Tuning geometrical parameters of lithium-mixed transition-metal oxide (LiTM) cathode materials is a promising strategy for resource-efficient design of high-performance Li-ion batteries. In this paper, we demonstrate that simple and facile geometrical tailoring of the secondary microstructure of LiTM cathode materials without complex chemical modification or heterostructure engineering can significantly improve overall electrochemical performance of the active cathode materials. An optimized LiTM with a bimodal size distribution of primary particles inside the secondary particles exhibits a 53.8% increase in capacity at a high discharge rate (10 C) compared to a commercially available reference and comparable rate capability after 100 charge/discharge cycles. The key concept of this approach is to maximize the beneficial effects arising from the controlled sizes of primary particles. Multimodal/multiscale microscopic characterizations based on electron tomography and scanning transmission electron microscopy, combined with electron energy-loss spectroscopy and energy-dispersive X-ray spectroscopy from the atomic level to the microscale level, were employed to elucidate structural origins of enhanced battery performance. This study paves the way for the resource-efficient microstructure design of LiTM cathode materials to maximize capacity and stability via simple adjustment of processing conditions, which is advantageous for mass-production applications. KEYWORDS: Li-ion battery, lithium mixed transition-metal oxides, core shell, Li(Ni x Co y Mn z )O 2 , STEM-EDX/EELS, electron tomography
For the removal of pollutants, a modified TiO2 photocatalyst is attracting attention. Fe-doped TiO2 nanofibers were prepared through a combination of electrospinning and calcination. Morphological characterization of the sample was conducted using field-emission scanning electron and transmission electron microscopy. The crystal structure of each sample was analyzed using high-resolution transmission electron microscopy, selected area electron diffraction, and Fast Fourier Transform imaging. The average diameter of the Fe-doped TiO2 nanofibers was measured to be 161.5 nm and that of the pure TiO2 nanofibers was 181.5 nm. The crystal phase when heat treated at 350 °C was anatase for TiO2 nanofibers and rutile for Fe-doped TiO2 nanofibers. The crystal phase of the TiO2 matrix was easily transitioned to rutile by Fe-doping. The photocatalytic performance of each sample was compared via the photodegradation of methylene blue and acid orange 7 under ultraviolet and visible light irradiation. In the Fe-doped TiO2 nanofibers, photodegradation rates of 38.3% and 27.9% were measured under UV irradiation and visible light, respectively. Although other catalysts were not activated, the photodegradation rate in the Fe-doped TiO2 nanofibers was 9.6% using acid orange 7 and visible light. For improved photocatalytic activity, it is necessary to study the concentration control of the Fe dopant.
The superior chemical and electrical properties of TiO2 are considered to be suitable material for various applications, such as photoelectrodes, photocatalysts, and semiconductor gas sensors; however, it is difficult to commercialize the applications due to their low photoelectric conversion efficiency. Various solutions have been suggested and among them, the increase of active sites through surface modification is one of the most studied methods. A porous nanostructure with a large specific surface area is an attractive solution to increasing active sites, and in the electrospinning process, mesoporous nanofibers can be obtained by controlling the composition of the precursor solution. This study successfully carried out surface modification of TiO2 nanofibers by mixing polyvinylpyrrolidone with different molecular weights and using diisopropyl azodicarboxylate (DIPA). The morphology and crystallographic properties of the TiO2 samples were analyzed using a field emission electron microscope and X-ray diffraction method. The specific surface area and pore properties of the nanofiber samples were compared using the Brunauer-Emmett-Teller method. The TiO2 nanofibers fabricated by the precursor with K-30 polyvinyl pyrrolidone and diisopropyl azodicarboxylate were more porous than the TiO2 nanofibers without them. The modified nanofibers with K-30 and DIPA had a photocatalytic efficiency of 150% compared to TiO2 nanofibers. Their X-ray diffraction patterns revealed anatase peaks. The average crystallite size of the modified nanofibers was calculated to be 6.27–9.27 nm, and the specific surface area was 23.5–27.4 m2/g, which was more than 150% larger than the 17.2 m2/g of ordinary TiO2 nanofibers.
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