Self-Assembly Synthesis And Energy Storage Properties Of Nano-carbon Coated Transition Metal Oxides

With the rapid development of portable electronic products and electric vehicles,the requirements for the energy density and power density of lithium-ion batteries have gradually increased.At present,the most common anode material of commercial lithium ion battery is graphite material,and the improvement of energy density and power density have been seriously restricted by the low theoretical specific capacity?372 mAh/g?.Transition metal oxides are considered as an ideal anode material for lithium-ion batteries due to their high theoretical specific capacity,easy access and environmental friendliness.However,its disadvantages such as poor conductivity,slow electron transfer rate,and severe volume effect have hindered its commercial application.Many researches have used nanotechnology and coating technologies to make transition metal oxide materials nano-sized or coated with conductive materials to improve the performance.However,the current coating modification techniques are multi-step methods,which synthesize transition metal oxides and phase-coated materials respectively,and then perform two-phase physical mixing.There are many disadvantages of this physical multi-step coating method,such as uneven mixing,incapability of precise control of coating thickness,and so on.In this paper,transition metal oxides?Fe₂O₃,SnO₂,ZnO?coated with elastic nano-carbon layer was synthesized by in-situ chemical self-assembly method,combined with nanotechnology and nano-carbon layer cladding technology.The nano-carbon layer has good electrical conductivity,which can promote the charge conduction.Moreover,it can act as an elastic buffer layer to buffer the mechanical stress resulted from the expansion/contraction of the electrode material,to slow down the volume change of the transition metal oxide nanoparticle,and maintain the structural stability of the material.The nano-carbon coated transition metal oxide prepared through chemical self-assembly method has porous structure and high specific surface area,which could increase the lithium storage active site and buffer volume change of the electrode material.In addition,the higher specific capacity and better cycling stability of the nanocomposite suggest a strong synergistic effect between the conductive nano-carbon layer and high capacity transition metal oxide components,which can catalyze the graphitization of carbon materials and improve their electrochemical performance.1.A core-shell structure of?-Fe₂O₃@graphite nano-flower composites was synthesized by a molecular electrostatic self-assembly method and applied to lithium ion battery anode materials.After 1000 cycles,the specific capacity of the?-Fe₂O₃@graphite electrode material is up to 518.5 mAh/g at high current density of1 C.2.The cocoon-like?-Fe₂O₃@C nanoparticles were successfully synthesized by a simple hydrothermal molecular self-assembly method and were further assembled into energy storage devices to study its energy storage properties.The specific capacity reaches up to 358 mAh/g at 1 C current density after 150 cycles.The sandwich structure electrode was composed of?-Fe2O3@C nanoparticles and Ni foam.The specific capacitance was as high as 406.9 F/g at a current density of 0.5A/g and the capacity retention was 90.7%at a high current density of 10 A/g after2000 cycles.3.The SnO₂@C nanorod composites were successfully synthesized by self-assembly using plant fibers as templates.The SnO₂@C nanoparticles exhibited a better cycle performance and rate performance.The 3D nanoporous SnO₂@C composites formed by the stacking of nanorods can reduce the distance of Li⁺intercalation/extraction and improve the electron mobility.In addition,the coated nano-carbon layer could slow down the volume change of the SnO₂ nanoparticles,improve the electrochemical performance of SnO₂.4.The ZIF-8 precursors were prepared by self-assembly of MOF with zinc salts and imidazoles.The N-doped,carbon-coated 3D porous network structure ZnO@C composites were prepared by calcining ZIF-8 precursors in high purity N₂ atmosphere.The N-doping ZnO@C has good thermal stability and its high specific surface area is449.3 m²/g.The N-doping nano-carbon coated layers could enhance the conductivity of ZnO and effectively improve the electrochemical performance of ZnO.