The Application Of MOFs-derived Carbon-based Metal Compound Composites In The Lithium Sulfur Batteries

Lithium-sulfur（Li-S）battery has the advantages of high specific capacity(1675m Ah g^-1),high energy density(2600 Wh kg^-1),rich sulfur resources and low price,making it the most potential next generation of new energy storage battery.However,the commercialization of Li-S battery is still hindered by many factors,such as the poor electronic conductivity of sulfur species,the shuttle effect of polysulfides and the slow redox reaction kinetics.In order to solve the above problems,the multifunctional carbon-based composites derived from MOFs,combined with heteroatom doping engineering,heterojunction engineering and defect engineering,were used to design and prepare sulfur host composites for high-performance Li-S batteries.The charge-discharge mechanism of Li-S battery was studied by electrochemical test and theoretical simulation calculations.NiS₂/C HSs composites with hollow spherical structure were prepared by hydrothermal synthesis and followed by sulfuration.The hollow structure of NiS₂/C HSs can increase the sulfur loading and buffer large volume changes during the redox reaction process of the sulfur species.The main carbon structure of NiS₂/C HSs can provide a fast electron transport channel for the charge-discharge process of Li-S batteries.NiS₂ polar sites can form effective chemical adsorption with polysulfides,which can alleviate the shuttle effect of polysulfides and reduce the irreversible loss of active sulfur species.In addition,the NiS₂ active site can reduce the energy barrier of the redox reaction of polysulfides and improve the electrochemical performance of Li-S batteries.The NiS₂/C HSs Li-S cells exhibit a considerable discharge specific capacity of 780 m Ah g^-1 at 1.0 C.Even under a high sulfur loading of 5.3 mg cm^-2,the discharge capacity of NiS₂/C HSs-based cell is 4.8 m Ah cm^-2.Although the NiS₂ active site can effectively chemically adsorb polysulfides,NiS₂/C HSs still cannot effectively and fully inhibit the shuttle effect of polysulfides due to the nonpolar surface of the main carbon material in NiS₂/C HSs and a limited number of NiS₂ polar sites.Bi₂Te_3-xtopological insulator composites rich in Te vacancies were prepared by hydrothermal synthesis and high temperature tellurization using ionic liquid as dopant.N,B co-doped carbon materials could effectively enhance the chemical adsorption capacity with polysulfides and improves the cycle stability of Li-S cells.In addition,the conductive topological surface state of Bi₂Te_3-x can accelerate the charge transfer during the charging and discharging process of Li-S cells and improve the electrochemical performance of Li-S cells.By introducing Te vacancies on Bi₂Te_3-x@NBCNs surface through defect engineering,the electronic structure distribution of topological surface states can be changed,the coordination unsaturation of local atoms can be increased,and the affinity for polysulfides can be improved.Theoretical simulation calculations results show that Bi₂Te_3-x can reduce the Gibbs free energy difference of Li₂S₂/Li₂S nucleation reaction,thus improving the electrochemical performance of Li-S cells.At 0.2 C current density,the initial capacity of Bi₂Te_3-x@NBCNs Li-S cell is 1264 m Ah g^-1.After 300 cycles,Li-S cells keep 87%of discharge capacity retention rate,and the average capacity degradation rate is 0.045%per cycle.At 1.0 C,the specific discharge capacity of Bi₂Te_3-x@NBCNs Li-S cell is 978 m Ah g^-1.Under the condition of high sulfur loading of 5.5mg cm^-2,the specific discharge capacity of Bi₂Te_3-x@NBCNs Li-S cell is 5.4 m Ah cm^-2.Although the abundant polar sites of heteroatoms and Bi₂Te_3-xactive sites on Bi₂Te_3-x@NBCNs surface can effectively adsorb polysulfides and promote the redox reaction.However,the dense Li₂S₂/Li₂S deposition layer will cover the surface of the catalytic active sites and prevent the continuous diffusion and adsorption of polysulfides.FeSe-Mn Se/NBC composites rich in heterojunction and Se vacancies were prepared by hydrothermal method and high temperature selenization.The theoretical calculation shows that the FeSe-Mn Se heterojunction exhibit enhanced adsorption capacity and catalytic activity for polysulfides.Polysulfides will preferentially adsorb at the FeSe-Mn Se heterojunction.FeSe-Mn Se/NBC induced the radial vertical growth and deposition of Li₂S₂/Li₂S in 3D mode through abundant heterojunction,which provided sufficient diffusion channels and adsorption active sites for the subsequent polysulfides.At a current density of 0.2 C,the specific discharge capacity of the FeSe-Mn Se/NBC Li-S cell is 1334 m Ah g^-1.At 1.0 C,the discharge capacity of the first cycle of FeSe-Mn Se/NBC Li-S cell is 1018 m Ah g^-1.After 1000 cycles,the average discharge capacity fading rate is only 0.03%per cycle.In addition,even under high-sulfur loading of 7.5 mg cm^-2,the specific discharge capacity of the FeSe-Mn Se/NBC Li-S battery is 6.4 m Ah cm^-2.