(The) effects of Fe as impurity element for sustainable resynthesis of Li[Ni1-x-yCoxMny]O2 cathode materials from spent lithium-ion batteries = 폐리튬이온전지로부터 삼성분계 양극활물질 Li[Ni1-x-yCoxMny]O₂ 재합성을 위한 불순물 수준의 철 영향 연구

다국어 초록 (Multilingual Abstract)

The electric vehicle market including hybrid electric vehicle (HEV), plug-in hybrid electric vehicle (PHEV), and electric vehicle (EV), as one of major field of lithium-ion rechargeable battery (LIB) applications has been growing rapidly over the recent years. Accordingly, the overall volume of LIB production has been also skyrocketing and this is mainly resulting from the future direction of global automobile industry in domestic and overseas that is moving toward eco-friendly electric car systems due to strict regulations of worldwide environment concern. Thereby, the demand in raw materials consisting of rechargeable batteries which are a core part of EV is also expected to surge. Therefore, given the finite nature of resources that comprises battery material and the maldistribution of countries for production of the raw materials, recycling spent LIBs is mandatory as a countermeasure for not only procuring the raw materials but also preventing the potential environment threat in the coming years.
With the raid development of EV market, the trend shifting especially in LIB cathode material was made from the conventional LiCoO2 (LCO) to nickel-based transition metal oxide materials such as Li[Ni1-x-yCoxMny]O2 (NCM), which is accelerated by the needs of higher energy density to satisfy more larger driving range. As a result, the existing LCO recycling process focusing on recovering mainly Li and Co simply by pyrometallurgical manner no longer effective in the case of NCM because of the difficulty in recovering Ni, Co, and Mn separately, thus leading to increase in additional purification costs. Meanwhile, the coprecipitating property of Ni, Co, and Mn in similar pH range in turn results in emerging the resynthesis process which is the concept of regeneration of cathode materials directly from leach liquor via coprecipitation method. However, the leach liquor treated by acid leaching process contains various impurity elements such as Li, Al, Cu, Fe, Na, Ca, Mg, etc. that significantly affect the electrochemical and structural properties of the resynthesized cathode materials. Therefore, dedicated efforts of investigating the impact of impurities in the resynthesized NCM on their LIB performance are necessary to determine the tolerable level of them from the aspects of industrial merit and material synthesis without any degradation of performance, respectively.
In an attempt to establish a lab-scale sustainable resynthesis process of NCM cathode active material from spent LIBs, an intensive research focusing on the effect of Fe as an omnipresent impurity element in NCM cathode material was systematically dealt with in this dissertation. Firstly, we synthesize Li[Ni1/3Co1/3Mn1/3]O2 (NCM) and Li[Ni1/3Co1/3Mn1/3]FexO2 (NCMF) cathode active materials with various amounts of Fe via hydroxide coprecipitation and calcination processes, which simulate the resynthesis of NCM in leach liquor containing Fe from spent lithium ion batteries (LIBs). The crystal structure and electrochemical performances of the synthesized NCMF (i.e., NCMF (0.05%), NCMF (0.25%) and NCMF (1.0%)), are investigated and compared with pristine NCM. The structural perfection of NCMF gradually deteriorates with increasing the amount of Fe because of undesirable cation mixing between Ni2+/Fe3+ and Li+ sites. In LIB performances, NCMF (0.05%) and NCMF (0.25%) present relatively reduced overpotential leading to superior rate performance at high C-rates to NCM with NCMF (1.0%) having the poorest. In terms of cycling stability, however, capacity retention improves as the Fe content in NCMF increases. The thermal stability of NCM and NCMF is also measured by differential scanning calorimetry, and the post-mortem analysis of X-ray photoelectron spectroscopy reveals that the ratio of Mn3+ becomes lower after cycling tests as the amount of Fe in NCMF increases. Moreover, the additional post-mortem analysis of energy dispersive spectroscopy on graphite surface after full cell cycling tests further confirms the positive effect of Fe on the improved capacity retention performance of NCMF.
Secondly, we synthesize trace amount of extra Fe (0.25%) incorporated Ni-rich NCM cathode material, Li0.97[Ni0.78Co0.12Mn0.10]Fe0.0023O2 (HNCMF), to develop the industrially feasible Ni-rich layered oxide cathode with improved electrochemical performance, especially rate capability. By exploiting the positive effects of Fe on the improved electrochemical property which is validated in NCM333 system, the modified structure of HNCMF significantly outperforms analogous bare material Li1.02[Ni0.78Co0.12Mn0.10]O2 (HNCMb). HNCMF shows lithium-deficient composition but reduced cation mixing ratio and the increase in initial Coulombic efficiency and charge/discharge capacity compared to HNCMb, notwithstanding the undesired nature of Fe3+ ions causing severe cation disordering between Ni2+/Fe3+ and Li+ sites detrimental to reversible capacity and rate performance. The advantage of the increased charge/discharge capacity and Coulombic efficiency of HNCMF is maintained at even in the 10 C-rate condition and it is noted that the discharge capacities at 1 C and 2 C for HNCMF are 163.73 mAh/g and 153.20 mAh/g, which are comparable as the values for HNCMb at 0.5 C (163.82 mAh/g) and 1C (153.90 mAh/g), respectively. Based on relatively lowered overpotentials, the improved structural and electrochemical property of HNCMF are scrutinized by conducting the analyses of in-situ X-ray diffraction and post-mortem high angle annular dark field scanning transmission electron microscopy.

