Bu tez çalışmasında lityum iyon piller için LiFePO4 (LFP) esaslı katotlar hammadde olarak düşük maliyetli demir nitrat içeren bileşikler ile baskı tekniğine uygun reolojik özelliklere sahip mürekkepler şeklinde sentezlenerek LFP/C/rGO elde edilmiştir. LFP esaslı pillerin mükemmel termal stabilitesi, çevre dostu olması ve yaygın kullanılabilirliği gibi avantajlara sahip olduğundan içerdikleri pozitif elektrotlar tercih edilen bir katot malzemesi olarak kabul edilmektedir. Fakat LFP pillerin doğası gereği zayıf elektriksel iletkenliği olduğundan karbon kaplama yöntemi uygulanmıştır. Dolayısıyla bu tez çalışmasında özgün olarak LFP'nin elektrokimyasal davranışını iyileştirmek ve iletkenliğine katkı sağlamak için karbon kaynağı olarak sükroz kullanılmıştır. Solüsyon yanma senteziyle farklı yakıt oranları denenerek önce LFP tozları üretilmiş ve üretilen bu tozlar yapısal ve morfolojik karakterizasyon yöntemlerine tabi tutularak faz yapısındaki ortaya çıkan sonuçlar sunulmuştur. LFP tozu sentezleme esnasında yakıt oranına göre gerçekleşen reaksiyon, giren yanma ürünleri az olduğu durumda yetersiz yanma oluşmuş fakat fazla olduğu durumda da reaksiyona daha fazla ısı girdisi sağladığından demirin de oksitlenmesine sebebiyet vermiştir. Denenen bu yakıt karışımlarından aralarında en iyi olan ürünler/yakıt oranı seçilerek (4:1) çevrim sonucu da gösterilmiştir. Bu sonuçlar doğrultusunda pil fonksiyonu her ne kadar iyi olsa da deşarj kapasitesi 170 mAh g-1 teorik kapasitesinden hala uzakta kalmaktadır. İhtiyaç olan bu noktada LFP tozlarına karbon kaplayarak iletkenliğinin arttırılması için farklı oranlarda karışımlar hazırlayarak parametre çalışması yapılmıştır. Temelinde solüsyon sentezinin kullanılmasının asıl sebebi de budur. Çünkü diğer yöntemlere nazaran üretimi daha kolay ve daha az hammadde kullanılarak parametrik olarak sonuçları diğer sentez yöntemlerine göre çabuk şekilde almak mümkündür. En iyi yakıt oranında sonuç alınan LFP tozlarını aynı hacimdeki ayrı krozelerde farklı sükroz karışımlı tozları hazırlanmış ve kalsinasyona tabi tutularak karbon kaplı LiFePO4/C yapısına sahip kompozit tozlar üretilmiştir. Karbon kaplı LFP tozlarının iletkenlik ve çevrim sonuçları EIS sonuçlarıyla da desteklenerek iletkenliğin arttığı, iç direncin düştüğü desteklenmiştir. Bu sayede hem LFP/C kompoziti elde edilmiş hemde deşarj kapasitesi gibi özelliklerin önemli ölçüde iyileştirilebildiği gösterilmiştir. Hazırlanan LFP/C yapısındaki iletken ağa ek bir karbon katkısıyla kapasitenin arttırılabilmesi ve döngü performansının iyileştirilebilmesi mümkün olabilmektedir. Bunun için LFP/C kompozitine indirgenmiş grafen oksit (rGO) eklemek iletken ağa makul bir katkı yapmaktadır. rGO'nun elde edilme süreci, modifiye edilmiş Hummers yöntemi ile başlar. Ayrıca rGO'nun karbon siyahı 'dan (CB) daha yüksek bir iletkenliği vardır. Elde edilen LFP/C/rGO kompoziti bu sayede hem iletkenliği optimize edilebildiği hemde deşarj kapasitesi, döngü performansı gibi özelliklerinin iyileştirilebildiği gösterilmiştir. Son aşamada ise optimize edilmiş LFP/C/rGO kompoziti ile elektrot üretimi için baskılanmaya hazır mürekkepler sentezlenmiştir. Baskı elektroniği, elektronik devrelerin basılmasını içeren bir teknolojidir. Sensörler, şeffaf elektrotlar ve ışık yayan cihazlar gibi bileşenleri farklı şekillerde standart veya özel baskı işlemleri kullanılarak kâğıt, plastik ve tekstil gibi yüzeyler ve ekipmanlara baskılanabilmektedir. Dolayısıyla farklı oranlardaki hazırlanan mürekkep formlarının simülasyon analizleri ve testleri yapılarak bu çalışmada kullanılan serigrafi yöntemiyle baskılanmaya uygun reolojide mürekkepler hazırlanmıştır. Elde edilen sonuçlar literatüre katkı sağlayarak serigrafi yöntemiyle üretilecek pil elektrotları basma çalışmalarına da ışık tutması hedeflenmektedir.
