Design And Preparation Of High Performance Metal (Fe、Sn、Co) Oxide-Based Composites And Their Lithium Storage

Currently,with the rapid development and wide application of electric vehicles and other electric equipments with high power consumption,the need of lithium-ion batteries for high energy and power density is becoming more and more urgent.However,the energy density of the traditional lithium-ion batteries is limited by the graphite anode material due to its low theoretical specific capacity.Therefore,research and development of new high-performance anode materials to replace graphite is necessary and imminent.Thanks to the advantages of high theoretical specific capacity,abundant source and environmental friendly,metal oxides are expected to be a promising new generation of lithium-ion battery anode materials to replace the graphite anode.However,their practical application in lithium-ion batteries is impeded by some shortcomings,such as poor electrical conductivity,large volume change,low ion diffusion kinetics,and so on.Herein,in view of these key problems,some effective strategies,including design and control of microstructures and morphologies of the materials,composite with other heterogeneous conductive nanomaterials,and optimization of material component contents,have been demonstrated to improve the lithium storage of Fe₂O₃,Sn O₂and CoO metal oxides.The main contents of this thesis are as follows:（1）Herein,to stabilize the lithium storage property of Fe₂O₃,a characteristically clustered composite composed of adjacent Fe₂O₃and Ti O₂secondary nanoparticles has been constructed by a well-designed proposal.It is suggested that the adjacent durable Ti O₂nanoparticles can contribute to improve the structural stability of Fe₂O₃nanoparticles by preventing the aggregation of Fe₂O₃nanoparticles and restricting the pulverization of Fe₂O₃nanoparticles.As a result,the as-prepared composite exhibits improved performance as a promising anode material for lithium-ion batteries,showing high reversible capacity of 1234 m Ah g^-1after 320 cycles at 200 m A g^-1and792 m Ah g^-1after 600 cycles even at high current density of 1000 m A g^-1.（2）With high theoretical specific capacity and relatively safe working potential,Sn O₂has drawn widespread attention as a promising candidate of advanced anode for next generation lithium-ion batteries.However,the practical application of Sn O₂anode in lithium-ion batteries is severely blocked by some shortcomings such as the inferior rate capability,fast capacity decay and low initial coulombic efficiency during charge/discharge process.Gratifyingly,there are many works which have demonstrated that the Sn O₂anode achieves extra improvement in lithium storage performance after being reduced to nanoscale and embedded into porous carbon matrixes.Herein,porous Sn O₂@C micro-/nanospheres with a sandwiched buffer zone resulted from unevenly radial distribution of pores in carbon micro-/nanospheres are prepared.These Sn O₂@C micro-/nanospheres consist of small Sn O₂nanoparticles embedded within porous carbon micro-/nanospheres with a sandwiched buffer zone.The results of this study indicate that the sandwiched buffer zone of carbon micro-/nanospheres and the confinement effect of nanopores on small Sn O₂nanoparticles synergistically contribute to outstanding structural stability and excellent electrochemical performance of thus porous Sn O₂@C micro-/nanospheres.Besides,it is found that the lithium storage performance of the Sn O₂@C micro-/nanospheres can be tuned by adjusting the Sn O₂contents.As a result,the as-prepared Sn O₂@C micro-/nanospheres with an optimalizing Sn O₂content exhibit the best performance,delivering a high capacity of 955 m Ah g^-1at 200 m A g^-1after250 cycles as well as a high capacity of 836 m Ah g^-1at 1000 m A g^-1after 350 cycles.（3）Herein,a distinctive dual-carbon-confined nanoarchitecture,composed of inner high conductive,robust CNTs supporting and outer well-designed porous carbon coating,has been demonstrated to efficiently improve the electrochemical properties of CoO nanoparticles for the first time.The CoO nanoparticles are rationaly confined between the CNTs and porous carbon.The well-designed porous carbon coating shows a significant superiority compared to the common non-porous carbon coating due to its distinctive characteristics such as high flexibility,rich free space and open tunnel-like structure.Therefore,the synergistic effect of the CNTs core and the porous carbon sheath not only endowsheats thus CoO-based composite（CNTs@CoO@PC）with improved electrochemical reaction kinetics but also superior structural stability.As a result,the CNTs@CoO@PC shows satisfactory performance with 1090,571 and242 m Ah g^-1at 200,1000 and even 5000 m A g^-1after 300,600 and even 1000 cycles,respectively.Moreover,this strategy may be extended to improvement of other metal oxide anode materials in lithium storage performance.To sum up,with the design and control of microstructures and morphologies of the materials,composite with other heterogeneous conductive nanomaterials,and optimization of material component contents,we prepared clustered composite composed of adjacent Fe₂O₃and Ti O₂secondary nanoparticles,porous Sn O₂@C micro-/nanospheres with a sandwiched buffer zone and distinctive dual-carbon-confined nanoarchitecture composite composited of inner CNTs,CoO nanoparticles and outer porous carbon coating.Through comprehensive comparison,the porous Sn O₂@C micro-/nanospheres with a sandwiched buffer zone exhibits the improved cycle stability and best rate capability,as well as the relatively simple synthesis method,thus have great promising for a high-performance anode of LIBs.