Design And Preparation Of Micro-Nano Hierarchical Composite Materials Based On Transition Metal Oxides And Their Li Storage Performance

Recently,the demands on energy density,power density and cycle life are becoming higher for secondary batteries,thus traditional lithium ion batteries(LIBs)have been facing great challenges.As a core component,the electrode material determines the overall LIB performance to a large extent,however currently wide-used graphite anode becomes difficult to satisfy the demands due to drawbacks including low specific capacity.By contrast,transition metal oxides(TMOs)have attracted plenty of attention as promising next-generation LIB anode materials,owed to advantages such as high specific capacity,flexible compositional and structural adjustability,rich sources,etc.Meanwhile,imitating the hierarchical architecture and composition arrangement of nature-inspired structures,has emerged as an efficient strategy for designing functional materials.Therefore,based on Zn-Co-based bimetal oxide and Co-based oxide systems,the researches of this dissertation centered on following main themes:design of nature-inspired hierarchical composite materials,facile and controllable preparation process,and efficient Li storage performance.A series of electrode materials with micro-nano hierarchical design and composite components were successfully synthesized,which delivered good electrochemical properties.Besides,the preparation mechanisms and building strategies of micro-nano hierarchical materials were comprehensively studied,accompanied with efficient Li storage processes,and their structural and compositional advantages were further verified by DFT calculations,in situ TEM,etc.The main research content contained five parts as follows:(1)A bimetal oxide dual-composite strategy based on 2D-mosaic&3D-gradient design is proposed:Zn1-xCoxO/ZnCo2O4 2D-mosaic-hybrid mesoporous ultrathin nanosheets inspired by natural mosaic-dominance phenomena,were served as building blocks to assemble into a 3D Zn-Co hierarchical framework.Moreover,a series of derivative frameworks with highly evolution in composition and structure were controllably synthesized,based on which a facile one-pot synthesis process can be developed.As verified by DFT calculations,the kinetics of electron/ion transport of both Zn1-xCoxO and ZnCo2O4 was greatly enhanced owed to bimetal mutual-doping.More importantly,unique 2D-mosaic-hybrid mode gave rise to ladder-type buffering and electrochemical synergistic effect,thus realizes mutual stabilization and activation between the mosaic pair.Besides,the inside-out Zn-Co concentration gradient,rich oxygen vacancies and mesoporous nature further enhanced Li storage capacity and stability.As a result,a capacity as high as?1000 mA h g-1 was attained with a current density of 100 mA g-1,more,while excellent capacity retention was observed during high-rate and long-term cycle(1000 mAg-1,800 cycles).(2)A neuron-inspired electrode material design with multi-scale synergistic and two-dimensional assembled structure was proposed,aiming at tackling the low tap density and poor rate capability problems of nanosized and micronsized materials,respectively.Desired structure was synthesized based on a fluorine-induced bimetal reaction system similar to above fibrous-root-inspired work.Multiple ZnxCo3-xO4 ultrafine regional-nanowire-arrays grow separately in limited two-dimensional directions,from the multiple top platforms of a Zn1-yCoyO micron-star.On one hand,unique two-dimensional assembly mode help to retain the tap density advantage originated from micron-star part.On the other hand,the high rate Li storage capability of micron-star prat could be significantly improved,benefiting from multi-scale synergistic effect,effective transport network and bimetal mutual-doping.As expected,superior areal specific capacities of?1.55 mA h cm-2(1.0 mA cm-2,500cycles)and largely enhanced rate capability were simultaneously obtained.In addition,in situ TEM and kinetic analysis further illustrated the mechanism of performance superiority.(3)Inspired by natural fibrous-root structure,hierarchical ZnxCo3-xO4/Zn1-yCoyO binary synergistic nanoarrays were designed and synthesized on Cu mesh substrates based on a facile one-pot,successive-deposition process,utilizing a fluorine-induced bimetal liquid phase reaction system.From the top platforms of Zn1-yCoyO nanorod arrays,ultrafine ZnxCo3-xO4 regional-nanowire-arrays grew orderly in a direction parallel to the nanorods.The two parts made up a self-supporting binary synergistic Li storage system with fibrous-root-inspired structure,and served as the supporting unit and and functional unit respectively.As an integrated LIB anode without binder or conductive additive,ZnxCo3-xO4/Zn1-yCoyO hierarchical nanoarrays could maintain a high capacity of more than 800 mA h g-1 at a current density of 500 mA g-1,and showed great advantages on specific capacity,rate capability and cycle stability over single zinc-based nanorod arrays or cobalt-based nanowire arrays.(4)Through a sequential transformation route,Si/C-modified-Co3O4 nanowire arrays were constructed on 3D Ni foam,to form a corn-inspired ternary composite hierarchical self-supporting system.An ionic liquid-assisted electrodeposition strategy was employed to realize a discrete in situ fabrication of ultrafine Si nanoparticles onto Co3O4 nanowire substrate,which followed a Volmer-Weber island growth mode.In this synergistic system,corn-kernel-like discrete silicon nanoparticles and corn-husk-like carbon coating layer functioned as the enhancing unit and protecting unit respectively,to improve the capacity and stability of the corn-cob-like Co3O4 nanowire which functioned as the basic unit.And a special capacity nearly 1000 mA h g 1 was obtained at a current density of 100 mA g-1.(5)Aiming at obtaining dense packed and high stable Li storage simultaneously,a "thousand-layer-cake" Co3O4/C nano-mciro hierarchical stack structure was prepared.As the building blocks,the nanosized porous lamellar-structures were densely stacked to assemble into micronsized overall dimension,which obviously improved the tap density and prevents excess formation of SEI film.The lamellar-structure provided shortened electron/ion transport pathway and buffered the volume expansion during lithiation.The carbon coating process further improved the electrode/electrolyte interface.This thousand-layer-cake structure exhibited a capacity of about 740 mA h g-1 with a current density of 100 mA g1,and maintained stable at neraly 700 mA h g 1 after 500 cycles with a high current density of 500 mA g-1,rending excellent long-term cycle stability and rate performance.