In this thesis, LiFePO4 (LFP) based cathodes for lithium-ion batteries were synthesized using low-cost iron nitrate-containing compounds as raw materials, and LFP/C/rGO composites were produced in the form of inks with rheological properties suitable for printing techniques. LFP-based batteries are considered a preferred choice due to their excellent thermal stability, environmental friendliness, and widespread availability. Consequently, the positive electrodes they contain are accepted as promising cathode materials. However, the naturally poor electrical conductivity of LFP limits its electrochemical performance. Thus, a carbon coating method was applied to improve its electrochemical properties. In this study, sucrose was uniquely employed as a carbon source to enhance the electrochemical behavior and electrical conductivity of LFP. Using solution combustion synthesis, LFP powders were initially produced with varying fuel ratios, and the resultant powders were subjected to structural and morphological characterization techniques to present the outcomes of the phase structures. During the LFP powder synthesis process, when the fuel ratio was too low, insufficient combustion occurred, while an excess of fuel resulted in additional heat input, leading to the oxidation of iron. Based on the outcomes, the optimal fuel ratio (4:1) was determined, and its performance in cycling tests was showcased. Despite the improved performance, the discharge capacity remained below the theoretical value of 170 mAh g-1. To address this limitation, the electrical conductivity of LFP powders was further enhanced by carbon coating, using different ratios to prepare various mixtures, followed by parametric studies. The primary reason for selecting the solution synthesis method was its relatively simpler production process and faster results with minimal raw material usage compared to other methods, enabling rapid optimization of synthesis parameters. In this study, LFP powders produced using the best fuel ratio were coated with varying sucrose-based mixtures in separate crucibles of the same volume, and calcination was carried out to produce composite powders with a LiFePO4/C structure. The conductivity and cycling performance of the carbon-coated LFP powders were analyzed, and the results, supported by EIS (Electrochemical Impedance Spectroscopy), confirmed that the conductivity increased, and internal resistance decreased. As a result, both the LFP/C composite was successfully obtained, and essential properties such as discharge capacity were significantly improved. By incorporating an additional carbon network into the prepared LFP/C structure, it was possible to further enhance the capacity and improve the cycle performance of the material. The addition of reduced graphene oxide (rGO) to the LFP/C composite played a significant role in improving the conductive network. The process of obtaining rGO begins with a modified Hummers method. Moreover, rGO offers superior conductivity compared to conventional carbon black (CB). The LFP/C/rGO composite thus demonstrated optimized conductivity, along with enhancements in discharge capacity and cycle performance. In the final stage, the optimized LFP/C/rGO composite was used to formulate inks suitable for printing, specifically tailored for electrode production. Printed electronics is a technology involving the fabrication of electronic circuits. Using standard or customized printing techniques, sensors, transparent electrodes, and light-emitting devices can be printed onto various substrates such as paper, plastic, and textiles. Therefore, the rheological properties of ink formulations prepared in different ratios were simulated and tested to develop screen-printing-ready inks in this study. The inks were evaluated for their suitability in printing processes, and the outcomes are expected to contribute significantly to the literature by providing valuable insights into the development of printable battery electrodes using the screen-printing method. To elaborate further, this thesis highlights several critical steps and considerations that are essential for successful LFP-based cathode synthesis and their adaptation to printable technologies. Initially, the study focused on achieving the optimal phase formation by varying the ratios of fuel during solution combustion synthesis. This approach enabled a thorough understanding of the impact of reaction conditions on the purity and morphology of LFP powders. Structural analysis, including X-ray diffraction (XRD), confirmed the formation of a pure LFP phase with well-defined crystal structures. Scanning electron microscopy (SEM) provided insights into the surface morphology, revealing the uniformity of the carbon coating and the integration of rGO into the final composite. Energy-dispersive X-ray spectroscopy (EDS) further validated the successful incorporation of all the desired elements into the composite material. The electrochemical performance of the synthesized materials was evaluated using techniques such as cyclic voltammetry (CV) and galvanostatic charge-discharge tests. The LFP/C/rGO composite demonstrated a substantial increase in specific capacity and rate capability compared to the uncoated LFP. The initial discharge capacity, which was below expectations in pure LFP, showed a marked improvement upon carbon coating and the addition of rGO, achieving values much closer to the theoretical maximum. This enhancement was attributed to the synergistic effect of the rGO addition, which provided a more extensive conductive network, facilitating faster electron transport and improving the overall electrode kinetics. The stability of the material over extended cycling was also a key outcome, showing minimal capacity degradation, which is crucial for practical applications. Furthermore, the formulated LFP/C/rGO inks were successfully tested in screen-printing processes, which confirmed their suitability for large-scale manufacturing of lithium-ion batteries. The printed films exhibited excellent adhesion to substrates, uniform thickness, and retained the bulk material's electrochemical properties. The development of printable inks not only holds promise for conventional battery manufacturing but also opens new avenues for the integration of these materials into flexible and wearable electronic devices, thereby broadening the scope of their application. The advancements presented in this thesis lay a strong foundation for future research aimed at optimizing ink formulations and exploring alternative printing methods, such as inkjet or gravure printing, to further enhance the resolution and performance of printed electrodes. In conclusion, the synthesis and application of LFP/C/rGO composites for lithium-ion batteries represent a significant leap forward in the field of energy storage. The strategies developed in this study address some of the critical limitations associated with LFP materials, particularly their poor conductivity. By leveraging carbon coating and the introduction of rGO, this research successfully enhanced the electrochemical properties of LFP, making it a viable candidate for use in advanced battery technologies. Future research may focus on scaling up the production process and investigating the integration of these advanced cathodes into full-cell configurations to evaluate their performance in practical scenarios. Ultimately, the findings of this thesis contribute to the broader effort of developing more efficient, reliable, and sustainable energy storage systems, meeting the growing demands of modern society.
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