Doping Strategy to Improve Stability and Performance of Polyanion Structured Cathode Materials for Lithium & Sodium-ion Batteries

다국어 초록 (Multilingual Abstract)

The energy storage and conversion technologies are paying attention to reduce carbon emission by green policy due to climate crisis. The secondary battery systems, represented by lithium-ion batteries (LIBS) are widely used in electric vehicles (EVs), mobile devices, and energy storage systems (ESS). In LIBs, the cathode active materials (CAMs) are the key component for determining its operating voltage and energy density. In a recent year, the layered structure, especially Li[NixCoyMn1-x-y]O2 are applied to industrially due to its high energy density, power density and excellent theorical capacity (~275 mAh g-1). However, As the Ni content increases to achieve higher capacity, the stability of the crystal structure decreases due to cation mixing and instability of Ni3+. This leads to lower thermal stability and leading thermal runaway. Additionally, inherent limitations such as the high cost of materials, phase transitions, and other structural challenges must be overcome. Nowadays, the olivine structured materials, represented by LiFePO4 (LFP) cathode materials are promising in the industry. It has excellent thermal stability, high operating voltage (~ 3.4 V) and relatively low-materials cost using iron. On the other hand, structural limitations, which are the only have 1 D diffusion channel leading low diffusion kinetics, lower conductivity, theorical capacity (~ 170 mAh g-1) and energy density are still challenges to improvement. Nevertheless, the alternative olivine CAMs, which is LiMnxFe1-xPO4 (LMFP) is considering for the increasing energy density than LFP. LMFP can reach the higher voltage due to use Mn2+/Mn3+ redox reaction around 4.1 V. nevertheless, it has same challenges as LFP, and the presence of Mn3+ has occurred the Jahn-Teller distortion (JTD), leading to lower structural stability and cycling performance. Meanwhile, sodium-ion batteries (SIBs) are promising candidates for alternative battery systems to replace LIBs for their abundant resource and good low-temperature performance. SIBs have almost the same mechanism as LIBs, despite, in layered cathodes, NaxMO2 face similar challenges as LIBs, such as moisture sensitivity, energy density, and structure stability for practical application. In addition, the polyanion cathode, Na3V2PO4 has required significant advancements for limited capacity and poor electrical conductivity. This dissertation suggests doping strategies to improve stability and electrochemical performance of polyanion structured cathode materials for LIBs and SIBs. Doping strategies have been attempted to address the inherent issues of polyanion cathode materials and achieve improved electrochemical performance. The specific studies are addressed below. In chapter 1, the basic principles, the properties of cathode active materials, and strategies toward high performances and stability polyanion cathode for LIBs and SIBs are briefly discussed. In chapter 2, cation doping modification in olivine cathode, LiMn0.5Fe0.5PO4 was conducted by facile solvothermal method for suppressing metal dissolution and improving stability of olivine structured cathodes. The Zn doping substitute Fe site and provides a larger Li+ diffusion pathway. Also, Zn has a role of structural stabilizer by the pilar effect. Therefore, improved electrochemical performances, including discharge capacities, Li+ diffusion coefficients, rate capability and high rate cycling performance is demonstrated. In chapter 3, Na3V2(PO4)3 was synthesized by solid-state method with spray-drying process. The doping effect of F and I were compared and investigated. Anion elements could substitute certain (PO4)3-sites leading the bond length of M-O-P and providing structural stability. Moreover, the spray-drying process is conducted for the spherical morphologies and improving cycling performance. Therefore, the enhanced electrochemical performance, including cycling performance, rate capability and post-mortem analysis 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. Brief on Cathode Active Materials for LIBs & SIBs 5
• 1.4. Doping Strategies to Improve Stability and Performance of Polyanion Cathodes 13
• 1.5. References 15
• Chapter Ⅱ. Providing High Stability to Suppress Metal Dissolution in LiMn0.5Fe0.5PO 4 Cathode Materials by Zn Doping 18
• 2.1. Introduction 18
• 2.2. Experimental 20
• 2.2.1. Materials synthesis 20
• 2.2.2. Electrode and CR-2032 coin-cell preparation 20
• 2.2.3. Electrochemical and physical characterization 21
• 2.3. Results and Discussion 23
• 2.4. Conclusion 41
• 2.5. References 42
• Chapter Ⅲ. Comparison about Anion-doping Effect for Improving Na+ diffusion Kinetics of NASICON-type Cathode Materials for Na-ion Batteries 47
• 3.1. Introduction 47
• 3.2. Experimental 49
• 3.2.1. Materials synthesis 49
• 3.2.2. Electrode and CR-2032 coin-cell preparation 49
• 3.2.3. Electrochemical and physical characterization 49
• 3.3. Results and Discussion 52
• 3.4. Conclusion 64
• 3.5. References 66