Rational Design And Electrochemical Performance Of Metal Oxide Based Separator Modified Materials For Lithium-sulfur Batteries

In order to cope with the changing climate and environmental pollution,and to achieve the ultimate goal of sustainable development and carbon neutrality,replacing traditional fossil fuels with renewable energy sources has become inevitable.Rechargeable batteries show great promising in this regard.Among them,lithium-sulfur batteries are considered the most promising secondary battery energy storage system because of their high theoretical energy density and the advantages of low cost,environmental friendliness and abundant resources.However,due to the negative effects of polysulfides shuttle,volume expansion,and lithium dendrites,the commercialization of lithium-sulfur batteries has to be slowed down.Currently,various strategies such as construction of composite sulfur cathodes,modified separator,and cathode protection have been proposed and proven to contribute to the enhancement of the electrochemical performance of lithium-sulfur batteries.Among many improvement methods,diaphragm modification is considered to be the most simple and effective method.The separator can prevent the short-circuiting of the positive and negative electrodes,and can allow the transport of ions,which is an important part of the lithium-sulfur battery.Carbon materials have been widely used in the modification of lithium-sulfur battery separators due to their high electrical conductivity and large specific surface area,but due to their non-polar properties,the adsorption of polysulfides is weak and can only inhibit a small amount of Li PSs,so finding materials with higher polarity has become a research hotspot.Among them,metal oxides have been proved to have strong adsorption and catalytic ability,and are an ideal candidate material.Therefore,based on the advantages of high conductivity of carbon materials and high adsorption and high catalysis of metal oxides,this paper designs metal oxide composite carbon materials and uses them to modify lithium-sulfur battery separators.The details of the study are as follows.（1）ZIF-8 precursors were synthesized using 2-methylimidazole and Zn（NO₃）₂ as raw materials,and C@Mn O₂ composites were synthesized by carbonizing ZIF-8 and further in-situ Mn O₂coating.The C@Mn O₂ prepared by using MOF as the precursor of the material has a large specific surface area,and its specific surface area is as high as 154.68 m²/g.BJH analysis also proved that the pore structure of the material is mainly composed of micropores and mesopores.The hierarchical porous structure is beneficial to the wettability of the separator to the electrolyte,and at the same time,it can also adsorb Li PSs through physical action.The C@Mn O₂ composite material used in lithium-sulfur batteries has the following advantages:（1）The carbon layer can improve the electrical conductivity of the material,promote ion and electron transport,and can have a domain limiting effect on polysulfides.（2）The outer layer of Mn O₂ has strong chemical adsorption capacity,which can adsorb dissolved polysulfides and alleviate the shuttle effect.When C@Mn O₂ is applied to LSBs,the 2 C current density can still maintain 464.3 m Ah g^-1 after 1000 cycles.（2）In order to solve the series of insufficient electrochemical performances caused by the severe shuttle of polysulfides in LSBs,in this paper,ZIF-67 precursors were synthesized by using 2-methylimidazole and Co（NO₃）₂ as raw materials,and Co₃O₄@C@Mn O₂ composites were synthesized by multi-step heat treatment and in-situ Mn O₂ coating.The Co₃O₄@C nanospheres are covered by an amorphous Mn O₂ layer with a thickness of about 30-50 nm,constituting the Co₃O₄@C@Mn O₂ composite.The large specific surface area of MOF-derived carbons facilitates ion and electron transport and has a certain physical confinement effect on polysulfides.The study shows that the outer layer of Mn O₂ has a high affinity for Li PSs and can anchor Li PSs through strong chemical adsorption.The internal Co₃O₄@C has admirable catalytic activity,which can accelerate the complete conversion of Li PSs to Li₂S.Benefit from the synergistic effects between the outer layer Mn O₂ on the adsorption of Li PSs and the inner Co₃O₄@C on the catalysis of Li PSs,the cells exhibit an outstanding electrochemical performance,including high reversible capacity of 734 m Ah g^-1 at 1 C,a low-capacity decay rate of 0.0406%per cycle around 500 cycles.