PhD.Dissertation Defense:Vahid Charkhesht
ADVANCED MATERIALS FOR HIGH ENERGY DENSITY ELECTRODES FOR LI-ION BATTERIES
Vahid Charkhesht
Materials Science and Nanoengineering, PhD Dissertation, 2024
Thesis Jury
Prof. Selmiye Alkan Gürsel (Thesis Advisor), Prof. Emre Erdem, Assoc. Prof. Mustafa Kemal Bayazit, Prof. Gülfeza Kardaş, Prof. Özgül Keleş.
Asst. Prof. Alp Yürüm ( Co-Advisor)
Date & Time: 23rd July, 2024 – 15:30
Place: FENS G025
Keywords : Lithium-ion Batteries, Electrode Materials, TiO2, Electrospinning, NCA, Hydrothermal Synthesis
Abstract
Improving the energy density of existing lithium batteries requires further development of electrode fabrication techniques as well as synthesis routes for new active materials. In this sense, focusing on the further development of cathodes and anodes will be crucial. One of the interesting anode materials is TiO2, thanks to its safety and economical advantages. However, in its bulk form, this material does not show promising electrochemical properties. Therefore, creating nanostructures from it will improve its electrochemical properties. By obtaining a nanotubular structure of this material using hydrothermal synthesis, a hollow structure capable of storing greater amounts of Li-ion can be achieved. Subsequently, further thermal treatment removes the existing water in the structure, resulting in a denser structure. However, the parameters involved in the thermal treatment of this material have not been optimized yet. In the second chapter of this thesis, after optimization of heat treatment temperature and duration, the 10 hours at 500 °C shows the highest electrochemical performance. Further improvement of the energy density of this material is possible by reducing the weight of the whole cell package. Electrospinning can fabricate free-standing electrodes by removing the Cu substrate in the anode. Therefore, the optimized heat-treated particles were taken and homogeneously embedded in the fibrous electrode structure. The resulting electrode improves the energy density by 14% compared to the similar cast electrode.
Another important factor in the electrochemical performance of a cell is the surface chemistry of the active material. In the third chapter of this thesis, to understand the effect of facets on the electrochemical performance of TiO2, two hydrothermal synthesis methods were utilized to develop different facets on the material. Three types of single crystals (with cubic, octahedral, and truncated morphologies) obtaining various facets with different energies were synthesized and electrochemically investigated. The effect of high-energy facets was highlighted on the mass transfer, interface formation, and cycling performance. High-energy facets facilitate the diffusion of Li-ion, resulting in high diffusivity coefficients. Cyclic voltammetry shows different electrochemical potentials for Li-ion insertion for different facets. For capacity retention, the high-energy facets show superior performance comparatively. Therefore, developing single crystals with high-surface energy will help improve electrochemical properties.
Finally, in the last chapter, the energy density of the cathode material was targeted. To do so, four different synthesis routes were employed to fabricate Ni-rich cathode materials (NCA). The Pechini method and colloidal synthesis method have been used with and without templates. After achieving a homogeneous elemental distribution in the material, the active materials were embedded into fibrous matrices to fabricate electrospun electrodes. The carbon-black supported NCA synthesized with colloidal synthesis performed well compared to the other synthesized active materials. The freestanding NCA electrode can reach the capacities of state-of-the-art active materials with negligible capacity fade during cycling. Therefore, by removing the Al substrate, not only was the energy density improved, but also the cost of substrates for the electrodes can be reduced.