https://scholarworks.unist.ac.kr/handle/201301/120 2026-04-19T15:43:36Z https://scholarworks.unist.ac.kr/handle/201301/91112 Title: Microstructure implementation and analysis of dry-processed thick electrodes for high-energy-density lithium-ion batteries Author(s): Oh, Hyeseong Abstract: The increasing demand for high-energy-density lithium-ion batteries has encouraged the adoption of thick electrodes that enhance cell energy density by reducing the ratio of inactive components. While this strategy effectively boosts energy density, thick electrodes inevitably exhibit longer lithium-ion transport paths that result in lower capacity at the same C-rate and degraded rate capability. Moreover, conventional wet-processed electrodes encounter additional challenges, including uneven distribution of carbon–binder domains, delamination, and increased internal resistance, which worsen with increasing electrode thickness. These limitations highlight the urgent need for alternative electrode manufacturing strategies that can simultaneously deliver high energy density, reliable structural integrity, and scalable processability. To overcome these issues, this study introduces a solvent-free dry electrode fabrication method that employs PTFE as a binder. The fabrication process is systematically divided into sequential unit operations—mixing, kneading, grinding, film formation, pressing, and lamination—each of which is tailored to a specific role by exploiting the unique fibrillation properties of PTFE. At each stage, intermediate product specifications are explicitly defined, ensuring reproducibility and process control. Through comprehensive physical and electrochemical analyses, we establish how PTFE content and fibrillation behavior dictate the microstructural evolution of dry electrodes. This study further demonstrates the decisive role of conductive agent selection in optimizing dry-processed electrodes for high-energy-density applications. By systematically applying various conductive agents, we show that porous spherical conductive agents are particularly effective in enhancing both electrical conductivity and lithium-ion diffusion, characteristics that are difficult to incorporate into conventional slurry-based wet processes. Electrode parameter analysis confirms that an optimized content of porous spherical conductive agents enables the fabrication of cathodes with areal capacities of 10–20 mAh/cm2 and a composite density of 3.65 g/cm3. These electrodes exhibit outstanding electrochemical performance, showing 88% capacity retention at 1 C and 80% retention after 418 cycles. Moreover, their scalability is validated through the successful demonstration of 1 Ah-class stacked pouch cells employing double- sided cathodes, fabricated entirely through sequential dry process equipment rather than manual assembly. In summary, this study presents a hierarchical framework that spans material-, electrode-, and cell- level perspectives to address the challenges of thick electrode fabrication. By integrating fibrillation- based PTFE binder engineering with conductive agent optimization, we establish fundamental design principles and practical strategies for dry-processed thick electrodes. The findings not only improve the microstructural and electrochemical properties of electrodes but also pave the way for environmentally friendly, scalable, and high-performance electrode manufacturing processes for next-generation lithium-ion batteries. Major: School of Energy and Chemical Engineering 2026-01-31T15:00:00Z https://scholarworks.unist.ac.kr/handle/201301/91111 Title: Interfacial Defect Passivation for High-Performance n–i–p Less Lead Perovskite Solar Cells Author(s): Song, Taehee Abstract: Metal halide perovskites are promising photovoltaic absorbers due to their strong optoelectronic properties and defect-tolerant electronic structure. However, n–i–p Sn–Pb perovskites exhibit pronounced interfacial instabilities that induce defect formation, hinder charge extraction, and accelerate Sn oxidation under thermal and environmental stress, ultimately limiting both efficiency and operational durability. To address these challenges, this dissertation proposes an interfacial engineering strategy comprising three complementary strategies: buried-interface stabilization, thermally resilient top surface passivation, and additive-driven surface modification for Sn–Pb and Sn-based perovskite systems. The first strategy focuses on controlling the buried SnO2/perovskite interface through thermally oriented fullerene passivation. Conventional phenyl-C61-butyric acid methyl ester (PCBM) layers suffer from disordered molecular packing and poor resistance to polar solvents, resulting