Graphene-based three-dimensional electrode material applications for energy storage

The importance of energy storage is increasing day by day due to developing technology and increasing population. Various types of research are planned in many areas, such as current generation electrochemical energy storage in the world, increasing environmental concerns and life cycle assessment. For the energy transition to a sustainable technology to be successful, significant progress in storage technology is essential. Lithium-ion batteries have shown unlimited potential for electrochemical energy storage and conversion, which is widely used in electric vehicles and energy storage. Li-ion batteries continue to grow rapidly in sales, thanks to their high energy density and long life compared to other rechargeable battery systems. To date, various cathode materials have received wide attention due to their structural evolution and electrochemical complexity in lithium-ion batteries. Among them, high nickel cathode materials are recognized as a promising candidates for producing batteries with high energy density and high operating voltage. Layered metal oxides LiNixCoyMnzO2 (NMC) and LiNi0.8Co0.15Al0.05O2 (NCA) with high nickel content are attracting more attention than traditional LiNiO2 due to their improved structural stability and electrochemical properties. There is a concern that the use of cobalt for electric cars could lead to depletion of its reserves. Recently, it has been observed that cobalt prices have been greatly affected by unexpected global developments. Leading battery manufacturers are trying to further reduce their production costs by eliminating the dependence on cobalt in their cathode materials. Therefore, alternative materials that can be used instead of cobalt are sought. High nickel content up to 80% by weight will lead to better structural stability, greater safety and better cycle life as it provides high capacity and replaces some of the nickel with small amounts of aluminum and manganese of similar ionic radius. It has been stated that Aluminum increases the structural stability and various combinations of Nickel, Manganese and Cobalt can be used to simultaneously optimize the thermal and electrochemical properties. In addition, Manganese is abundant compared to Cobalt and does not harm the environment. Therefore, the trend is to gradually increase the Nickel content in the layered structures and destroy the Cobalt content, so that the capacity can be increased while the cost is reduced. Therefore, a natural combination of Manganese and Aluminum in high Nickel layered oxides is considered to be a good compound. Here, a new class of Nickel-rich, Cobalt-free Li-ion battery cathode material with the general formula LiNixMnyAlzO2 (x ≥ 0,8, x + y + z = 1) is presented for the development of next-generation Li-ion batteries. These new cobalt-free LiNixMnyAlzO2 cathodes reduce cost and increase capacity while maintaining the structural stability and safety advantages of manganese and aluminum and the high capacity offered by nickel. In doing so, the advantages of NCA and NMC cathodes can be transferred to the LiNixMnyAlzO2 grade without resorting to cobalt. The capacities and voltage of NMA materials are similar to NMC and NCA, but the material cost will be significantly lower due to replacing expensive cobalt with cheap manganese and aluminum. It attracts particular attention due to its high nano-sized layered Nickel ratio, excellent speed performance, high potential, low cost, low toxicity and high safety. When comparing micro-sized and nano-sized layered oxide materials, the reduction in particle size leads to a significant increase in the deintercalation/interference ratio of Lithium ions, as the diffusion distances of Lithium ions and electrons within the particles are shortened. The reduction in particle size also results in a larger contact area with the electrolyte and a larger surface area, allowing a greater Li ions flow across the interface. In addition, given the poor conductivity of Nickel-rich layered cathode materials and the tendency of nanoparticles to aggregate, conductive substrates are highly needed to promote charge transfer and support active materials. Graphene aerogel (GA) has a three-dimensional (3D) interconnected framework of interesting properties of graphene sheets, including its macroporous architecture, high electrical conductivity, and good mechanical stability. The interlocking three-dimensional macroporous conductor structure can facilitate rapid diffusion of both Li ions and electrons, improve electrolyte storage, enrich the transport pathway of Li+, and increase electrode capacity and reversibility. Moreover, the three-dimensional conductive network intertwined with graphene greatly accelerates electron transfer, which is beneficial to further increase the speed capacity of lithium-ion batteries. In this thesis, LiNi0.8Mn0.15Al0.05O2 (NMA), which does not contain cobalt and has high nickel content, together with LiNi0.8Mn0.1Co0.1O2 (NMC-811) and NCA with high Nickel content, was synthesized by the sol-gel method. Composite structures (NMC-811/GA, NCA/GA, NMA/GA) were produced by self-forming and L-ascorbic acid reduction process with 3D GA in order to increase the conductivity, practical energy density and performance capability of the produced layered structures with high Nickel content. With an easy freeze-drying approach, the active materials nanoparticles were uniformly dispersed on the graphene sheets and significantly improved the structure and conductivity thanks to GA, creating a strong and conductive framework. Thanks to its 3D structure, it increases the large specific surface of the composite structure and widens the contact surface of the cathode and the electrolyte. Characterization data of cathodes with techniques such as X-ray diffraction (XRD), Raman spectroscopy, Fourier transform infrared spectra (FT-IR) and X-ray photoelectron spectroscopy (XPS) are discussed. The field clearly showed that it is homogeneously distributed between and among the GAs by emission scanning electron microscopy (FE-SEM). In addition, composite structures were investigated using transmission electron microscopy (TEM) to determine the dispersion mechanisms. Electrochemical loop performance testing was evaluated by rate capacitance, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). According to the results of the electrochemical tests, GA formed a conductive network with its porous structure, formed a protective layer on the surface, prevented side reactions between the cathode and the electrolyte, decreased the impedance of the cathode and increased the redox kinetics. In addition, at the end of the 50th, 250th and 500th cycles, the changes in the structure of the composites with the cathode were examined by XRD, FE-SEM and Raman analyzes. According to the results, it was observed that the 3D porous structure of the produced composites facilitated the mobility of Li+ ions and the excellent electrochemical performance of the cathodes was improved due to the increased defects as well as the electrical conductivity of the GA. The electrochemical performance of the cathodes was evaluated by galvanostatic charge-discharge measurement between 2,5 V and 4,5 V for NMC-811 and NMA, and 2,8V - 4,3V for NCA at room temperature using a computer-controlled battery tester. The galvanostatic charge/discharge curves of the nNMC-811, NCA, NMA, nNMC-811/GA, NCA/GA and NMA cathods electrochemical characterization was first performed in coin half cells paired with Li-metal anodes. Electrochemical tests were carried out at C/2 constant current density. While the initial capacity of NMC-811, NCA, NMA, NMC-811/GA, NCA/GA, NMA/GA were 186,72 mAh g-1, 182,84 mAh g-1, 174,8 mAh g-1,191,58 mAh g-1,188.88 mAh g-1, 183,8 mAh g-1 respectivley, they decreased to 155,38 mAh g-1, 148,8 mAh g-1, 143,58 mAh g-1, 172,82 mAh g-1 , 168,17 mAh g-1 and 165,08 mAh g-1 after 500th cycle. Capacity retention were measured as %83,21%, %82,38, %82,13, 90,20%, 89,42% and 89,81% for NMC-811, NCA and NMA nanoparticles, NMC-811/GA, NCA/GA and NMA/GA composite electrodes after 500th cycle, respectively. Silicon nanoparticles used to form full cells were produced by magnesiothermic reduction methods. It was then obtained in three-dimensional GA form prepared by a simple one-step freeze-drying process using L-ascorbic acid. Silicon/graphene aerogel (Si/GA) nanocomposite was investigated by XRD, FE-SEM, TEM and XPS. Si/GA nanocomposite shows a superior capacity of 550 mAh g-1 after 500th cycle. Si/GA anodes showed improvement in cycle stability compared to pure Si.