Strategies for Enhancing the Stability of Layered Cathodes in Advanced Lithium & Sodium-ion Batteries

다국어 초록 (Multilingual Abstract)

Energy storage systems are attracting considerable attention to address climate crisis stemming from fossil fuels. Rechargeable batteries, represented by lithium-ion batteries (LIBs), are being widely utilized in daily life as sustainable energy storage systems. The key components determining the energy and power density of LIBs are the cathode active materials. Currently, the olivine structure represented by LiFePO4 (LFP) and the layered structure represented by Ni-rich cathodes, Li[NixCoyMnzAl1-x-y-z]O2 are predominantly utilized industrially. Although LFP benefits from the low cost of iron and high stability, it has limited theoretical energy density for their high molecular weight by PO4 framework. In this regard, the layered cathode active materials have been widely studied for their high theoretical energy density. Nevertheless, their stability is undermined by challenges such as phase transitions and interactions with the electrolyte.
Meanwhile, sodium-ion batteries (SIBs) are receiving significant attention as a candidate for next-generation energy storage system to replace LIBs for their abundant resource and environmentally friendly properties. However, SIBs using layered cathodes, NaxMO2 face similar challenges as LIBs, and additionally require improvements in moisture sensitivity and energy density for practical application.
This dissertation addresses strategies for enhancing stability of layered cathode active materials for LIBs and SIBs. The strategies of surface engineering, doping, electrolyte engineering and high-entropy substitution have been conducted into the cathode active material to achieve the specified goal, and the improvements resulting from these approaches have been discussed.
In chapter 1, the basic principles, the properties of cathode active materials, and strategies toward high stability of layered structure for LIBs and SIBs are briefly discussed.
In chapter 2, the surface engineering on Ni-rich cathode, LiNi0.8Co0.15Al0.05O2 was conducted by wet chemical process for improving stability of LIBs. The oxygen-deficient perovskite which has high physicochemical stability and ability to prevent oxygen gas emission to electrolyte was utilized as coating materials. Then, stability improvement at high cut-off voltage was examined.
In chapter 3, Li[Li0.2Ni0.13Co0.13Mn0.54]O2 (LLO) and Li[Li0.2Ni0.13Co0.13Mn0.54Zr0.02]O2 (Zr-doped LLO) are synthesized using solvothermal method. It is unraveled that the Li+ transportation channel is wider and more compact agglomeration of secondary particle is exhibited by Zr-doping. Therefore, the enhanced electrochemical performance, including initial Coulombic efficiency, rate capability and capacity retention is demonstrated.
In chapter 4, Na2/3Cu2/9Fe1/9Mn2/3O2, an air-stable P2-type layered cathode is newly synthesized using solution combustion method and investigate its electrochemical performance using triethyl phosphate (TEP), a well-known fire-retardant solvent. It is confirmed that an air and moisture stability of cathode and nonflammable property of electrolyte. In addition, compared to a cell comprising a carbonate-based electrolyte, the TEP cell exhibits a higher capacity retention of 76.28% after 200 cycles at 2 C rate in a wide voltage range (1.5–4.6 V vs. Na/Na+). To find out the reason of superior electrochemical properties of the TEP cell, morphology and components of the cathode-electrolyte interphase (CEI) layer are characterized. This study was published in Journal of Power Sources, 2023, 580: 233266.
In chapter 5, high-entropy substitution is conducted in O3-type layered cathodes. The correlation between configurational entropy and morphology is determined that Na+ channels is enlarged as configurational entropy is increased. In addition, interlayer spacing of O-Na-O is wider by Li+ substitution. These properties synergistically enhance the Na+ transport kinetic and serve high stability. Also, the electrochemical results are well matched with the properties. Furthermore, the relatively stable moisture sensitivity is demonstrated.

목차 (Table of Contents)

• Chapter Ⅰ. Introduction 1
• 1.1. Background of Lithium-ion Batteries (LIBs) 1
• 1.2. Background of Sodium-ion Batteries (SIBs) 3
• 1.3. Cathode Active Materials for LIBs 5
• 1.4. Cathode Active Materials for SIBs 9
• 1.5. The Strategies for Enhancing Stability of Layered Cathodes 13
• 1.6. References 17
• Chapter Ⅱ. High Voltage Operation of Ni-rich Cathode Through Surface Engineering Using Oxygen-deficient Perovskite for Li-ion Batteries 20
• 2.1. Introduction 20
• 2.2. Experimental 22
• 2.2.1. Materials synthesis 22
• 2.2.2. Materials Characterizations 22
• 2.2.3. Electrochemical measurements 22
• 2.3. Results and Discussion 24
• 2.4. Conclusion 33
• 2.5. References 34
• Chapter Ⅲ. Improving the Cycling Stability of Li-rich Layered Cathodes via One-step Zr-doping Strategies for Li-ion Batteries 36
• 3.1. Introduction 36
• 3.2. Experimental 38
• 3.2.1. Materials synthesis 38
• 3.2.2. Materials Characterizations 38
• 3.2.3. Electrochemical measurements 38
• 3.3. Results and Discussion 40
• 3.4. Conclusion 50
• 3.5. References 51
• Chapter Ⅳ. The Effect of Nonflammable Electrolyte on Cu-substituted P2-type Layered Cathode for High Safety Sodium-ion Batteries 53
• 4.1. Introduction 53
• 4.2. Experimental 56
• 4.2.1. Materials synthesis 56
• 4.2.2. Materials Characterizations 56
• 4.2.3. Electrochemical measurements 56
• 4.3. Results and Discussion 58
• 4.4. Conclusion 77
• 4.5. References 78
• Chapter Ⅴ. Constructing Li-substituted High-entropy O3-type Layered Oxide for Sodium-ion Batteries 83
• 5.1. Introduction 83
• 5.2. Experimental 86
• 5.2.1. Materials synthesis 86
• 5.2.2. Materials Characterizations 86
• 5.2.3. Electrochemical measurements 86
• 5.3. Results and Discussion 88
• 5.4. Conclusion 101
• 5.5. References 102