in partial dissolution during perovskite deposition and the generation of structural voids. By annealing PCBM above its glass transition temperature, this work achieves a highly ordered PCBM interlayer that possesses enhanced solvent resistance and improved electronic percolation pathways. The oriented fullerene network effectively suppresses interfacial recombination by preventing direct chemical interactions between SnO2 and the perovskite precursor, thereby reducing oxygen-related defect formation at the buried interface. Devices incorporating high-temperature-annealed PCBM exhibit enlarged perovskite grain sizes, reduced trap-filled limit voltages, and more balanced charge transport, leading to improved operational reproducibility and significantly enhanced thermal durability. These results demonstrate that buried-interface stabilization is an essential prerequisite for overcoming the intrinsic instabilities of n–i–p Sn–Pb devices. The second strategy addresses the thermally vulnerable top perovskite surface by introducing an ionic-liquid PEAFo passivation layer. Unlike phenethylammonium iodide (PEAI), which induces quasi-2D surface crystallization, PEAFo forms a uniform, amorphous coordination layer without generating insulation 2D domains. The formate anion strongly binds to undercoordinated Sn2+ and Pb2+, effectively eliminating dominant trap states and markedly suppressing Sn oxidation that typically accelerates at elevated temperatures. Owing to its ionic-liquid characteristics, PEAFo undergoes mild softening and partial volatilization during thermal processing, enabling surface reconstruction that ensures complete coverage while preventing the accumulation of residues. The n–i–p Sn–Pb devices treated with PEAFo achieve a champion efficiency of 18.5% and retain 90% of their initial efficiency after 500 h at 85°C, establishing one of the highest intrinsic thermal-stability benchmarks reported for n–i–p Sn–Pb perovskite solar cells. The third strategy extends surface defect passivation to Sn-only perovskites by introducing a fluorinated benzylammonium iodide derivative (F3-BAI) that undergoes surface-selective crystallization due to its relatively low solubility. Rather than remaining dissolved in the precursor solution and distributing throughout the bulk, the additive rapidly reaches supersaturation during the antisolvent step and nucleates preferentially at the film surface. This crystallization sequence, combined with the tendency of the -CF3 group to orient toward the air interface, leads to its enrichment at the growing surface instead of incorporation into the bulk lattice. The resulting surface-localized molecular layer stabilizes surface Sn2+ against oxidation and provides hydrophobic protection against moisture. As a result, F3-BAI substantially improves resistance to moisture- and oxygen-induced degradation, demonstrating an interfacial passivation pathway that extends the overall interface engineering strategy to highly reactive Sn-based perovskites. Collectively, the three approaches presented in this dissertation demonstrate that interfacial defect passivation is a decisive factor governing efficiency, reproducibility, and long-term stability in Sn- containing perovskite solar cells. Buried-interface stabilization through thermal fullerene alignment, thermally resilient top surface reconstruction via ionic-liquid formate coordination, and solubility- governed surface passivation enabled by F3-BAI together establish a coherent and generalizable strategy for enhancing the durability of n–i–p Sn–Pb and Sn-only perovskite solar cells. These findings provide fundamental insights and practical design rules for advancing low-bandgap perovskite photovoltaics toward stable, high-performance device operation. Major: School of Energy and Chemical Engineering 2026-01-31T15:00:00Z https://scholarworks.unist.ac.kr/handle/201301/91110 Title: Crystallization Control via Alkylammonium Halides for Efficient and Stable Perovskite Solar Cells Author(s): Kim, Jongbeom Abstract: From fire and agriculture to the industrial revolution, electrical energy has been essential to human civilization. However, global demand has grown unsustainably with the rise of artificial intelligence, while conventional power generation continues to cause pollution and safety issues. To ensure sustain- ability, halide perovskites with superior optoelectronic properties offer great promise for harvesting light, heat, and mechanical energy. This research focuses on interfacial engineering and crystallization control using alkylammonium halides to improve the stability, efficiency, and multifunctionality of de- vices in both conventional (n–i–p) and inverted (p–i–n) perovskite solar cell architectures. 