| Metal oxides(sulfdes)are used as anode material for lithium ion batteries with high theoretical capacity,which have gradually become a potential substitute for commercial graphite anode.Metal oxides(sulfdes)have a safe lithium-intercalation potential,which guarantees a relatively independent lithium uptake and release process.However,the volume expansion of metal oxides in the process of uptake and release will lead to the agglomeration of the electrode materials,and eventually result in a large irreversible capacity loss.In addition,the low electrical conductivity of metal oxides greatly affects their electrochemical reaction and limits their further application.In view of these problems,nanostructured metal oxides(sulfdes)and utilized good conductive matrix are efficient way to improve the electrochemical performance of metal oxides(sulfdes).Nanotechnology mainly refers to control the metal oxides in different dimensions to optimize the electrochemical properties.Although graphene has special physicochemical properties,the disorderly stacking structure is not conducive to the diffusion of lithium ions and electrolytes.The unique networks of three-dimensional graphene can effectively inhibit stacking and agglomeration of graphene in the drying process.The micropore and mesoporous structure can make full use of the physical and chemical properties of graphene,which can effectively improve the electrochemical properties of traditional electrode materials.In this paper,the low cost and high theoretical capacity of metal oxides(sulfdes)are used as the research object and a series of newmultidimensional metal oxides(sulfdes)/three-dimensional graphene composites were prepared by using a simple and efficient hydrothermal method and a chemical reduction process.Three-dimensional graphene as conductive substrate have high specific surface area and high conductivity.Multi-dimensional metal oxides/three-dimensional graphene composites as anode materials for lithium/sodium ion batteries exhibit good cyclic and rate performance.At the same time,the basic scientific problems of electrode materials in electrochemical applications,such as electrode reaction and electrochemical behavior in the process were studied by various electrochemical methods.The specific research contents are as follows:(1)We developed facial and high effective hydrothermal and chemical reduction approachs for synthesizing ZnFe2O4/GAs and Fe304/GAs composites,respectively.The nanoparticles are homogeneous anchored into the graphene networks.Here,nanoparticles as spacers can effectively inhibite the restacking of graphene.Meanwhile,the GA with porous structure can buffer volume expansion and improve the electronic conductivity of the composite.The ZnFe2O4 exhibits high initial charge capacity with 1068 mAh g-1.However,it suffered from severe capacity degradation with only 458 mAh g-1 retained after 100 cycles at 100 mA g-1.While,the ZnFe2O4/GAs composite possesses outstanding cycling stability with a reversible capacity of 1049 mAh g-1 after 100 cycles at 100 mA g-1.The ZnFe2O4/GAs composite also possesses extraordinary high-rate cycling performance,which can still deliver a reversible capacity of 565 mAh g-1 even at a high current density of 1000 mA g-1.Also,Fe304/GAs composite with porous architecture was obtained by in situ chemical reduction approach using Fe2+ as reducing agent and Fe source.The Fe3O4/GAs composite possesses outstanding cycling stability with a reversible capacity of 1016 mAh g-1 after 100 cycles at 100 mA g-1.The Fe3O4/GAs composite also possesses extraordinary high-rate cycling performance,which can still deliver a reversible capacity of 434 mAh g-1 even at a high current density of 2000 mA g-1.(2)To further overcome the re-stacking of graphene sheets in closely-interwined structure,metal oxides quantum dots(QDs)and graphene aerogel composite with porous architecture was obtained by situ hydrothermal method without any additive agents.The ultrafine particle size of metal oxides could dramatically mitigate the volume changes during conversion process and the graphene substrates could provide large electrode/electrolyte interface area and inhibit particle aggregation,resulting in high reversible capacity.The as-synthesized Fe3O4/GA composite delivers a high specific capacity of 1078 mAh g-1 at 100 mA g-1 after 70 cycles as LIBs.Even at a high current density 1000 mA g-1 can still maintain a stable reversible capacity of 562 mAh g-1.SnO2/GA delivers a high specific capacity of 319 mAh g-1 at·50 mA g-1 after 50 cycles as SIBs.Even at a high current density 800 mA g-1 can still maintain a stable reversible capacity of 150 mAh g-1.Based on the above methods,we developed a solvothermal-induced self-assembly approach to construct three dimensional macroscopic CoO QDs/nitrogen-doped graphene aerogel as anode materials for LIBs.DMF acted as both reducing reagent and nitrogen sources.The as-prepared composite.deliveries high specific capacity of 896 mAh g-1 at 200 mA g-1 after 200 cycles.It still retains specific capacity of 373 mAh g-1 at 1600 mA g-1.On one hand,the doping of nitrogen can increase surface defects in the graphene and introduce Co-N-C bonds which is indispensable for the good electrochemical performance.On the other hand,ultrafine CoO nanoparticles disperse onto macroscopic graphene could mitigate the agglomeration of nanoparticles as well as shorten Li+ diffusion length.(3)To date,these metal oxides in graphene present uncrafted geometries,and in most cases,they are nanoparticles in the rough without structure design.The geometric shape and surface features could significantly influence the physiochemical properties of metal oxides.So controlled aqueous growth of well-defined morphologies inside 3D graphene still remains a challenge.Therefore,we present a novel synthetic strategy to construct the purposely designed,SnO2 nanorods(SnO2 NRs)embellished GA heterostructures with enhanced lithium/sodium-storage performance.Rational integration of the two advanced structures,SnO2 NRs/GA shows enhanced lithium storage properties(869 mAh g-1 for the 50th cycle at 100 mA g-1)and sodium-storage properties(232 mAh g-1 for the 50th cycle at 50 mA g-1).It was also well-demonstrated that this hybrid material was capable of delivering high specific capacity at rapid charge/discharge cycles(1001 mAh g-1 at 100 mA g-1,925 mAh g-1 at 200 mA g-1,781 mAh g-1 at 500 mA g-1,643 mAh g-1 at 1000 mA g-1,and 458 mAh g-1 at 2000 mA g-1).1D metal oxides Fe3O4 nanowires/GA exhibited high specific capacity and enhanced rate capability.(4)The micron level transverse dimension of two-dimensional metal oxides can enhance the integrality of the material structure,and effectively buffer the volume change during Li charge/discharge process.On the other hand,it can promote the Li+ and electron transmission,and improve the rate performance.In addition,large surface area increases electrode-electrolyte contact area and is benefcial for access to the electrochemically active surface.The high-level dimensional metal oxides(sulfdes)/three-dimensional graphene composites such as flower-like Fe304/GA composites and flower-like MoS2/GA composites were prepared for the first time.The structure is benefcial for the synergistic effect between two-dimensional materials and three-dimensional graphene,which showing excellent electrochemical properties.The flower-like Fe3O4/GA composites as anode for LIBs exhibit outstanding cycling stability(1016 mAh g-1 after 200 cycles at 200 mA g-1)and superior rate performance(534 mAh g-1 at 3.2 A g-1).Applied as anode materials of sodium ion batteries,the flower-like MoS2/GA composites exhibit high initial discharge/charge capacities of 562.7 and 460 mAh g-1 at a current density of 100 mA g-1.The initial coulombic efficiency is 81.7%for the first cycle.The discharge capacities were 208 mAh g-1 when cycled at current densities of 500 mA g-1. |
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