리튬이온전지 열폭주 메커니즘 분석을 통한 산소 방출 억제형 고니켈계 양극 설계 = Design of Oxygen-Release-Minimized Ni-Rich Cathodes Based on Thermal Runaway Mechanism Analysis in Lithium-Ion Batteries

다국어 초록 (Multilingual Abstract)

Ni-rich layered oxide cathodes remain indispensable to high-energy lithium-ion batteries, yet their intrinsic thermal instability continues to pose critical safety challenges in electric vehicle (EV) and energy storage system (ESS) applications. Despite extensive efforts to enhance surface robustness and interfacial stability, the fundamental origin of thermal runaway in these materials’ oxygen activation at deep delithiation has not been fully clarified or quantitatively validated at the pouch-cell scale. This dissertation establishes an oxygen-centered degradation framework that connects atomic-scale electronic instability to macroscopic fire behavior and proposes practical materials strategies to suppress oxygen-induced thermal hazards.
Multiscale characterization using (S)TEM-EELS, XPS, and TG-DSC reveals that oxygen-hole formation at high states of charge destabilizes the lattice, driving oxygen-vacancy generation, slab gliding, spinel/rock-salt reconstruction, and nano-void growth. These transformations accelerate electrolyte decomposition and produce oxygen-derived radicals that initiate rapid exothermic reactions. To validate this mechanism at the cell scale, a DI-water-accelerated pouch-cell platform was developed, enabling complete retention of evolved gases and highly reproducible quantification of gas composition, ignition onset, heating rate, and flame propagation (σ ≤ 0.9). Pristine Ni88 cells exhibited early voltage collapse, sharp dT/dt spikes, and high concentrations of CO2, CO, and unsaturated hydrocarbons (C2H4, C3H6), confirming that oxygen-activated radical cracking dominates thermal runaway chemistry.
Building on these insights, two oxygen-stabilizing cathode architectures were designed using LiFePO4 (LFP): a 1 wt.% surface-coated structure and a 5 wt.% blended composite. LFP coating mitigated surface oxygen activation, delayed ignition, and reduced gas reactivity, while LFP blending provided the strongest bulk-scale stabilization, suppressing void formation, reducing heating rates, and minimizing unsaturated hydrocarbon formation during fire exposure. Among all samples, LFP-blended cathodes exhibited the lowest gas evolution, the most saturated GC profiles, and the weakest flame propagation, demonstrating effective interruption of the oxygen-driven degradation chain.
Overall, this work establishes a mechanism-validated and industrially scalable design principle for oxygen-release-minimized Ni-rich cathodes. By integrating fundamental mechanistic analysis, reproducible safety evaluation, and practical materials engineering, this dissertation provides a coherent pathway toward safer high-energy lithium-ion batteries and offers valuable guidance for future development of Co-lean and Co-free Ni-rich chemistries for EV and ESS systems.

목차 (Table of Contents)