3D perovskite crystallization and surface morphology in alkylammonium chloride (RACl)-doped FAPbI₃ were controlled by tuning the alkyl chain length of RA⁺ to adjust volatility and binding affinity. This approach directed the δ-to-α phase transition, producing high-quality, low-defect film. Further- more, systematic correlation between chain length, volatility, and lattice distortion revealed the molec- ular-level origin of enhanced phase stability and charge transport. Top interface of 3D perovskite layer between perovskite absorbers and hole transport layers (HTLs) were designed using organic cation susceptibility analysis and machine learning–guided screening. By incorporating alkylammonium halides with different electron-donating properties, methoxy-phene- thylammonium iodide (M-PEAI) and cyclohexylammonium bromide (CHABr), strong intermolecular interactions spontaneously formed a thermally stable quasi-2D interlayer, enabling efficient charge ex- traction and surface passivation. Combining targeted molecular structures with Bayesian optimization further identified thermally stable interfacial materials with enhanced charge transport and defect pas- sivation. Buried interface induced by methylammonium chloride (MACl) was further controlled by incorporat- ing a low-solubility chlorine-containing additive to promote heterogeneous nucleation. This approach maximizes the chloride (Cl⁻) effect in the crystallization of perovskite layers during the transition from inorganic ETLs to organic HTLs in inverted PSCs (p–i–n). Additionally, the cooperative role of chloride and cation species was found to regulate interfacial crystallization, resulting in improved morphological uniformity and device reproducibility. This work demonstrates that controlling nucleation, crystallization, and interfacial chemistry is key to advancing stable, reliable, and multifunctional halide perovskite devices for next-generation energy harvesting and optoelectronics. Major: School of Energy and Chemical Engineering 2026-01-31T15:00:00Z https://scholarworks.unist.ac.kr/handle/201301/91109 Title: Self-Assembled Monolayers as Platforms for Analyzing Molecular Interaction Mechanisms Author(s): Kwon, Haeun Abstract: Understanding the mechanisms of intermolecular interactions is essential for designing and controlling the performance of biomaterials and functional polymers. In particular, adhesion and molecular behavior at interfaces directly influence biocompatibility, mechanical stability, and the properties of composite materials. Therefore, quantitatively elucidating interfacial interactions is indispensable for material design and applications. Self-assembled monolayers (SAMs) provide well- defined model surfaces that enable systematic investigation of interactions with specific functional groups. In this thesis, a Surface Forces Apparatus (SFA) was employed to quantitatively analyze the interactions between functionalized SAMs and polymers/biomolecules, with particular focus on the effects of pH, contact time, and salt concentration. Chapter 1 introduces the fundamental concepts, structures, and preparation methods of SAMs, along with the importance and potential applications of mixed SAMs. Chapter 2 examines the adhesion behavior of high molecular weight chitosan with four functionalized SAMs (COOH, NH₂, CH₃, Phenyl) under different pH, contact time, and salt conditions, confirming hydrophobic interactions as the dominant mechanism. Chapter 3 investigates the interactions of chitosan with mixed NH₂/CH₃ SAMs, demonstrating that adhesion is maximized at a specific mixing ratio and revealing synergistic effects at mixed interfaces. Chapter 4 explores the interactions between a DSPHTELP peptide derived from M13 bacteriophage and functionalized SAMs under varying pH and contact time, identifying hydrophobic and cation–π interactions as governing specific binding behavior. Chapter 5 analyzes the adhesion mechanisms of polyurethanes (PUs) synthesized from XDI and H6XDI with different polyols, elucidating the relative contributions of hydrogen bonding and hydrophobic interactions, and clarifying structure–property relationships. This thesis demonstrates the utility of SAM-based model interfaces in elucidating the interaction mechanisms of diverse polymers and biomolecules, including chitosan, polyurethanes, and peptides. The results highlight the possibility of precisely tuning intermolecular interactions under different environmental conditions, thereby providing fundamental insights and design strategies for advanced biomaterials and functional composites. Major: School of Energy and Chemical Engineering 2026-01-31T15:00:00Z