Modification Of Metal-organic Framework ZIF-8 And Its Application In Lithium-sulfur Battery Cathode Materials

Lithium-sulfur batteries are potential candidates for next-generation secondary batteries because of their high theoretical capacity and energy density,as well as the abundance of sulfur in the earth.However,there are still many challenges in the commercialization of lithium-sulfur batteries.For example,the conductivity of sulfur and discharge end product（Li₂S）is low;the intermediate product lithium polysulfide dissolves in the organic electrolyte during the operation of lithium-sulfur batteries,thus causing the shuttle effect;the reaction kinetics is slow,and the active material cannot be fully converted into the discharge end product Li₂S during the discharge of lithium-sulfur batteries,resulting in the failure of the battery specific capacity to reach the theoretical capacity.In response to the above challenges,some corresponding modification strategies are made in.Firstly,we chose a metal organic framework as the carbon material for the cell to improve the poor conductivity of monomeric sulfur.Metal organic framework are obtained by self-assembly of metal ions and organic ligands,and are also known as porous coordination polymers.In this paper,ZIF-8,a typical member of metal organic framework,was chosen as a precursor to obtain the derived porous carbon material Zn@NPC by calcination.Firstly,Zn@NPC has an open porous structure and a large specific area,which provides abundant space for the accommodation of sulfur.Secondly,the mesopores in Zn@NPC can provide strong physical adsorption to trap polysulfides,and the intermediate atom zinc in Zn@NPC can effectively adsorb polysulfides as Lewis acidic sites to slow down the shuttle effect during the operation of lithium-sulfur batteries.However,using only Zn@NPC as the cathode material for lithium-sulfur batteries is less effective in enhancing the reaction kinetics of lithium-sulfur batteries.Based on this paper,three different metal compounds were used to modify it for enhancing its electrochemical performance as follows.1.The zeolite-imidazole framework ZIF-8 was prepared by using zinc nitrate hexahydrate as the zinc source and dimethylimidazole as the organic ligand,which was annealed at 800°C to form the ZIF-8-derived porous carbon Zn@NPC@Ti O₂ was successfully coated on Zn@NPC by in-situ hydrolysis of tetrabutyl titanate,resulting in a core-shell structure of Zn@NPC@Ti O₂.Titanium dioxide has a strong adsorption capacity for polysulfides.Through electrochemical tests,it is found that the titanium dioxide coating can well inhibit the occurrence of the shuttle effect and improve the cycle stability of lithium-sulfur batteries.The specific discharge capacity of the cell at0.1 C reached 1351 m Ah/g,and the Coulomb efficiency of the composite was always maintained above 98%in the long cycle test at 0.2 C.2.In view of the slow reaction kinetics and shuttle effect,the composite Zn@NPC-CeO₂ was prepared by one-step hydrothermal method by successfully attaching cerium dioxide particles to Zn@NPC surface and investigating the optimal cerium dioxide composite ratio.Because of the extranuclear valence electron configuration of cerium element is 4f¹5d¹6s².The hollow f orbital of cerium accepts sulfur lone pairs and promotes the deposition of uniform polysulfide ions.The addition of cerium dioxide particles can promote the diffusion of lithium ions as demonstrated by CV tests with different scanning speeds,and also according to the Uv-vis visualization experiments to illustrate that cerium dioxide particles due to enhance the adsorption effect of polysulfides.The prepared Zn@NPC-CeO₂-2 electrode was assembled into a battery with a capacity retention of 88%after 200 cycles at 2 C,indicating that the cerium dioxide particles are beneficial to enhance the reaction kinetics of the battery.3.Zn@NPC and carbon nanotubes were mixed by simple mechanical stirring,and then Ni₁₂P₅ particles were grown on the surface of the mixed carbon material by hydrothermal method to generate Zn@NPC-Ni₁₂P₅CNT.Ni₁₂P₅ has good electrical conductivity similar to that of metals（conductivity>5000 S/cm）.Meanwhile,the inherent polarity of Ni₁₂P₅ facilitates the absorption of polysulfides.The addition of carbon nanotubes can enhance the electrical conductivity of the composite,and the synergistic effect of the three is beneficial to enhance the electrochemical performance of lithium-sulfur batteries.The adsorption energy of Li₂S₆ on the（312）crystalline surface of Ni₁₂P₅ is-5.2 e V calculated by density flooding theory,which indicates that Ni₁₂P₅ has a strong affinity for polysulfides and can improve the shuttle effect during the operation of the battery.The first discharge specific capacity reaches 1453.5 m Ah/g at a rate of 0.1 C.In addition,after 500 cycles at a current density of 1 C,the coulombic efficiency of the battery remains above 96%and the specific capacity retention rate also remains above 90%.It shows that the Zn@NPC-Ni₁₂P₅-CNT composite can effectively improve the electrochemical performance of the battery.