Anatase TiO2 and spinel Li4Ti5O12 (LTO) have attracted ever-increasing attention as anode materials for LIBs due to their high and flat intercalation potentials and stable structures. The application of both materials in the high-power LIBs, however, is restricted by their low electronic conductivity and the sluggish Li+-ion diffusion in them. In this thesis, nanostructuring and doping were used to cope with these issues. Additionally, the major attention is paid to investigate the impacts of crystallite size, porous structure and doping on their electrochemical charge storage.Niobium doped TiO2 (Nb:TiO2) nanocrystals with a crystallite size of 6 nm were prepared by a novel non-aqueous method based on tert-amyl alcohol and 1-hexanol under the assistance of microwave radiation. The doping level can be tuned from 2 to 10 at% by the niobium precursor concentration. The crystallite size can be tailored from 4 to 20 nm by the reaction parameters. The electrochemical properties of Nb:TiO2 as the anode material were evaluated, and the results indicate Nb:TiO2 with the doping level of 5 at% shows the best performance owing to its highest conductivity.It has been proven that nanocrystals with a crystallite size of several nanometers will exhibit a significant pseudocapacitive contribution in previous studies. Up until now there are no direct studies on the conductivity effect on the pseudocapacitive contribution. Here mesoporous Nb:TiO2 films were selected as model materials to investigate it. In order to obtain mesoporous Nb:TiO2 films with the similar crystallinity and specific surface area, these films were prepared based on the self-assembly of preformed nanocrystals by dip-coating. The results reveal that the 5 at% Nb:TiO2 presents the highest conductivity of 9.8×10-5 S/cm and the pseudocapacitive contribution varies in the same trend with the conductivity of these films.LTO nanocrystals were prepared by a benzyl alcohol-based non-aqueous process and the crystallite sizes can be tuned from 3 to 7 nm by changing the reaction temperature. The other reaction parameters including reaction concentration and precursors have little effect on crystallite sizes. The crystalline structure, surface components and the formation mechanism of these nanocrystals were investigated in detail. The unreacted alkoxide, carbonate and the benzoate are found on the surface. The liquid reaction products such as toluene, benzaldehyde and dibenzyl ether imply a reaction path of the analogous Meerwein-Ponndorf-Verley reaction and a side reaction of the ether elimination.In order to improve the conductivity of LTO, Nb5+ and Al3+ ions were used to substitute Ti4+ ions in TiO6 octahedrons. However Al3+ ions prefer to substitute the Li+ in LiO4 tetrahedrons. The Al:LTO nanocrystals present the highest conductivity due to the high doping level. LTO nanocrystals possess the higher specific capacity at the small C-rates (200 mAh/g at 1C to 153 mAh/g at 10C) than Al:LTO and Nb:LTO nanocrystals. The investigation of the crystallite size effect on electrochemical performance was conducted on LTO and Al:LTO nanocrystals with different crystallite sizes. The results indicate that nanocrystals can enhance the rate capacity and specific capacity on account of the short diffusion distance of electrons and Li+ ions. On the other hand, the LTO nanocrystals with a crystallite size of 3 nm possess a large amount of pseudocapacitive contribution and this effect results in a large irreversible energy loss at the small C-rates owing to the surface restructuring.Mesoporous LTO films and powders can be successfully prepared based on the sol-gel process, but a well-controlled heat treatment is required to avoid the formation of anatase TiO2. The study of the porosity impact on the electrochemical performance was carried out on untemplated, mesoporous and macroporous LTO films. These films were prepared based on their preformed nanocrystals under the assistance of soft or hard templates. Macroporous LTO films exhibit the best rate capacity (150 mAh/g at 160C) compared with dense and mesoporous films. Mesoporous LTO nanofibers can be successfully prepared via electrospinning and it can deliver a specific capacity as high as 145 mAh/g at 10C.
Verknüpfung zu Publikationen oder weiteren Datensätzen
