Preparation Of Dual MOFs Core-shell Structure And Study On Lithium Storage Properties Of Its Derivatives

The demand for clean energy in human society has increased day by day since the21st century.Lithium-ion batteries,as a good carrier of electric energy,have been widely used in various fields due to their advantages of efficient and repeated charging.The theoretical specific capacity of transition metal oxides/sulfides is several times the of graphite so they have high application value in anode materials.However,due to the disadvantages of poor conductivity and large volume expansion in the charge/discharge process,they need to be modified to meet the needs of practical applications.Metal-organic framework?MOFs?derivatives are a class of products with special morphological structures or functional properties obtained by using MOFs as precursor templates,which can maintain or convert the advantages of MOFs precursors,such as hollow porous,high specific surface area or the introduction of certain elements.MOFs derivatives are often developed as functional materials for applications such as energy storage and catalysis.In this paper,a new type of heterogeneous dual MOFs material MIL-125@ZIF-67with core-shell structure was designed and synthesized.The precursor template was transformed into two kinds of MOFs derivatives:bubble-like graded hollow TiO₂@Co₃O₄ and cobalt sulfide/sulfur-doped carbon CoS_x/SC,through different subsequent treatment processes then they were used as anode materials for lithium ion batteries.The microstructure and structural composition of the two anode materials were characterized by related test methods,and combined with electrochemical performance tests,along with the kinetic analysis test,explained out the relationship among“experimental design”,“composition structure”,“lithium-storage performance”.The heterogeneous dual MOFs precursor MIL-125@ZIF-67 with core-shell structure was synthesized by simple chemical precipitation method by adjusting the feeding sequence,feed ratio and reaction time.The precursor has a unique shape:the dense outer shell of ZIF-67 is coated on the surface of MIL-125 cubic core.It also has the advantages of rich mesoporous structure,large specific surface area and high thermal stability.Finally,the"surface functional group adsorption method"is proposed to explain the synthesis mechanism of this heterogeneous core-shell double MOFs material.The TiO₂@Co₃O₄ anode material was transformed into a bubble-like hierarchical hollow structure by a one-step calcination method by adjusting the heat treatment temperature.The MOFs-derived oxide is composed of an inner hollow TiO₂ nanobox supporting a bubble-like Co₃O₄ aggregated outer shell,and a CoTiO₃ transition layer is present at the interface of the two phases.This special structure greatly enhances its surface capacitive contribution and lithium ion transport inner the anode,which result in a lithium storage performance far higher than the theoretical specific capacity,stable cycle performance,and good rate performance.Cobalt sulfide/sulfur-doped carbon CoS_x/SC composites synthesized by inert atmosphere high-temperature solid phase method were screened by adjusting different vulcanization routes.It was confirmed by correlation characterization test that it was composed of sulfur-doped carbon skeleton composite CoS/CoS₂.The sulfur-doped carbon skeleton can maintain good conductivity and structural stability of cobalt sulfide during delithiation/lithium intercalation,which makes CoS_x/SC composites have better lithium storage performance and stable cycle after activation.performance.