Design and production of biomass-derived anode active materials for lithium ion batteries

Abstract

As technology advances, societies have seen a rise in energy demands, leading to a heavy reliance on fossil fuels to meet this growing need. However, the carbon dioxide and greenhouse gases released during these processes have contributed to global warming and climate change, becoming increasingly significant issues. According to the 2023 report by the U.S. Environmental Protection Agency, the largest portion of greenhouse gas emissions—28%—comes from gasoline and diesel transportation. In response, many countries have implemented policies to promote the widespread adoption of electric vehicles (EVs) as a means to mitigate the climate challenges posed by greenhouse gas emissions. The two most critical factors influencing the adoption of electric vehicles are the range per charge and the charging time, both of which are directly tied to the lithium-ion batteries used in these vehicles. To support the sustainable growth of electric vehicles, it is also essential that the components of lithium-ion batteries are sourced sustainably. Graphite is the predominant anode material used in commercial lithium-ion batteries. With its high electrical conductivity, graphite is valuable across various industries and is listed among the Critical Raw Materials by the European Union. However, as the demand for electric vehicles rises, experts predict a shortage of graphite supply beginning in 2026. Therefore, finding environmentally friendly and sustainable alternatives that can perform similarly to graphite is becoming increasingly crucial. This thesis aims design and production of an anode active material for lithium-ion batteries using rice husk, an organic waste. Rice husk is a promising alternative to graphite due to its high silica content and worldwide annual production. However, the electrochemical performance of silica-based materials is often limited by their poor electrical conductivity. To enhance their performance, strategies such as reducing particle size to shorten the lithium diffusion path or creating composites with materials that have higher electrical conductivity can be employed. In this thesis, submicron silica particles and SiOx/C nanocomposite biochars were synthesized using wet chemical methods and fast pyrolysis by induction heating. High During pyrolysis the organic components in rice husk decomposes and produces reducing gases such as CO, CH4, and H2, which partially reduce the silica to SiOx. The high heating rates promote the formation of these reducing gases. For the first time in the literature, rice husk-derived anode active materials are synthesized using fast pyrolysis with induction heating. This study is significant because it has the potential for scalability and commercial application. In the first part of the experimental studies, submicron silica particles were produced using leaching and precipitation methods using rice husk. The effects of pre-calcination, the solid-to-liquid ratio, and the addition of ethanol as a co-solvent on the silica particle sizes were examined. In this process, both rice husk and rice husk ash were dissolved in a sodium hydroxide solution to obtain a sodium silicate solution, which is commonly used in silica production. By utilizing organic waste to derive the sodium silicate solution, the process aimed to reduce the overall carbon footprint. In the next stage of production, sulphuric acid was added to the sodium silicate solution to precipitate silica particles at a pH of 7. The resulting samples were characterized based on their structural, morphological, and electrochemical properties. FTIR and XRD analyses confirmed the existence of Si-O bonds and the amorphous silica structure. SEM images revealed that the particles were irregularly shaped and agglomerated. The sample that underwent calcination before leaching, with a solid-to-liquid ratio of 1/20 and no ethanol added, exhibited the smallest particle size. It was noted that low solid-to-liquid ratio leads to low SiO2/Na2O ratio, which is the key parameter to control particle growth. Moreover, adding ethanol increased particle size and shifted the distribution from monodisperse to polydisperse. This shift was attributed to the immiscibility of sodium silicate in ethanol, which led to local supersaturations. The sample with the smallest particle size delivered discharge capacities of 503, 167, and 145 mAh/g at the 1st, 100th, and 200th cycles, respectively. The improvement in electrochemical performance was attributed to the shorter lithium diffusion path. Additionally, the capacity drop observed after the 1st cycle in all samples is attributed to the formation of the solid electrolyte interface (SEI) layer and irreversible lithium silicates. In the second part of the experimental studies, SiOx/C nanocomposite biochar samples were synthesized using rice husk via induction-heated fast pyrolysis method with the aim of preserving the maximum carbon content to improve electrical conductivity. Three different pyrolysis temperatures (specifically, 700-750-800 °C) and two pyrolysis atmospheres (argon and argon-hydrogen mixture) were tested. Additionally, the sample with the highest carbon content was ball milled to reduce its particle size and achieve a more homogeneous elemental distribution. FTIR analysis confirmed the successful synthesis of silica, revealing the presence of Si-O-C, Si-O-Si, and Si-O bonds. XRD patterns showed that samples pyrolyzed at temperatures below 800 °C exhibited an amorphous phase, while above this temperature, crystallization began, and the cristobalite phase was formed. SEM images indicated that the samples had irregular morphologies; however, milling after pyrolysis resulted in smaller, more spherical particles. Raman spectroscopy confirmed the presence of both defective and graphitic carbon structures. As the pyrolysis temperature increased, the ratio of graphitization also increased, and milling further increased the ID/IG ratio by introducing additional defects into the structure. XPS analysis confirmed the formation of SiOx, displaying peaks of Si+2, Si+3, and Si+4 in the Si 2p spectrum. TEM images showed that silicon-rich particles were uniformly distributed within the carbon matrix. The pyrolysis conducted at 700 °C in the argon atmosphere resulted in the highest carbon content as 41.3 wt.% and delivered the best electrochemical performance as 1366, 340, and 275 mAh/g in the 1st, 100th, and 200th cycles, respectively. An increase in carbon content was associated with improved electrochemical performance. However, all samples exhibited a drop in capacity after the first cycle, attributed to the formation of the solid electrolyte interphase (SEI) layer and irreversible lithium silicates.