of metal oxides,metal sulfides and biomass carbon materials are compounded to prepare a high-performance separator modified coating material,thus achieving outstanding the long-cycle stability.The research of this thesis can be divided into the following two parts:（1）The physical adsorption of silkworm sand carbon and chemical adsorption of metal oxides（TiO₂,ZnO,and MnO₂）were combined to capture polysulfides and effectively suppress its shuttle effect.Silkworm sand biomass carbon@metal oxide composites were prepared for functional modified separators to enhance electrochemical performance.In this work,SC@TiO₂,SC@ZnO and SC@MnO₂ composites were synthesized through calcination,reflux,and in-situ transformation reactions,respectively.The first-cycle discharge capacities of lithium-sulfur batteries with SC@TiO₂,SC@ZnO and SC@MnO₂modified separators were 1527 m Ah/g,1511.7 m Ah/g and 1492.3 m Ah/g at 0.2 C,and the remaining capacity after 100 cycles are 896.1 m Ah/g,770.2 m Ah/g and 697.8 m Ah/g,and the average capacity decay rates per cycle were 0.41%,0.49%and 0.53%.The silkworm sand biocarbon material acts as a good physical barrier to capture polysulfides,and metal oxides chemical adsorption polysulfides through forming metal-sulfur bonds or lithium-oxygen bonds.The physical adsorption of silkworm sand carbon and the chemical adsorption of metal oxides are used to achieve efficient capture of polysulfides to block its migration.The research proves that the lithium-sulfur batteries assembled with SC@TiO₂modified separators have higher initial capacity and better cycle performance.TiO₂ has a lower binding energy（2.30 e V）between polysulfides compared with ZnO and MnO₂,thus SC@TiO₂ composites have a strong binding interaction with polysulfides to alleviate its shuttle effect,the synergistic effect of them achieve the long-cycle stability and high-rate performance of the battery.（2）The physical interception of silkworm sand carbon and catalytic conversion of metal sulfides（SC@CoS₂,SC@ZnS and SC@Mn S）can synergistically inhibit polysulfides shuttle effect.Silkworm sand biomass carbon@metal sulfide composites were prepared for functional modified separators to enhance the electrochemical performance of batteries.In this work,SC@ZnS,SC@CoS₂ and SC@Mn S composites were synthesized through co-precipitation,reflux methods,respectively.The first cycle capacities of batteries with SC@CoS₂,SC@ZnS and SC@Mn S modified separators were 1599 m Ah/g,1531.3m Ah/g and 1417.3 m Ah/g at 0.2 C,and the remaining capacity after 100 cycles are 1208.3m Ah/g,1039 m Ah/g and 966.7 m Ah/g.The silkworm sand biocarbon material acts as a physical barrier to block polysulfides,and metal sulfides adsorb polysulfides and perform efficient catalytic conversion of polysulfides.The effective combination of the physical interception of silkworm sand carbon materials and the catalytic conversion of metal sulfides can not only effectively block the shuttle of polysulfides and enhance the electrochemical properties of the battery.The research proves that the batteries assembled with SC@CoS₂ modified separator has more outstanding electrochemical performance.Compared with ZnS and Mn S,the CoS₂ with high conductivity has a lower lithium ion diffusion barrier（0.12 e V）,which can realize the rapid transport of lithium ions and accelerate the redox process of Li PSs,thus SC@CoS₂ composite has a higher efficiency catalyzed conversion and demonstrates better electrochemical performance.