Optimization Design Of Niob-based Oxides And Its Electrochemical Lithium Storage Behavior

Facing with continuous fossil energy consumption and increasing environmental pollution,unprecedented importance is attached to green and renewable energy sources.Meanwhile,energy conversion and storage devices are considered as the key to renewable energy utilization.Whether Li-ion battery or Li-ion capacitor and other energy storage devices,the performance of the anode material is one of the most important factors affecting its overall performance.Niobium-based oxide,as one of the anode materials with superior specific capacity and high cycling stability,has prominent research value.However,its electrochemical performance and commercial application are limited by the following problems:1)The limited lithium ion transport rate of niobium-based oxides still cannot fully meet the increasing actual demand;2)In the energy storage process,the Li+exchange rate and stability at the interface between the niobium-based oxide and the electrolyte need to be improved;3)the electrons conduction in the anode material is severely influenced by its inherent low conductivity.In response to above questions,this thesis aims to enhance kinetic advantage of niobiumbased oxides by using doping defects and vacancy defects to construct good ion transport channels.At the same time,multifunctional interfacial coating is created to coordinated optimization of material interface stability.Moreover,a variety of scientific solutions have been utilized to prepare niobium oxides with various morphologies as Li+storage anode materials.The research of this thesis is divided into five main parts as follows.1.Li+storage performance of carbon tube confined Nb2O5 compositesFirst,this chapter characterizes the different lattice lithium storage mechanisms of the niobium-based oxide,and optimizes its design by combining spatially limited domain effects.Specifically,the nitrogen-doped heteroatomic carbon tubes were synthesized by using MnO2 as template and dopamine as a raw material.This work focuses on planting Nb2O5 on both inner and outer surface of the carbontube to form domain-limiting effect and protect function for the grown Nb2O5.At the same time,various lattices Nb2O5 were investigated to understand the mechanism of its lattice effect on lithium ion transport.In fact,the orthorhombic Nb2O5 material shows desirable mass specific capacity(up to 194 mAh g-1 at 0.25C)and cycling stability(89%specific capacity after 1000 cycles)in LIBs.Meanwhile,the applicability of TNb2O5@NC for LICs anode material is also demonstrated.2.Lithium storage properties of fibrous T-Nb2O5-x@NC under vacancy defect modificationBased on the above studies,the oxygen vacancy defects were introduced into the study of lithium storage in niobium-based mono-oxides,trying to show that oxygen vacancy defects can optimize the lithium ion transport rate and construct them as one-dimensional necklacelike fiber materials.Unique constructed necklace-like fiber,modified with oxygen vacancies,can further optimize T-Nb2O5 electrochemical properties.This is manifested by the following facts:oxygen vacancies in the T-Nb2O5-x@NC reduce its lithium ion transport energy barrier.In this process,the independent unmatched electrons are produced,which optimize its electrical conductivity.The oxygen vacancy defects also enhance the lithium ion transport rate and lithium ion storage capacity of niobium-based monoxide.3.Lithium storage performance of capillary microspheres N-Nb2O5-x@CNTs under double defect and interfacial coating optimizationBuilding on previous work,both nitrogen doping and oxygen vacancy defects were imported into the niobium-based monoxide,and their synergistic optimization is confirmed.In addition,ultra-thin carbon was coated on niobium oxide surface to suppress the lattice disorder caused by defects.Highly stable microspheres N-Nb2O5-x@CNTs,with carbon tubes as "skeleton" and Nb2O5 as "cement",were constructed by spray drying.The carbon tubes running through the bulk phase of the microspheres ensure that the electrolyte reach the interior of the bulk phase at high speed by the capillary effect.Moreover,the presence of more pores on the microsphere surface provides multiple channels for the electrolyte infiltration.As for the doping defects and derived vacancies,they provide abundant active sites for the fasttransporting Li+.Thus,N-Nb2O5-x@CNTs achieves good transport rate and reaction rate,closer to the non-Faraday reaction.Furthermore,carbon tubes act as structural scaffolds to provide strong structural toughness and effective strain buffering for N-Nb2O5-x@CNTs.Interestingly,the carbon tubes also effectively block the direct contact between Nb2O5 and electrolyte,inhibiting some of the undesirable reactions and enhancing the electrochemical stability.The highly conductive carbon tubes and the unmatched electrons in the lattice(derived from vacancies and doping defects)synergy improves the electrical conductivity of Nb2O5.4.Amorphous carbon and lattice defects synergistically optimizing lithium storage performance in MnNb2O6In terms of the synergistic effect of lattice defects and interfacial coatings in our previous study.An attempt was made to apply these two optimization approaches to niobium-based binary oxides in order to analyse the synergistic optimization effect.Specifically,electrostatic spinning of mixed Niobium-manganese precursors was used to produce one-dimensional MnNb2O6 nanofibers.Subsequently,N-MnNb2O6-x@C fibers with ultrathin amorphous carbon(by in situ polymerization of dopamine)and defects(N-doping and oxygen vacancy)was prepared by simultaneous carbonization and nitridation in high-temperature melamine steam atmosphere.The one-dimensional N-MnNb2O6-x@C possesses satisfied lithium ion transport kinetics and interfacial stability.The ultra-thin amorphous carbon shell relieves the chaotic situation of Li+ embedding/transport,and suppresses the undesirable reaction between the electrolyte and the energy storage body.The heteroatom and oxygen vacancy defects enhance the electrical conductivity and active site density of MnNb2O6,and also reduce the ionic embedding energy barrier.N-MnNb2O6-x@C offered admirable electrochemical performance under the synergistic optimization of both.The Li-ion battery maintained 83.6%capacity after 2000 cycles at a low current density of 0.1 A g-1,displaying good stability.5.Defects and interfaces synergic optimize the Li+/electron transport kinetics of Ti2Nb10O29-xAlong with the above-mentioned defects and interface optimization,we also tried to improve the utilization of the bulk phase active site.The bowl-shaped ppy@TNO-x@NC with high stability and bulk density was prepared with a combination of interfacial protection and lattice defect.That is,biomass hollow carbon bowl was used as conductive substrate,on which Ti2Nb10O29-x nanoparticles(TNO)with vacancy defects were loaded as the energy storage body.After that,a thin layer of polypyrrole(ppy,theoretical capacity 64.5 mAh g-1)was coated,thereby preparing ppy@TNO-x@NC as an anode material for Li-ion capacitors.The hollow bowl structure shows high bulk density and corresponding volumetric energy density based on the conventional hollow spheres.Moreover,its biological origin is in line with the green development theme.the abundant oxygen vacancy defects with unmatched electrons in TNO enhances its ion transport kinetics and conductivity.In addition,the present work mitigates the lattice mismatch due to defective structure using ppy coating,and enhances its electrochemical stability.ppy@TNO-x@NC composites present good electrochemical stability(91.7%after 5000 cycles),electric capacity and energy density in both Li-ion half batteries and Li-ion capacitors.