Focused ion beam-based structural characterization of energy materials

다국어 초록 (Multilingual Abstract)

Microstructural characterization of materials is a key factor for performance improvement in the development of energy conversion and storage technologies. In this study, methodologies for high-precision structural analysis of various energy materials using Focused Ion Beam-Scanning Electron Microscopy (FIB-SEM) technology were developed and applied.
Using advanced FIB-SEM 3D tomographic analysis, this study investigated the internal microstructure of secondary particles in high-nickel cathode material Li [Ni0.75Mn0.25]O2, with specific focus on pore nucleation and crack propagation mechanisms. During this structural analysis process, the effect of cathode-electrolyte interface (CEI) stabilization on the microstructural integrity of secondary particles was quantitatively evaluated. Experimental findings revealed that optimized CEI layers substantially decreased internal porosity of secondary particles from 8.43% to 6.14%, while simultaneously preventing structural deterioration and crack initiation throughout extended cycling operations, greatly enhancing the structural stability of the particles.
Additionally, a damage-minimizing Xe⁺ plasma FIB characterization technique was developed for precise structural analysis of fuel cell catalyst electrode-ionomer electrolyte interfaces. Compared to conventional Ga⁺ FIB, Xe⁺ PFIB more accurately preserved the microstructure of PFSA ionomer electrolytes, increasing the number of micropores from 81 to 1,380 and decreasing the average pore diameter from 0.324 μm to 0.04 μm, enabling precise characterization of the actual porous structure.
The FIB-SEM based structural analysis methodologies developed in this study provide an in-depth understanding of the microstructure-performance relationship of energy materials and are expected to serve as important guidelines for material design and optimization for next-generation energy devices.

국문 초록 (Abstract)

본 연구에서는 집속 이온 빔(Focused Ion Beam, FIB) 기법을 이용하여 에너지재료의 삼차원 미시구조를 체계적으로 규명하는 분석법을 개발하였다. 에너지 변환 및 저장 시스템의 효율성은 구성 소재의 내부 구조적 특성에 직접적으로 좌우되기때문에, 고정밀 구조 해석이 시스템 성능 개선의 결정적 인자로 작용한다.
연구의 첫 번째 부분에서는 Co-free Li[Ni0.75Mn0.25]O2 고니켈 양극재의 이차입자 내 기공 및 균열 형성을 FIB-SEM 3D 토모그래피를 통해 분석하였다. 전극전해질 계면(CEI) 안정화가 이차 입자의 구조적 안정성에 미치는 영향을 정량적으로 평가한 결과, 1 wt% BS 첨가제를 통한 CEI 안정화 처리는 100회 사이클링 후에도 이차 입자의 기공도를 6.14%로 유지시켜 미처리 시료(8.43%)보다 현저히 낮은 구조적 열화를 보였다. 또한 CEI 안정화된 전지는 90%의 높은 용량 유지율을 보여, 구조적 안정성과 전기화학적 성능 간의 명확한 상관관계를 입증하였다.
연구의 후반부에서는 제논 플라즈마 집속 이온 빔(Xe⁺ PFIB)을 활용하여 시료손상을 최소화하는 새로운 3차원 전자 토모그래피 분석 방법론을 개발하였다. IrOx 촉매 전극과 PFSA 이오노머 전해질 계면의 정밀한 구조 분석을 위해, 전통적인 갈륨 이온 빔(Ga⁺ FIB)과 새로운 Xe⁺ PFIB 방법의 특성을 종합적으로 비교 평가하였다. Xe⁺ PFIB 방법은 연성 고분자인 PFSA 이오노머의 미세구조를 효과적으로 보존하여, PFSA 층에서 미세기공의 수를 81개에서 1,380개로 증가시키고, 평균 기공 직경을 0.324 μm에서 0.04 μm로 감소시켰다. 이는 Xe⁺ PFIB가 100 nm 미만의 미세기공을 더 잘 보존하여 실제 미세다공성 구조를 정확히 분석할 수 있음을 보여준다.
본 연구에서 개발된 FIB-SEM 기반 구조 분석 방법론들은 에너지 재료의 미세구조-성능 관계에 대한 심층적 이해를 제공하며, 차세대 에너지 장치 개발을 위한 재료 설계와 최적화에 중요한 지침이 될 것으로 기대된다.

목차 (Table of Contents)

• Chapter 1. Introduction 1
• 1.1. Importance of Structural Characterization of Energy Materials . 1
• 1.2. Understanding the Structure-Performance Relationship in Energy Conversion and Storage Devices . 2
• 1.3. Advancements and Limitations of Focused Ion Beam Technology . 5
• 1.4. Application of FIB-SEM Technology for Multi-scale/Multi-modal Imaging in Battery Research . 13
• 1.5. Research Objectives . 17
• 1.6. References 19
• Chapter 2. Background 23
• 2.1. Physical Fundamentals and Imaging Mechanisms of FIB-SEM Systems 23
• 2.1.1. Principles and Applications of FIB Technology . 23
• 2.1.2. Ion-Solid Interactions and Damage Mechanisms 28
• 2.1.3. Basic Principles of 3D Electron Tomography 35
• 2.1.4. Image Processing and Analysis Methodology 39
• 2.1.5. Current Trends in FIB-SEM Technology for Energy Materials Analysis . 43
• 2.2. References 48
• Chapter 3. Result & Discussion . 54
• 3.1. 3D Tomographic Analysis of Secondary Particle Microstructure in High-Ni Cathode Materials . 54
• 3.1.1. Introduction: Correlation Between Secondary Particle Structure and Battery Performance 54
• 3.1.2. Experimental Methods . 56
• 3.1.2.1. Synthesis and Characterization of Co-free Li[Ni0.75Mn0.25]O2 (NM7525) 56
• 3.1.2.2. Cathode-Electrolyte Interface (CEI) Stabilization Strategies . 59
• 3.1.2.3. FIB-SEM Based Serial Sectioning Analysis and 3D Reconstruction . 61
• 3.1.3. Results and Discussion 66
• 3.1.3.1. Effect of CEI Stabilization on Electrochemical Performance. 67
• 3.1.3.2. Quantitative Analysis of Porosity and Crack Formation in Secondary Particles 69
• 3.1.3.3. Pore Distribution and Size Characterization 73
• 3.1.4. Conclusion: Importance of Secondary Particle Structure Control Through CEI Stabilization 75
• 3.2. Development of Damage-minimized 3D Electron Tomography using Plasma Focused Ion Beam for Energy Materials . 78
• 3.2.1. Introduction: Importance of 3D Electron Tomography for Energy Materials Characterization . 78
• 3.2.2. Experimental Methods . 81
• 3.2.2.1. Preparation of IrOx Catalyst Electrode-Ionomer Electrolyte Single Cell 81
• 3.2.2.2. Comparative Methodology of Ga FIB and Xe PFIB Sectioning . 81
• 3.2.2.3. Image Processing and 3D Reconstruction Techniques . 83
• 3.2.3. Results and Discussion 87
• 3.2.3.1. Effect of Sectioning Methods: Ga FIB vs. Xe PFIB 87
• 3.2.3.2. Quantitative Analysis of Pore Structure and Surface Area 92
• 3.2.3.3. Pore Distribution and Size Analysis . 95
• 3.2.4. Conclusion: Superiority of Xe PFIB for Damage Minimization in PFSA Ionomer Electrolyte 98
• 3.3. References 101
• Conclusion . 106
• Summary (in Korean) 108