In-situ Synthesis And Lithium-ion Battery Performance Of Transition Metal Oxides@Carbon Nano-composites

Transition metal oxides-based anodes have been limited in application due to their poor electrical conductivity and volumatric expansion.Nowadays,preparing nano-sized transition metal oxides and/or compositing transition metal oxides with conductive carbon materials can improve the plight effectively.Noteworthily,the composites with metal oxides dispersing homogenously in three-dimensional carbon network,can exerts better cycling performance when used as anodes.While,such homogenous metal oxides@carbon composties can be easily prepared by in-situ growth methods.This dissert focus on preparation various metal oxides@three-dimentional carbon composites by chemical vapor deposition process based on the nickel catalysis function and have effectively improved the electrochemical performance of TMO-based anodes.Firstly,drawing on the preparation method of graphene aerogels,three-dimensional composite NiO@C/pRGO was realized by utilizing the good catalytic performance of nickel to in-situ grow carbon layers,and its formation mechanism and electrochemical performance was studied.The results showed that the formation of porous RGO is derived from RGO cracking catalyzed by nickel;the cracked RGO provided the carbon source for carbon layers' in-situ growth.Meanwhile,the NiO@C/pRGO with more stable structure,presents higher specific capacity?1003 m Ah/g?,better rate performance and superior cycle stability?perform well at 2000 m A/g after 1000 cycles?.Compared with the structure dedorating by RGO alone,the in-situ growth of carbon materials on mateal oxides brings about a great improvement on the electrochemical stability.Secondly,the catalyst seeds were uniformly uploaded on the surface of the iron oxide nanopowders by impregnating in the nickel nitrate solution,and then the Fe₃O₄/graphitic carbon composites were in-situ achieved by the CVD process.The process parameters were optimized and their electrochemical performances were also tested.The results showed that the uploading of nickel catalysts was necessary for the in-situ growth of carbon materials;the nickel seeds uploaded by this method were uniformly distributed in the composite material and the in-situ grown carbon also adhered to the surface of the iron oxide evenly;the CVD treated temperature was chosen at 450?,the obtained composite material maintained not only the morphologies of the original iron oxides,but also the oxidized phase avoiding massive formation of iron carbide,and the main phase of the product is triiron tetroxide;the amount of carbon source added in the CVD process can be used to regulate the carbon content of the final composite,so as to regulate the electrochemical properties;thereinto,the composite material FNPls@GrC-8 with a carbon content of 22.64% at 8 m L carbon source,exhibited superior specific capacity?1008 m Ah/g?and cycle stability than FNPls@GrC-2 and FNPls@GrC-15;the iron oxide nano-powders with various morphologies can all be used as a substrate to in-situ growing carbon materials;graphitic carbon-coated on iron oxide powders have superior electrochemical properties than amorphous carbon-coated ones.Finally,inspired by the good catalytic effect of nickel/cobalt metal on the formation and decomposition of SEI films during electrochemical processes,metal/metal oxide/carbon ternary composites NiCo-NiCo₂O₄@CNTs were designed by adding transition metal components to metal oxide/carbon system and prepared by controlling the specific parameters of CVD,and their electrochemical properties were tested.The results showed that the ternary composite material presented a high capacity of 1405 m Ah/g after 500 cycles at a current density of 200 m A/g and 1324 m Ah/g and 945 m Ah/g at 500 m A/g and 1000 m A/g both after 1000 cycles,respectively,indicating that the ternary composite exhibited good capacity,excellent rate performance,and superior cycling performance as long as 2200 cycles.The ternary composite NiCo-NiCo₂O₄@CNTs showed a gradual increase in capacity during cycling.However,the contribution in capacity from nickel-cobaltate conversion reaction decreased gradually as the testing proceeding.While the capacity growth mainly came from the formation and decomposition of SEI films.All components in the ternary composites NiCo-NiCo₂O₄@CNTs played different roles in electrochemical processes: NiCo₂O₄ provided part of the capacity and effectively refined the catalyst particles into nanoparticles with a higher catalytic function;refined NiCo metal effectively catalyzed the reversible reaction of SEI film and provided the capacity increase;the carbon nanotubes provided a good conductive network and sufficient surface for nano-particle dispersing,avoiding reagglomeration of the refined catalyst.