| Lithium-sulfur batteries are a leader in the new generation of secondary electronics,with the unique advantages of high theoretical specific capacity(1675 m Ah·g-1)and high energy density(2600 Wh kg-1),as well as the advantages of environmental friendliness and abundant sulfur reserves.However,its commercialization is still hampered by three main problems:（1）the low conductivity of sulfur and its sulfur monomers can reduce the availability of active substances,（2）the significant volume expansion that exists in the sulfur cathode during charging and discharging,which can easily cause electrode collapse,（3）the shuttle effect caused by the polysulfide intermediates generated during charging and discharging,which limits long-term battery cycling.To address the above problems,in this paper,Co3O4/TiO2 polyhedral cathode composites with hollow structure,Co3O4/Ti O2/PPy cathode composites with a conductive network structure,and P-Co3O4/Ti O2/PPy cathode composites with abundant active sites were prepared using Co-MOF（ZIF-67）as a template.The microstructure,chemical composition,and electrochemical properties of three cathode composites were studied,and their performance enhancement mechanisms were analyzed,with the main work as follows.（1）ZIF-67 was heat-treated to obtain Co3O4 hollow polyhedral,and a layer of Ti O2was coated on the surface of Co3O4 by chemical oxidation to obtain Co3O4/Ti O2 cathode composites.An investigation was conducted to assess the influence of heat treatment on the morphological structure of Co3O4/Ti O2 composites,with systematic electrochemical tests also conducted.The results indicated that the microstructure of Co3O4/Ti O2composites obtained at 450℃was more stable and the dimensions were more uniform（heating rate:1℃/min）.At 0.1 C,the S@Co3O4/Ti O2 anode’s initial capacity is 1152.7m Ah·g-1;however,at 1.0 C,it is 657 m Ah·g-1,with a capacity decay rate of 0.072%in500 cycles.The coulomb efficiency remains above 98%and has good multiplicative performance.The heterojunction formed at the interface between Co3O4 and TiO2accelerates the surface reaction kinetics and charge migration;the hollow structure accommodates the volume expansion;and has good polysulfide adsorption.（2）To further increase the capacity of the battery,the conductive polymer PPy nanotubes were prepared by the template method and chemical oxidation method,and the Co3O4/Ti O2/PPy cathode composites were obtained by compounding them with hollow polyhedral Co3O4/Ti O2.The effects of the mass ratio of polyhedra to nanotubes in the composites on the morphological structure and electrochemical properties of Co3O4/Ti O2/PPy composites were investigated,and the results showed that when the mass ratio of polyhedra to nanotubes in the composites was 4:1,the dispersion of polyhedra and nanotubes was more uniform and the polyhedra could better adhere to the conductive network of nanotubes.The prepared S@Co3O4/Ti O2/PPy cathode has an initial discharge capacity of 1151.9,994.8,and 789.1 m Ah·g-1 at 0.1,0.2,and 1.0 C,with capacity retention rates of 59.66%,55.81%,and 77.84%,respectively,and the Coulomb efficiencies are all maintained above 98%with good cycling stability.PPy nanotubes’conductive network not only hastens electron and ion transport and boosts the electrical conductivity of composites,but also permits spatial confinement and chemisorption of polysulfides,thus diminishing the shuttle effect.（3）To improve the capacity retention of the battery,the Co3O4/TiO2/PPy composites were phosphorus doped to obtain P-Co3O4/Ti O2/PPy cathode composites.At current densities of 0.1,0.2,1.0,and 2.0 C,the S@P-Co3O4/Ti O2/PPy anode exhibits a high capacity retention of 78.89%,72.13%,78.93%,and 64.28%at all current densities throughout its long cycle,with an initial specific capacity of 1281.9,975.0,857.9,and616.8 m Ah·g-1.Compared with phosphorus-doped Co3O4/Ti O2 materials,the positive contribution of the conducting polymer to the battery performance can also be verified.Phosphorus doping can introduce more active sites and form chemical bonds with polysulfides,which in turn accelerates electron conduction and electrochemical reactions,better anchors the polysulfides,improve the utilization of active substances and effectively solves the problem of cyclic stability. |
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