• PART I. LITERATURE REVIEW 1
• CHAPTER 1. Introduction 1
• 1.1 Overview of Lithium-Ion batteries' safety 1
• 1.2 Scientific Background of Ni-Rich Cathode 5
• 1.3 Limitations of Current Safety Strategies 10
• 1.4 Objectives of the Dissertation 14
• 1.5 Research Framework and Contributions 16
• CHAPTER 2. Fundamental Principles of Thermal Runaway in Lithium-Ion Batteries 18
• 2.1 Introduction 18
• 2.2 Thermodynamic and Kinetic Basis of Thermal Runaway 19
• 2.2.1 Coupling of temperature and reaction rate 19
• 2.2.2 Multi-stage nature of heat generation 20
• 2.3 Transition from Self-Heating to Thermal Runaway 20
• 2.4 Role of Oxygen Evolution in Triggering Runaway 21
• 2.4.1 Energetics of lattice oxygen release 21
• 2.4.2 Chemical amplification through radical pathways 22
• 2.4.3 Gas evolution as a runaway accelerator 22
• 2.5 Summary and Connection to Mechanistic Analysis 24
• 2.6 Structural Instability and Oxygen Mobility in Ni-Rich Layered Oxide Cathodes 24
• 2.7 Defect Chemistry and the Evolution of Oxygen-Vacancy Networks 26
• 2.8 Surface Reconstruction, Rock-Salt Formation, and Oxygen Activation 27
• 2.9 Comparative Instability of Ni-Rich Cathodes Relative to Other Chemistries 29
• 2.10 Multiscale Theoretical Models of Oxygen Instability in Ni-Rich Layered Oxides 30
• 2.11 Models of Thermal Runaway Propagation in Cells, Modules, and Packs 31
• 2.12 Remaining Research Gaps 33
• 2.13 Thermal Runaway Mechanisms in Ni-Rich Layered Oxides 34
• 2.14 Guiding Principles for Mechanistic Investigation and Material Design 36
• PART II. Objectives 38
• CHAPTER 3. Research Objectives 38
• 3.1 Overview of the Research Strategy 38
• 3.2 Rationale for an Oxygen-Centered Research Framework 39
• 3.3 Hypothesis Development 40
• 3.4 Material-Design Strategy for Oxygen-Stabilized Ni-Rich Layered Oxide Cathodes 41
• 3.4.1 LFP Coating 42
• 3.4.2 LFP Blending 44
• 3.4.3 Complementarity of Coating and Blending in a Unified Oxygen-Stabilization Strategy 45
• 3.5 Mechanistic Characterization for Oxygen-Driven Degradation 46
• 3.5.1 Atomic-Scale Electronic and Chemical Probes 46
• 3.5.2 Nanoscale Structure and Defect Evolution 47
• 3.5.3 Microstructural Evolution 48
• 3.5.4 Thermochemical Pathways 49
• 3.5.5 Interfacial Chemistry and Gas-Evolution Pathways 49
• 3.5.6 Integration of Multiscale Data into a Unified Mechanistic Interpretation 50
• 3.6 Development of the DI-Water Accelerated Gas and Fire Evaluation Method 50
• 3.7 Integrated Material–Mechanism–Safety Validation Loop 52
• PART III. Preparation of Experiments 54
• CHAPTER 4. Experimental Methodology and Technical Approaches 54
• 4.1 Overview 54
• 4.2 Materials 54
• 4.3 Electrode Preparation and Cell Fabrication 56
• 4.4 Structural and Chemical Characterization 59
• 4.5 Gas-Composition Analysis via DI-Water Accelerated Activation 61
• 4.6 Ignition and Fire-Propagation Evaluation 65
• PART IV. Experimental Results and Discussions 69
• CHAPTER 5. Mechanistic Analysis of Degradation and Oxygen Release in Ni-Rich Layered Oxide Cathodes 69
• 5.1 Introduction 69
• 5.2 Early Electronic Destabilization and Oxygen-Hole Formation 71
• 5.3 Structural Reconstruction: Evolution from Layered to Spinel and Rock-Salt Phases 75
• 5.4 Formation and Evolution of Nano-Void Networks in Ni-Rich Cathodes ·· 79
• 5.5 Thermochemistry of Oxygen Release and Its Consequences for Thermal Stability 86
• 5.6 Interfacial Degradation and Electrolyte Reactivity in Ni-Rich Layered Cathodes 90
• 5.7 Integrated Multiscale Degradation Model 97
• CHAPTER 6. Optimization of Gas and Fire Evaluation for Reliable Assessment of Oxygen-Controlled Ni-Rich Cathodes 102
• 6.1 Introduction 102
• 6.2 Rationale for Optimizing Pouch-Cell Gas Evaluation 104
• 6.3 Preliminary Evaluation of Conventional Gas-Inducing Triggers 107
• 6.3.1 High-voltage charging as a trigger 107
• 6.3.2 Thermal Soaking and High-Temperature Charging 110
• 6.3.3 Electrochemical/Electrical Abuse Conditions 112
• 6.3.4 Mechanistic Interpretation and Fundamental Limitations 113
• 6.4 Adoption of DI-Water Triggering as a Mechanistically Relevant and Controllable Accelerator 114
• 6.5 Optimization Strategy 119
• 6.6 Environmental Control, Experimental Apparatus, and Calibration 122
• 6.7 Validation of Reproducibility and Statistical Convergence 126
• 6.8 Mechanistic Interpretation of Gas Evolution in Pristine and Modified Cathodes 129
• 6.9 Fire Behavior and Ignition Characteristics Under Optimized Evaluation Conditions 133
• 6.10 Critical Evaluation and Practical Limitations of the Optimized Fire-Propagation Platform 136
• CHAPTER 7. Safety Performance Evaluation of LFP-Modified Ni-Rich Cathodes 142
• 7.1 Introduction 142
• 7.2 Electrochemical Performance of LFP-Modified Ni-Rich Cathodes 144
• 7.3 Gas Evolution Characteristics of LFP-Modified Ni-Rich Cathodes 154
• 7.4 Ignition and Thermal Propagation Behavior 158
• 7.5 Real-Time Gas Evolution During Ignition· 161
• 7.6 Gas Composition Analysis Using Gas Chromatography (GC) 163
• 7.7 Discussion & Summary 166
• PART V. Conclusions 168
• CHAPTER 8. Conclusions 168
• References 173
• 국문초록 185