목차 (Table of Contents)

• Chapter 1. Introduction 1
• 1.1. Present status of lithium-ion rechargeable batteries (LIBs) 1
• 1.1.1. Cathode active materials applied to LIBs 8
• 1.2. Research background and challenges for recycling spent LIBs 24
• 1.2.1. Recycling process of LIBs 27
• 1.2.2. Resynthesis of cathode materials from spent LIBs 41
• 1.2.3. Influences of impurity elements in Li[Ni1-x-yCoxMny]O2 cathode materials on their LIB performance 44
• 1.3. Objectives and scope of the dissertation 51
• Chapter 2. Experimental procedures 54
• 2.1. Material synthesis 54
• 2.1.1. Background for synthesis of Ni1-x-yCoxMny(OH)2 hydroxide precursors via coprecipitation method 54
• 2.1.2. Li[Ni1/3Co1/3Mn1/3]FexO2 (x = 0, 0.0005, 0.0025, 0.01): NCM333 system 56
• 2.1.3. Li[Ni0.78Co0.12Mn0.10]FexO2 (x = 0, 0.0025): Ni-rich NCM system 57
• 2.2. Material characterization 58
• 2.2.1. Scanning electron microscope 59
• 2.2.2. A focused ion beam 59
• 2.2.3. Scanning transmission electron microscopy 60
• 2.2.4. High-angle, annular dark field detector 61
• 2.3. Analyses of electrochemical and thermal property 62
• 2.4. Cell assembly 65
• 2.4.1. Fabrication of pouch-type half/full cell 65
• Chapter 3. The effect of Fe as impurity element for sustainable resynthesis of Li[Ni1/3Co1/3Mn1/3]O2 cathode material from spent LIBs 66
• 3.1. Introduction 66
• 3.2. Synthesis and characterization of the precursors and the cathode active materials 69
• 3.3. Electrochemical performance of the cathode active materials 72
• 3.4. Post-mortem analyses for improved cycleability and thermal stability 75
• 3.5. Conclusions 80
• Chapter 4. Understanding the role of trace amount of Fe incorporated in Ni-rich Li[Ni0.78Co0.12Mn0.10]O2 cathode material 99
• 4.1. Introduction 99
• 4.2. Synthesis and characterization of the cathode active materials 103
• 4.3. Electrochemical properties of the cathode active materials 108
• 4.4. Elucidating the improved structural and electrochemical stability 113
• 4.5. Conclusions 120
• Chapter 5. Conclusion 146
• Bibliography146
• 국문초록 173