Structural Induction Of Metal Oxides By Oxidized Carbon Nanotubes And The Electrochemical Behavior Of Their Nanocomposites

With the booming development of new energy industry,the demand for high energy density and strong power density lithium-ion batteries continues to grow.As the core component of lithium-ion batteries,the selection and optimization of electrode materials is crucial.Transition metal oxides stand out among many materials due to their high theoretical specific capacity and natural abundance,but their shortcomings such as low electrical conductivity and poor structural stability have seriously hindered the commercialization process.One of the effective ways to solve the above problems is to construct composite materials with unique nanostructures in combination with carbon materials.Based on the above research background,this thesis takes manganese,cobalt-manganese mixed and vanadium oxides with variable valence and tunable structure as the research object,and find that the oxygen-containing functional groups of oxidized MWCNTs have a structure-inducing effect on the metal oxides formed at MWCNTs surface.Then a series of novel transition metal oxides/carbon nanocomposites with excellent performance are prepared through precisely regulating this inductive effect.The research results show that the type and number of oxygen-containing functional groups on oxidized MWCNTs have a great influence on the chemical composition and morphology of the transition metal oxides,which should originate from the formation of chemical bonds between transition metal oxides and oxidized MWCNTs and the structure-inducing effects.This interfacial structure-induced design synthesis strategy proposed in this thesis is universal and provides a new avenue for the development of transition metal oxide/carbon composites with efficient energy storage and conversion.The main research contents are as follows:1.MnO_x bound on oxidized MWCNTs as anode for lithium-ion batteries.Firstly,the MWCNTs is oxidized by piranha solution,and the type as well as amount of oxygen-containing functional groups on the surface of MWCNTs are regulated by changing the oxidation time.Then,MnO_x/MWCNTs-raw,MnO_x/MWCNTs-1h,MnO_x/MWCNTs-6h,and MnO_x/MWCNTs-24h nanocomposites are synthesized by in-situ hydrothermal method.The XRD,SEM,and TEM results show the composition and morphology of the above four nanocomposites were different.Among them,MnO_x/MWCNTs-6h possesses the most uniform morphological structure with the binary manganese oxides（38%MnO₂ and 62%Mn₃O₄）nanosheets growing vertically and uniformly on the oxidized MWCNTs.FT-IR and XPS results show that Mn-O-C bonds are formed at the interface between MnO_x and MWCNTs,and MnO_x/MWCNTs-6h nanocomposite has the highest number of Mn-O-C.This strong interfacial interaction can not only effectively improve the structural stability of the nanocomposite,but also provide a fast channel for electron transfer.Furthermore,we conduct an in-depth study on the formation mechanism of Mn-O-C bonds,and find that ether-oxygen bond plays an essential role in the formation of the strong interfacial connection.The electrochemical performance test results show that the MnO_x/MWCNTs-6h nanocomposite with the most uniform morphology and the highest number of Mn–O–C bonds give the best electrochemical properties with high capacity(1353.2 mAh g^-1 at 0.1Ag^-1),outstanding rate capability(384.9mAh g^-1 at 20 Ag^-1),and durable long-term cyclability(1000 cycles with 70%retention at 2 Ag^-1).2.Chemical coupling of manganese–cobalt oxide and oxidized MWCNTs for enhanced lithium storage.By using the oxidized MWCNTs,a novel MnO₂/（Co,Mn）（Co,Mn）₂O₄/o MWCNTs（MO/CMO/o MWCNTs）nanocomposite with a flower-like open-porous structure is prepared via in-situ hydrothermal method.The results show that a large number of metal-oxygen-carbon（Me-O-C）bonds are also formed in MO/CMO/o MWCNTs.Benefiting from the rational structural design and strong interfacial connection,the first prepared MO/CMO/o MWCNTs electrode displays superior long-term durability with the capacity of 897 mAh g^-1 over 1000 cycles at 2 Ag^-1 and ultrafast charging/discharging capability of 673 mAh g^-1 at 5 Ag^-1.In addition,quantitative electrochemical analysis results show that over 70%of the energy storage in the MO/CMO/o MWCNTs electrode is dominated by the pseudocapacitive behavior,which is the main reason for its ultrafast Li-ion migration rate.3.Strongly bonded VO_x/oMWCNTs nanocomposite significantly for enhanced Li-ion kinetics.Similarly,VO_x/oMWCNTs nanobelts are successfully fabricated by composite VO_x with oxidized MWCNTs using in-situ hydrothermal method.The results of the study show that the formation of mixed valence vanadium oxides can be successfully induced by the modulation of oxygen-containing functional groups on MWCNTs to increase the redox reaction activity.More importantly,through the connection of oxygen atoms in the oxygen-containing functional groups of oxidized MWCNTs,V-O-C bonds can also be formed between vanadium oxides and MWCNTs,providing a faster path for electron transfer and ion diffusion while improving the stability of the electrode structure.As a result,the VO_x/oMWCNTs nanocomposite exhibits excellent electrochemical performance:its reversible specific capacity can reach256.1 mAh g^-1 and 179.1 mAh g^-1 at 0.1 Ag^-1 and 2 Ag^-1,respectively,and the capacity retention after 150 cycles is as high as 88.6%.