Synthesis Of MOF-Derived Materials For Lithium-Ion/Lithium-Oxygen Batteries

The demand for energy from social development have promoted the rapid development of renewable energy technologies(such as solar and wind energy).Owing to the intermittent nature of these renewable energy sources,large-scale and efficient energy storage and conversion systems need to be developed.The battery converts chemical energy into electrical energy(and vice versa)as needed,and has great flexibility.Lithium-ion batteries(LIBs)have attracted much attention as energy storage devices due to their long cycle life,high energy density,and light weight,and have been widely used in many applications.Lithium-oxygen batteries are considered to be one of the most promising next-generation energy storage devices due to their ultra-high theoretical specific capacity and relatively simple structure.During the development of electrode materials for these energy storage systems,metal-organic frameworks(MOFs)and their derivatives have become research hotspots in this field due to their unique characteristics,such as high specific surface areas,rich pore structures,and controllable active sites and morphological structures.Based on the advantages of MOFs derivatives,two types of electrode materials were synthesized in this paper by using MOFs with different morphologies and functions,which were applied to lithium ion batteries as anodes and lithium-oxygen batteries as cathode electrocatalysts,respectively.The main research contents of this paper are as follows:(1)At room temperature,sheet Zn MOFs precursors were synthesized,followed by carbonization and vulcanization to synthesize ZnCoS@CNSs-1.The sheet structure stability of the Zn MOFs was investigated by changing the amount of Co²⁺incorporation.The results showed that with the increase of the amount of Co²⁺incorporation,the metastable state of Zn MOFs was gradually destroyed,leading to the formation of irregular morphology.When less Co²⁺ions were doped,a small amount of ZnCoS active species were formed.The synthesized materials were used as anode material of lithium ion batteries to assemble half-cells,and their lithium storage performance were tested.When the mass ratio of Co(NO₃)₂·6H₂O to Zn MOFs was 1,the obtained ZnCoS@CNSs-1 has both abundant ZnCoS active sites and stable sheet morphology.ZnCoS@CNSs-1 material delivered a higher first discharge specific capacity of 1172.0 mAh g^-1 at 200 mAg^-1,stable cycle ability(200 cycles),as well as excellent rate performance.(2)Two morphologies of ZIF-8/67@CNTs were synthesized by solvothermal method.In Synthesis Path 1,Zn/Co-ZIF@CNTs precursor was prepared by Zn²⁺and Co²⁺as metal centers coordinated with ligands and combined with CNTs.Then the precursor was pyrolyzed at 800? and acid-treated to obtain carbon matrix with rich N doped uniformly coated on the surface of CNTs,a earhead-like NC@CNTs material was formed.The rich N active sites of NC@CNTs facilitate the ORR kinetics,and the porous carbon framework with large surface area can provide space to accommodate Li₂O₂.The NC@CNTs and pure CNTs as cathodes for Lithium-oxygen batteries were undergone electrochemical tests.The results showed that NC@CNTs cathodes exhibited higher initial discharge capacity and longer cycle life than pure CNTs cathode.In addition,the improvement of the batteries performance of NC@CNTs cathode may include the catalytic contribution from very few Co single atoms on N-doped carbon.In Synthesis Path 2,ZIF-67@CNTs composite was synthesized by Co²⁺,2-methylimidazole and CNTs,then calcined at 800? for 2 h to prepare Co-NC@CNTs material with 3D conductive carbon network.In the Co-NC@CNTs material,ZIF-67 derived Co-NC nano-particles as carbon network nodes have micro-mesoporous structures,as well as a large number of Co species catalytic sites for ORR and OER.And CNTs as carbon skeletons transfer electrons to enhance the overall conductivity;in addition,the interconnected carbon network provides abundant space for accommodating the discharge products.Therefore,Co-NC@CNTs as cathode catalysts for lithium-oxygen batteries showed a high initial discharge capacity of 12570 mAh g^-1 at 200 mAg^-1,good rate capability(5865 m Ah g^-1 at1000 mAg^-1),and excellent cycling stability(145 cycles at 250 mAg^-1)superior to that of Co-NC and pure CNTs cathodes.