Multi-shelled Hollow Polyhedral Synthesized By ZIF-67 As Template And Their Lithium Battery Performance

Transitional metal oxides can meet the requirements of next-generation LIBs with high energy and high power density,and have been suggested as promising advanced anodes.In particular,Ni Co2O4 and Co3O4 possess high theoretical capacity and have been reported a lot.Recently,hollow micro/nanostructures have attracted intensive interest for their intriguing structural features and great potential in a myriad of applications including energy storage,catalysis and drug delivery.Hollow materials are appropriate electrode materials for LIBs because hollow structures possess large interfacial surfaces that allow Li+to easily access the nanoshells from both sides,resulting in a dramatically shortened diffusion path and improved kinetics that are favorable for high rate capacity,better cycle performance,and improved storage capacity.Accordingly,multishelled Co3O4,Ti O2 hollow microspheres,hollow Ni Co2O4 nanoboxes,and so on,have been extensively reported as anode materials for LIBs.Shell quantity and shell composition are the most important factors influencing the electrochemical performance of hollow materials.The multiple layers of shells and multiple compositions of hollow materials could bring about structure merits and synergetic effects,which might offer vast opportunities and hold great potentials for improved physical/chemical properties.The main contents of this dissertation are as follows:(1)Uniform Cu2 O nanocubes were large-scale synthesized through a liquid-phase reduction method.The Ti O2 was further coated onto Cu2 O nanocubes to form Cu2O@Ti O2 core-shell composite based on TIP hydrolysis reaction.The TEM image clearly shows that there is no gap between the Cu2 O and Ti O2 cladding layers,and the core-shell composite structure is still a nanocube.The average discharge capacity of Cu2O@Ti O2 which is based on 100 cycles is 358 m Ah g-1and increases 22.6% in comparison with pure corresponding value of the latter is only292 m Ah g-1.As anode materials for lithium ion batteries,Cu2O@Ti O2 core-shell composite showed higher discharge capacity(390 m Ah g-1)and more stable cycling performance than the uncoated Cu2 O nanocubes,exhibiting the remarkable coating effect.The complete and uniform Ti O2 layer effectively controls the internal stress and volume fluctuation of Cu2 O during chargeand discharge,which makes the structure stable and slows the rapid decay of capacity caused by pulverization.(2)zeolitic imidazolate framework-67(ZIF-67)-derived Ni Co2O4@Co3O4 double-shelled hollow polyhedrals(DSHPs)was coated by a layer of Ti O2 shell,forming a heterogeneous triple-shelled cage-in-cage structure.The three-shelled polyhedral structure is Ti O2,Ni Co2O4,and Co3O4 from the outside to the inside.The outermost Ti O2 is closely attached to the Ni Co2O4 shell,and the Co3O4 shell inside is independent.The average discharge capacity of triple-shelled hollow polyhedrals(TSHPs)based on 200 cycles is 852 m Ah g-1 while DSHPs is 691 m Ah g-1.The increasing rate is 23.3%.As anode materials for lithium ion batteries,triple-shelled nanocages show improved cycling stability and discharge capacity,which could be due to the cage-in-cage hollow nanostructure and beneficial effect of the Ti O2 shell.The structurally stable Ti O2 layer makes Ni Co2O4 and Co3O4 slow down the material pulverization caused by internal stress and volume fluctuation during charge and discharge,thus achieving the purpose of improving performance.(3)Ni Co2O4 DSHPs are prepared through a new strategy that relies on the unique reaction of ZIF-67 with Ni(NO3)2 and H2 O.It is found that Ni Co2O4 DSHPs are composed of nanocrystallines,showing the cage-in-cage hollow polyhedron structure with large specific surface area of 193.8 m2 g-1 and abundant mesopores whose size si between 2-20 nm.The inner casing measures approximately 500 nm and the outer casing is 800 nm,and the inner and outer casings are closed with a wall thickness of approximately 20 nm.As anodic materials for lithium-ion batteries,Ni Co2O4 DSHPs demonstrate remarkable lithium storage properties with high discharge capacity,stable cycling performance and excellent rate performance,surpassing Ni Co2O4single-shelled hollow polyhedrons(SSHPs).At 0.1C,the average discharge capacity over 100 cycles is 1132 m Ah g-1.After 100 cycles,a reversible capacity as high as 1138 m Ah g-1 is still delivered and coulombic efficiency is 98.2%.The discharge capacity at 2C reaches 811 m Ah g-1.The enhanced Li-storage performance should be attributed to their nanocrystalline,mesopores and double-shelled hollow nanocage structure.The larger specific surface area and porous structure promote ion diffusion at the electrode-electrolyte interface and buffer mechanical stresscaused by volume changes during the discharge-charge cycle,resulting in improved material properties.(4)Ni Co2S4 single-shelled hollow polyhedrons and double-shelled hollow polyhedrons are synthesized by the same metal-organic-framework-engaged strategy that takes advantages of the unique reactions of ZIF-67 and Ni(NO3)2,H2 O,thiacetamide.It is found by XRD,SEM,TEM that the synthesized Ni Co2S4 materials show the hollow polyhedron structure assembled with nanocrystallines(the size is around 10-15 nm),and numerous mesopores exist among these nanocrystallines.The polyhedron has a size of approximately 800 nm and a wall thickness of 30 nm.When used as anode materials for lithium-ion batteries,Ni Co2S4 SSHPs and DSHPs exhibit excellent cycling stability,but DSHPs are remarkably superior to SSHPs in terms of capacity performance and rate performance.At 0.1C,DSHPs still deliver the discharge capacity of 770.3m Ah g-1 at the 100 th cycle,and the average discharge capacity over 100 cycles reaches 745.5m Ah g-1.When the current comes back to 0.1 C after rate measurements,Ni Co2S4 DSHPs can stably deliver the discharging capacity of 712.0 m Ah g-1,reaching 92.1% of that at the initial0.1C.The superior Li-storage performance should be attributed to the nanocrystallines,mesopores,and multi-shelled hollow polyhedron structure.The unique Ni Co2S4 multi-shelled hollow polyhedrons exhibit great potential as an anode materials for high-performance lithium-ion batteries.(5)ZIF-67-derived yolk-shelled ZIF-67/Ni-Co layered double hydroxides were further vulcanized,forming Ni Co2S4/Co3S4 heterogeneous cage-in-cage hollow polyhedron.The outer casing is Ni Co2S4,the inner casing is Co3S4,the inner casing is independently movable,and the casing exhibits the characteristics of porous nanocrystals.The thickness of wall is 30 nm.The average discharge capacities at 1,2,4 and 6 m A are 158.8,133.1,110.6 and 90.9 m Ah g-1 which can be converted to be 1430.1,1197.2,995.1,818.0 F g-1 according to the pseudocapacitance calculation rule.As active materials for super capacitor,Ni Co2S4/Co3S4 showed relatively high discharge capacity and cycling stability,large current adaptability and reversibility,which could be due to the cage-in-cage hollow nanostructure and high electrochemical activity of transition metal sulfide.The internal hollow structure and rich mesopores can effectively reduce the volumechange caused by the Li+ insertion/deintercalation process.Numerous mesopores facilitate the electrolyte to enter the interior,so that the performance of the material can be fully exerted.