Preparation And Electrochemical Properties Of Hierarchically Structured Titanium-oxide-based Materials

Lithium-ion batteries (LIBs) are one of the most promising electrochemicalpower sources, widely used in portable electronics, electric vehicles, andstationary energy storage systems. Being inherently safe and chemicallycompatible with the electrolyte, titanium-oxide-based materials, includingboth Li-titanates and various TiO2polymorphs, are considered to bealternatives to carbonaceous anodes in LIBs. However, the rate performanceof titanium-oxide-based compounds is not satisfactary due to the lowelectronic conductivity, limiting their practical application in LIBs demandingfor high power densities. Much effort has been focused on morphologytailoring and size controlling of the active material, to improve their ratecapability. It is envisaged that the introduction of a hierarchical structure andthe increase of crystallographic defect concentration and grain boundarydensity are feasible strategies to improve the electrochemical performance ofelectrode materials. Hierarchically structured materials with small particlesize and large specific surface area can provide relatively shortened mass andcharge transport pathways and charming surface activities, resulting inenhanced rate capabilities. The presence of defects in the crystal structuresmay enhance the lithium ion transport properties and electronic conductivityof the electrode materials. Therefore, it is promising to enhance the rateperformance of lithium ion batteries to a great extent by introducing defects tothe electrode materials. A large amount of lithium ions can be stored on theinterfacial area of the hierarchically structured materials with a high grainboundary density, increasing the specific capacities of these materials. It is ofinterest that the capacity introduced by the interfacial lithium storage is almost independent on the charge-discharge rate due to its thermodynamiccharacteristics. Dual-and multi-phase materials are expected to provideincreased grain boundary density.In this thesis, new titanium-oxide-based hierarchically structured materialsare fabricated, their electrochemical performance as anode for lithium ionbatteries has been significationly improved by morphology tailoring andmicrostructure control．1．Hierarchically structured titania rods as an anode material for highperformance lithium-ion batteries.Hierarchically structured titania materials composed of anatasenanoparticles have been prepared via a template-free light-driven fabricationroute by employing titanium glycolate (TG) as a precursor. The volume ratiosof ethylene glycol and tetrabutyl titanate during the synthesis of the TGprecursors determine the morphology and texture properties of the titaniamaterials. As evaluated by CV and galvanostatic charge/dischargemeasurements, the morphology, texture property, and crystallinity give animportant influence on the electrochemical performance of the anatase titaniamaterials. Among these materials, hierarchically structured titania rods,derived from a TG precursor prepared with an ethylene glycol over tetrabutyltitanate ratio of100:1(v/v), exhibits high initial discharge capacity of262mAh g–1at0.1A g1. Cycling at1.0A g1, a capacity of161mAh g–1isretained even after40cycles, indicating excellent cycling stability. The highelectrochemical performance of the mesoporous TiO2rods is attributed to theunique mesoporous structure and the crystalline nature of the rods, whichfacilitate the transport of the electrolyte and improve the lithium-ioninsertion/extraction kinetics.2．Fabrication of hierarchical Li4Ti5O12/TiO2hollow spheres and theirlithium-ion storage property.Hierarchical Li4Ti5O12/TiO2hollow spheres composed of nanoflakes havebeen successfully fabricated via the calcination of a hydrothermal product ofamorphous TiO2precursor and lithium hydroxide monohydrate. These nanoflakes exhibit a quite complex submicroscopic structure, including ahuge number of grain boundaries and defects. The lithium ion storageproperty of these hollow spheres is evaluated by galvanostaticdischarge/charge experiment. The product shows good electrochemicalperformance, including high specific capacity (266mAh g1at0.1A g–1),andexcellent rate capability (149mAh g–1at1.0A g–1up to100cycles). The highspecific capacity, good rate capability and cycleability of the Li4Ti5O12/TiO2hollow spheres are attributed to the intrinsic nature of the defects and thestructure feature of the spheres. The defects existed in the nanoflakes providea large number vacancies with similar chemical environment that benefit tolithium-ion storage property of the spheres. The high porosity structure of thematerial facilitates the transport of electrolyte within the electrode andshortens the diffusion distance for both electron and lithium-ions. The highspecific surface area of the material facilitates the surface Li storage and theLi insertion/extraction in/from the material. High grain boundary density withlarge interfacial areas is favorable for the fast electrochemical lithiuminsertion and extraction processes. These structural features offer fast lithiuminsertion/extraction kinetics, leading to high rate capability. In addition, thedistinct hollow interior allowing the material to effectively buffer the stressinduced during the discharge/charge process, contributing to the cyclingstability.3．Hierarchical Li4Ti5O12/TiO2composite tubes with regular structuralimperfection for lithium-ion storage.Hierarchical Li4Ti5O12/TiO2tubes composed of ultrathin nanoflakes havebeen successfully fabricated via the calcination of the hydrothermal productof amorphous TiO2rods and lithium hydroxide monohydrate.. The structuralimperfection formed in the nanoflakes during the calcination has proved to beadvantageous for lithium ion storage of these tubes. The obtained hierarchicalLi4Ti5O12/TiO2tubes exhibit a high initial discharge capacity of420mAh g–1at0.01A g1. Cycling at1.0A g1, a capacity of139mAh g–1is retained evenafter100cycles, indicating excellent cycling stability. The high electrochemical performance of the tubes is attributed to their uniquenanostructure, including the3D porous channel system, the structuralimperfection in Li4Ti5O12nanoflakes and the high grain boundary densityamong the nanoparticles. The3D porous channel system facilitates the masstransport of the electrolyte and lithium ions and provides sufficient void spaceto accommodate the volume change during the discharge/charge process,ensuring the good cycling performance. Such elaborate combination of crystalstructure and architectural mode could be helpful for designing new high ratecomposite electrodes.