Surface Engineering of Single-Crystalline Ni-rich Cathode Materials for High-energy Lithium-ion Batteries

다국어 초록 (Multilingual Abstract)

As the global electric vehicle (EV) market grows rapidly, lithium-ion batteries (LIBs) with high energy densities are becoming more important. Recently, several research efforts have focused on enhancing LIBs and all-solid-state batteries (ASSBs) to enable the long-distance driving of EVs (exceeding 700 km). In particular, LIBs have exhibited improvements in energy density through the development of high-capacity materials. However, their use is still restricted to large-scale applications because of their inherently flammable liquid electrolytes and thermal instability. To solve these problems, various studies have recently been conducted using nonflammable solid electrolytes. ASSBs have many advantages over LIBs in terms of their superior thermal and electrochemical stability, enhanced energy density, and ease of processing. However, several challenges must be addressed to ensure stable operation, including issues related to cathode–solid electrolyte interfaces.
Developing cathode materials with high reversible capacities is crucial to achieve high-energy-density batteries. Specifically, Ni-rich cathode materials offer the advantage of high reversible capacity through the double redox reactions of Ni2+. However, these materials face limitations owing to their increased thermal and structural instability, leading to particle collapse and rapid degradation of cycle stability. Recent studies have focused on single-crystalline Ni-rich cathode materials (SNCMs) to overcome these challenges. SNCMs have the advantage of superior cycling stability compared to polycrystalline cathode materials (PNCMs) because they do not have defects such as grain boundaries and voids, which enhances mechanical strength. Unfortunately, SNCMs still have difficulties, such as surface degradation, owing to their high reactivity with electrolytes in LIBs, and problems, such as solid electrolyte decomposition and space charge layer formation in ASSBs.
Addressing these issues requires a functional coating layer to protect the cathode material from the electrolyte. The desired properties of such a functional coating layer include thermodynamic stability, a wide electrochemical stability window, and high ionic conductivity. Among them, the LiTaO3 and LiYO2 functional coating layers were selected because they possess both thermodynamic stability and a broad electrochemical stability window. In Chapter 1, we control the crystallinity of LiTaO3 to derive optimal structures for both the amorphous and crystalline phases, thoroughly validating their physicochemical properties, such as ionic conductivity, electronic conductivity, mechanical strength, and voltage stability. Additionally, research has been conducted to understand the correlation between physicochemical properties and electrochemical performance of cathode materials. In Chapter 2, research was conducted to enhance the ionic conductivity of LiYO2 by introducing La to optimize its composition. Changes in composition were analyzed to understand the corresponding variations in physicochemical properties, considering the increase in the lattice volume, which can lower the activation energy of lithium ions. This study aimed to interpret the electrochemical performance of LIBs and ASSBs.
As a pivotal technology, the research into developing highly ionic conductive functional coating layers for stabilizing the surfaces of single-crystalline Ni-rich cathode materials is expected to enable the early commercialization of high-energy-density batteries in the future.

국문 초록 (Abstract)

세계 전기 자동차(EVs) 시장이 급격히 성장하는 가운데, 고에너지밀도 전지 기술에 대한 수요가 급증하면서 리튬이온전지(Lithium-ion batteries)와 전고체전지(All-solid-state batteries)의 기술 개발이 더욱 가속화되고 있다. 리튬이온전지는 현재까지 고용량의 소재 개발을 통해 에너지밀도를 향상시켜 왔으나, 현재 300 Wh/kg 수준으로 이론적인 한계치에 직면하고 있다. 한편, 전고체전지는 가연성의 불연성의 고체전해질을 사용함으로써 셀안정성을 향상시킬 뿐만 아니라 무게와 부피 두 측면에서 더 높은 에너지밀도를 구현할 수 있다. 하지만 양극소재 계면에서의 높은 반응성 셀 성능의 빠른 저하를 초래하는 문제점이 있다.
이를 해결하기 위해서는 불안정한 양극소재를 전해질로부터 보호해줄 수 있는 기능성 코팅층이 필요하다. 기능성 코팅층의 요구물성으로는 열역학적 안정성, 넓은 전위안정창 그리고 높은 이온전도도가 있으며, 이 세 가지를 모두 충족하는 후보군으로서 LiTaO3 및 LiYO2 기능성 코팅층을 도출하였다.
제 1장에서는 고이온전도도를 가진 기능성 코팅층을 최적화하기 위해 LiTaO3 소재의 결정성을 제어하여 비정질과 결정질의 최적 구조를 도출하였다. 결정성에 따른 물리화학적 특성 변화를 종합적으로 검증했을 뿐만 아니라, 각각의 성능 지표가 양극소재의 전기화학특성의 개선과 어떤 상관관계를 가지는지에 검증하고 도출하는 연구를 진행하였다. 제 2장에서는 LiYO2 소재의 이온전도도를 개선하기 위해 La원소를 도입하여 조성을 최적화하는 연구를 진행했다. 격자 부피의 증가는 리튬이온의 활성화에너지를 낮출 수 있기 때문에 조성 변화에 따른 물리화학적 특성 변화를 고찰하였고, 리튬이온전지와 전고체전지에서의 전기화학적 성능을 해석하는 연구를 수행하였다.
이와 같이 고니켈계 단일입자 양극소재의 표면안정화를 위한 기능성 코팅층의 고이온전도성 개발 연구는 향후 고에너지밀도 전지의 조기 상용화를 실현할 수 있는 핵심기술로 기대된다.

목차 (Table of Contents)

• Figure contents iii
• Table contents . x
• Abstract (English) xi
• 1. Introduction 1
• 1.1 Global EV market trend 1
• 1.2 Lithium-ion battery 4
• 1.3 All-solid-state battery 6
• 1.4 Cathode materials for LIBs and ASSBs 8
• 1.5 Challenges and modifications of cathode materials 12
• 2. Surface Engineering of Single-Crystalline Ni-rich Cathode Materials with Amorphous LiTaO3 coating layer for LIBs and ASSBs 16
• 2.1 Introduction 16
• 2.2 Experiment 17
• 2.3 Results and discussion 19
• 2.4 Conclusion 54
• 3. Surface Engineering of Single-Crystalline Ni-rich Cathode Materials with LiY0.9La0.1O2 coating layer for LIBs and ASSBs 55
• 3.1 Introduction 55
• 3.2 Experiment 56
• 3.3 Results and discussion 58
• 3.4 Conclusion 85
• 4. References 86
• List of publication 101
• List of presentation 102
• Abstract (Korean) 103