Browsing by Author "Shin, Ji-Yong"

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    Effects of ionic and electronic charge carriers in nanostructured TiO2 on lithium storage
    (2012) Shin, Ji-Yong; Maier, Joachim (Prof. Dr.)
    As far as efficient energy storage device in electric vehicles or smart grids are concerned lithium batteries came to the fore due to their high energy and power densities. However, it is still challenging to develop reasonably high rate and energy density batteries because of the limitation in intrinsic electrochemical properties of electrode materials. For electrode materials in Li-batteries, excellent performance can be only achieved by short diffusion length (L) and high chemical diffusivity (D) of Li. In recent years, therefore many efforts to control the time dependence have been conducted in the field of Li-batteries. As operation of Li-batteries involves both ionic and electronic charge carriers, fundamental understanding of the role of each charge carrier is of great importance. Using titanium dioxide (TiO2) as a model material, the main aim of this study therefore is to primarily understand effects of size and morphology and defect chemistry on the overall lithium transport and storage properties and furthermore to propose a strategy to design high performance electrode materials from the standpoint of defect chemistry. In the first part, effects of size and morphology of titania (variation of transport length of mainly ionic charge carriers (i.e. Li+)) on the Li storage are studied. Hierarchical nanoporous TiO2 particles with a large interfacial proportion were successfully prepared by hydrolysis and its electrochemical performance investigated. Due to the significantly shortened transport length of Li+ ion by nano-structuring, the materials showed excellent Li storage performance (showing both enhanced bulk and interfacial Li storage contributions) compared to nonporous materials. Especially, the nature of interfacial storage behavior is systematically discussed and its great importance for Li-batteries emphasized. The second part of the thesis mainly focuses on effects of charge carriers concentration (variation of electronic charge carrier concentration) on the Li storage properties. To increase electronic charge carrier concentrations in nanostructured TiO2, two different strategies are applied: (1) formation of frozen-in native defects (oxygen-deficient TiO2-d nanoparticles) and (2) homogeneous n-type doping by extrinsic defects (Nb5+-substituted mesoporous TiO2 nanoparticles). The results are discussed on the basis of multiple experimental techniques as well as a theoretical defect chemical analysis. These different strategies not only allow to develop relevant adjusting screws for optimizing the performance, they also allow a deeper understanding of ionic and electronic charge carriers in the context of Li-batteries.