MOFs Confined Growth And Derivatization In Carbon Nanocages And Their Lithium Storage Properties

The large-scale use of fossil fuels urges humans to face up to serious energy and environmental problems.It is the inevitable choice for the sustainable development of society to use renewable energy.However,efficient energy storage and conversion technology are the key to renewable energy utilization.As a high-voltage,high-energy-density rechargeable battery,lithium-ion secondary battery has become a widely used energy storage device.At present,potential lithium battery anode materials include metal oxides,sulfides and phosphides,etc.,with a high theoretical specific capacity.However,the volume effect of such electrode materials severely limits the cycle life and rate performance of the device.Hollow mesoporous carbon spheres(Carbon Nanocages,CNCs)have a certain volume of confined space,which can be used as a nanoreactor to realize the confined growth and storage of lithium in electrode materials,provides ample expansion space for the electrode material.The simultaneous introduction of metal and carbon source in nanocages can construct a multi-carbon layer protection which could further alleviate the volume effect of electrode materials and improve the lithium storage performance of lithium ion batteries.Metal-organic frameworks(MOFs)are a class of porous nanocrystalline materials with both carbon and metal sources.Sulfide,selenide,phosphide and other materials synthesized from MOFs can inherit the unique nano-skeleton and pore structure of parent units,providing channels for rapid ion migration.The surface of MOFs derivatives forms a carbon layer protection,which can effectively improve the electrochemical stability of the electrode material.Therefore,the application of MOFs confined in carbon nanocages as the framework and in-situ pyrolysis derivatization to construct the MOFs-based derivative@carbon nanocages composite electrode materials can alleviate the volume effect of lithium electronegative materials and improve the cyclic stability and rate performance of electrode materials.The research content of this paper is mainly divided into three parts:1.Synthesis of ZnP4@CNCs by ZIF-8 confined derivatization in carbon nanocages and lithium storage properties.The hollow carbon nanocages(CNCs)were used as nanoreactors to realize the internal ZIF-8 confined growth at room temperature to form ZIF-8@CNCs composites,which were phosphated at a high temperature in the argon atmosphere downstream of red phosphorus to obtain ZnP4@CNCs composite nanomaterials.The synthesized materials were characterized by transmission electron microscopy(TEM),scanning electron microscopy(SEM),X-ray diffractometry(XRD)and X-ray photoelectron spectroscopy(XPS).The results show that the nitrogen-doped carbon layer encapsulates ZnP4 nanoparticles in the CNCs.The lithium storage performance of ZnP4@CNCs composite nanomaterials was studied by electrochemical voltammetry,constant current charge and discharge and AC impedance.The experimental results show that the ZnP4@CNCs composite materials reduce the powdering and shedding of the materials and improve the reversible capacity and cycle performance of the materials.Comparing with the ZnP4/C-N,ZnP4@CNCs show excellent electrochemical performance.After circulating 500 cycles at a current density of 2 A g-1,the reversible specific capacity is still up to 462.5 mA h g-1.After cycling for 1000 cycles at a current density of 0.2 A g-1 the reversible specific capacity can still reach 773.4 mA h g-1.By observing the structural changes of ZnP4@CNCs after cycling,the unique core-shell structure of the composite materials can effectively alleviate the volume effect and improve the cycle stability and rate performance of the electrode material.2.Synthesis of CoP@CNCs by ZIF-67 confined derivatization in carbon nanocages and their lithium storage and electrochemical hydrogen evolution properties.In order to further prove the universality of the growth of MOFs within the carbon nanocages(CNCs),the confined growth and derivatization of ZIF-67 in carbon cage were studied.The experimental results show that ZIF-67 can grow inside the carbon cage and the cage structure ZIF-67@CNCs can be obtained.Due to the confinement of carbon cage,the size of ZIF-67 decreases significantly.The confined growth mechanism shows that oxygen-containing groups and defects on the surface of carbon cages are beneficial to the enrichment and preferential nucleation of metal ions.CoP@CNCs nanocomposites were obtained by phosphating at high-temperature under argon atmosphere.The performance of electrochemical lithium storage shows that the reversible specific capacity can still reach 714.1 mA hg-1 after 500 cycles at2 A g-1 current density.After 1000 cycles at a current density of 0.2 A g-1,the reversible specific capacity can still maintain 1215.2 mA h g-1.Its excellent lithium storage performance can be attributed to:(a)high specific surface area increases the active site of lithium storage and the reversible capacity of lithium storage;(b)The cage structure provides a buffer volume for the conversion reaction of CoP and improves cycle stability;(c)The interconnected 3D frame improves the conductivity of the composite to enhance the rate performance of the electrode materials.In addition,the catalytic hydrogen production performance of CoP@CNCs under acidic conditions was studied.The results show that the overpotentials are 144 and 217 mV at current densities of 10 and 50 mA cm-2,respectively.The synergistic effect between CoP nanoparticles and N,P co-doped carbon shells promotes the catalytic performance of the materials.3.Synthesis of Binary Metal Sulfide Co9Ss/MoS2@CNCs in Carbon Nanocages and lithium storage propertiesIn order to further enhance the lithium storage capacity of lithium ion anode materials,MoS2 nanosheets and Z1F-67 were sequentially confined growth in carbon nanocages(CNCs)to construct ZIF-67/MoS2@CNCs nanocomposites.The binary metal sulfide Co9S8/MoS2@CNCs nanocomposite was obtained by subsequent high temperature vulcanization under an argon atmosphere.The experimental results show that the introduction of MoS2 nanosheets does not affect the confined growth of ZIF-67.After vulcanization,both Co9S8 nanoparticles and MoS2 components have good confinement in the carbon nanocage to obtain stable microstructured nanosheets to form heterostructures.The Co9S8 nanoparticles can effectively block the aggregation of MoS2 nanosheets and obtain a richer lithium active site.After 1000 cycles of the high current density of 2 A g"1,the composites have a reversible specific capacity of 728.55 mA h g-1.The improvement of the electrochemical performance of Co9S8/MoS2@CNCs is mainly attributed to the unique core-shell structure of the composite materials,which alleviates the volume effect during charging.The introduction of binary metal sulfide increases the filling rate of the active component and effectively increases the volume energy density of the electrode material.It has high specific capacity and rate performance and excellent cycle stability,which is derived from the good synergistic lithium storage effect of Co9S8 and MoS2.