Preparation And Electrochemical Performance Study Of CO-MOF-based Cathode Materials For Lithium-sulfur Batteries

Lithium-ion batteries,which currently dominate the energy market,are subject to the limited theoretical specific capacity of their materials,and it is hard for the capacity to get a greater breakthrough.However,the theoretical specific capacity of the sulfur cathode in lithium-sulfur battery is 1675 m Ah g^-1and the energy density is 2600 Wh kg^-1.In addition,sulfur abundance,environmentally friendly and low cost,so it is expected to become a new generation candidate.However,it is difficult for lithium-sulfur batteries to avoid the“shuttle effect”,the electronic/ion insulation of sulfur and the volume expansion.Therefore,this article aims to select and synthesize polar cobalt sulfide and construct microstructure for achieving the design of material composition and structure to alleviate the problems and improve the long-term stable cycle performance of lithium-sulfur batteries.In this paper,a self-assembled cobalt-based metal organic framework(Co-MOF)was selected as the template,and a sulfur host of C/Co₃S₄hybrid hollow polyhedron was obtained through a simple solvothermal and annealing.Then,2D C/Co₉S₈-C@S fiber network was obtained with electrospinning and 3D GO@C/Co₃S₄@S nanocage was gotten by strong electrostatic adsorption,which were employed as lithium-sulfur cathodes to explore electrochemical performance.The research contents are as follows:(1)The effects of solvent type,absolute ethanol content and different molecular weight of PVP for the microstructure of self-assembled ZIF-67 were studied.Finally,two complete structure of Co-MOFs(1?m,350 nm)were selected to get the C/Co₃S₄hollow polyhedron after sulfurizing and annealing.The polyhedron was impregnated with sulfur by melting diffusion.The polar Co₃S₄chemically adsorbed polysulfides and the hollow structure stored sulfur,making the initial capacity of the C/Co₃S₄@S(S)was1100.3 m Ah g^-1,and the capacity remaining 608.5 m Ah g^-1after 120 cycles.The initial capacity of C/Co₃S₄@S(L)was 933.7 m Ah g^-1.As a whole,the small size C/Co₃S₄@S is better than the large size C/Co₃S₄@S in cycle stability and rate performance.(2)Using electrospinning to string hollow polyhedrons containing Co₉S₈nanocrystals on carbon nanofibers as the sulfur host of lithium sulfur batteries,the 2D interconnecting network has a stable microstructure support.It can achieve the functions of accommodating,immobilizing and catalyzing sulfur as a cathode.At the same time,the carbon fiber skeleton and C/Co₉S₈polyhedra effectively accelerate the ion/electron transmission and redox.Under these synergistic effects,the initial capacity of C/Co₉S₈-C@S cathode reached 1013.7 m Ah g^-1,and it was stable at 694.9 m Ah g^-1after 150 cycles.It was always higher than that of C/Co₉S₈@S polyhedra,C@S fiber and sulfur for discharge capacity and coulomb efficiency?100%.(3)GO directly packages the dispersed Co-MOF-derived C/Co₃S₄@S hybrid polyhedron to form a 3D nanocage unit.The GO layer and the polyhedral carbon shell jointly establish a double conductive channel as a continuous conductive network.Part of the sulfur attached on the outer surface of the polyhedron can be blocked by the GO.The polar Co₃S₄anchoring cooperates with the physical restraint of the GO network,effectively preventing the loss of S and giving a structural buffer.At 0.1 C,the initial specific capacity of GO@C/Co₃S₄@S cathode was 1110 m Ah g^-1,and the capacity decay rate per cycle was only 0.166%after 200 cycles.Compared with dispersed C/Co₃S₄@S polyhedron,the capacity was significantly improved.At 1C,the decay rate per cycle for 500 cycles was only 0.099%,and the coulomb efficiency was?100%.