| Lithium-ion batteries(LIBs)have been widely applied as the major power sources to fulfill the demands of portable electronics,electric vehicles and renewal energy storage systems due to their high energy density and long cycling life.Nevertheless,owing to the low theoretical specific capacity of graphitic anodes,the commercial LIBs hardly meet the ever-growing requirements for next-generation high-performance batteries.Transition metal oxides have attracted much attention due to their high specific capacity and environmental friendliness.In this paper,Co3O4 and TiO2 were used as research objects to synthesize a unique hollow/frame structure based on zeolitic imidazolate framework-67(ZIF-67)by hybridization,coating,carbonization,oxidation and other means,so as to solve the common shortcomings of transition metal oxides,such as poor cycling performance,poor electrical conductivity,significant volume expansion effect and so on,and enhance the cycle stability and rate capability of the materialThe main contents of this paper can be shown in the following aspects:(1)A jujube cake-type ZIF-67 hybrid was formed by embedding PVP-modified TiO2 nanospheres into ZIF-67.The hybrid is subjected to carbonization and controlled oxidation to form Carbon coupling ultrafine Co3O4 hollow nanosphere-assembled mesoporous polyhedron with inner vesicles encapsulating TiO2 nanospheres.The structure was characterized by SEM,TEM,XRD,BET and other means.Co3O4 hollow nanospheres were assembled to the mesoporous polyhedron together with carbon.Within the Co3O4 polyhedrons,the bubbles 300-500 nm in diameter were uniformly generated.Every bubble further encapsulated one TiO2 nanosphere.forming a unique sphere-bubble structure.The specific surface area and the pore volume were calculated to be 97.85 m2 g-1 and 0.31 cm3 g-1.The as-prepared material exhibited superior lithium storage properties with great specific capacity,stable cycling life and good rate capability.After 400 cycles,the discharge capacity of 609 mAh g-1 was still delivered at current density of 335 mA g-1.Even the reversible capacity reached 296 mAh g-1 at a high current density of 2000 mA g-1.The outstanding electrochemical performance was attributed to the unique hybrid structure,which avoids nanomaterial aggregation,promotes ion diffusion and electron transfer,accommodates volume change of Co3O4 during(de)lithiation process,enhances structure strength,cycling stability and space utilization ratio of the hollow material.(2)The jujube cake of ZIF-67 is coated by a layer of hydrous titania by TIP hydrolysis.Afterwards,the controllable oxidization in air at appropriate temperature and ramp rate is performed.TiO2/hydrous titania polyhedron nanocage further encapsulates the Co3O4 framework,forming a unique TiO2@Co3O4 framework capsule.Through SEM,TEM,XRD,FTIR,BET and other material characterization methods,it was confirmed that zeolitic imidazolate framework-67 changed into Co3O4 three-dimensional mesoporous framework structure by oxidation.The thickness of the nanocage wall is about 10 nm.The size of these nanoparticles that make up Co3O4 nanoframework is 10-20 nm.The specific surface area and the pore volume were calculated to be 136.3 m2 g-1 and 0.5173 cm3 g-1.It is indicated that the framework capsule structure has a large specific surface area and abundant mesoporous structure.This strategy relies on the oxygen-limitation effect of the closed nanocage and the affinity effect of PVP towards ZIF-67.As anode materials for lithium-ion batteries,TiO2@Co3O4 framework capsules show superior lithium storage properties with high reversible capacity,stable cycling life and good rate capability.The reversible capacity of 1042 mAh g-1 can be delivered at a current density of 300 mA g-1 for 200 cycles.The average discharge capacity over 200 cycles reaches 926 mAh g-1.These excellent lithium storage properties demonstrate the structural advantage of the TiO2@Co3O4 framework capsule,which mainly benefits from the ultrafine nanocrystalline of Co3O4 framework shortens the electronic/ionic diffusion pathway;High specific surface area of the nanoparticles offers more active sites during discharge-charge process;TiO2/hydrous titania nanocage provides an independent and stable space for every Co3O4 framework and well accommodates the strain induced by the volume change of Co3O4 during(de)lithiation process;The embedded TiO2 nanospheres as the unique junctions can significantly protect structure stability of Co3O4 3D mesoporous framework.This work may open a new avenue for the structure design and synthesis process of MOFs-derived materials.(3)Co3O4 nanocage coated Co3O4 nanoparticles-assembled 3-dimension mesoporous nanoframework with TiO2 spheres as framework nodes was designed and synthesized through Co(NO3)2 treatment of jujube cake-type ZIF-67 hybrid followed by oxidation.Through SEM,TEM,BET and XRD,the shell of the material was determined to be Co3O4 nanocage.The internal Co3O4 nanoframe was mainly due to the dominant effect of PVP on the adsorption of ZIF-67.The embedded TiO2 nanospheres as the unique nodes can significantly strengthen structure stability of Co3O4 nanoframework,forming the unique nanocapsule structure.The size of these nanoparticles that make up Co3O4 nanoframework is 15-20 nm.The gap between the framework and the shell is around 80 nm.The thickness of the Co3O4 nanocage is around 10 nm.The specific surface area and the pore volume were calculated to be 100.7 m2 g-1 and 0.494 cm3 g-1 As anode material for lithium ion batteries,the nanocapsules exhibited superior lithium storage properties and rate capability.The reversible capacity of]409 mAh g-1 can be delivered at a current density of 200 mA g-1 for 19 cycles.The superior lithium storage performance was benefited from large specific surface area and amounts of mesopores offers more active sites during discharge-charge process. |
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