Compositional change of surface film and failure mechanisms of LiNi0.5Mn1.5O4 electrode at elevated temperatures

학위논문 (박사)-- 서울대학교 대학원 : 화학생물공학부, 2014. 8. 오승모.초 록 LiNi0.5Mn1.5O4는 리튬의 환원 전위 대비 4.7 V라는 높은 작동 전압을 구현 할 수 있기 때문에 에너지/출력 밀도의 향상을 요하는 최근의 수요에 적합한 물질이라 할 수 있다. 또한 스피넬 구조로서 층간 산화물에 비해 충전 시 구조적으로 안정하며 망간이 산화, 환원에 참여하지 않기 때문에 LiMn2O4의 약점으로 지목되는 Jahn-Teller distortion이나 불균화(disproportionation) 반응 으로부터 자유롭다는 장점이 있다. 그러나 높은 작동 전압은 이 물질의 가장 큰 매력인 동시에 가장 큰 문제점이기도 하다. 즉, 전해질이 전기화학적으로 안정할 수 있는 영역을 벗어난 전위에서 작동하기 때문에 전해질의 산화 분해가 불가피하다. 만약, 흑연 음극의 경우처럼 전해질이 분해되어 전극 표면에 부동태막을 만들어 줄 수 있다면 추가적인 전해질 분해를 막을 수 있으나, 아쉽게도 상용 유기 전해질은 산화 분해 되었을 때 부동태막을 만들어 주지 않는다고 알려져 있다. 그러나 산화 분해되었을 때 생성된 피막이 왜 부동태막의 역할을 할 수 없는지에 대한 이유에 대해서는 알려져 있지 않아서 근본적인 이유는 모른 채 성능 개선을 위한 방법들이 여러 가지로 시도되고 있다. 본 연구에서는 전해질이 산화분위기에서 분해 되었을 때 생기는 피막의 성질을 알아보고 이렇게 생성된 피막이 왜 고전압에서 추가 전해질 분해를 막아주지 못하는지에 대해 알아보았다. 이를 위해 미세 저울인 EQCM(electrochemical quartz crystal microbalace)을 통해 산화 분위기에서 피막의 생성 여부에 대해 알아보고 이때 생긴 피막의 화학적 조성의 특징을 분석하기 위해 XPS(X-ray photoelectron spectroscopy)를 이용하였다. 백금 전극과 LiNi0.5Mn1.5O4 전극의 피막 비교 연구를 통해 4.2 V에서 생기는 피막 중 LiF나 LixPFyOz와 같은 무기물들이 4.9 V에서 사라지는 것을 확인할 수 있었다. 저전압 영역에서 생성된 피막의 일부가 고전압 영역에서 녹아 없어지고 새로 피막이 만들어지는 과정을 거쳐야 하므로 안정적인 피막이 만들어지기 까지 LiNi0.5Mn1.5O4의 초기 쿨롱 효율이 낮은 것을 확인하였다. 피막의 무기물 성분을 용해시키는 성분으로 전해질 산화 생성물인 HF(hydrofluoric acid)를 지목하였고 이를 증명하기 위해 활물질 위에 HF scavenger로서 Al2O3를 코팅하거나 활물질과 직접 혼합하여 전극을 제조하고 이때의 전기화학 성능을 확인해 보았다. 상온에서의 피막 특성과 쿨롱 효율의 관계에 더하여 고온에서의 열화 메커니즘 연구도 수행하였다. 고온에서는 모든 전기화학 반응, 화학 반응의 속도가 빨라지므로 상온에서 영향이 미미했던 반응들이 중요한 퇴화 메커니즘으로 작용할 수 있다. 고온에서 싸이클에 따른 전압-용량 곡선의 변화와 XRD(X-ray diffraction), 전극 단면의 FE-SEM 이미지를 종합했을 때, 전극 내부의 접촉 저항이 크게 증가하면서 전극의 성능이 퇴화 되는 것을 확인할 수 있었다. 전극의 내부의 전자 전도를 향상 시키기 위한 방편으로 에칭된 알루미늄 호일을 집전체로서 이용하거나 바인더, 도전재를 충분히 넣어 혼합해 보았고, 이때 전지의 고온 성능이 향상되는 것을 관찰하였다. 또한 집전체의 퇴화를 막고 활물질 층과 집전체 사이의 전도도를 확보하기 위해 graphene oxide를 집전체에 코팅했을 때도 고온 성능이 개성되는 것을 확인하였다.Lithium-ion batteries, LiNi0.5Mn1.5O4, high-voltage positive electrode, surface film, failure mechanismsThe crisis of fossil fuel economy triggered the demand for lithium ion battery (LIB) whose energy and power density are high enough for the criteria of electric vehicles. Among others, spinel LiNi0.5Mn1.5O4 is one of the most promising positive electrode materials for LIBs because its working voltage is over 4.7 V (vs. Li/Li+), which could deliver higher energy and power density. Spinel structure of LiNi0.5Mn1.5O4 has excellent structural stability even at highly delithiated state, contrast to other positive electrode materials of layered structure such as LiMO2 (M=Co, Ni, Mn……). Furthermore, the oxidation state of Mn ions remain intact during galvanostatic charge and discharge to eliminate the structural deformation of Jahn-Teller distortion or disproportionation from which LiMn2O4 suffer. The advantages of its high working voltage, however, is largely offset due to electrolyte decomposition. Electrolyte could be easily oxidized on the surfaces of LiNi0.5Mn1.5O4 particles since the working potential is beyond the electrochemical stability window of electrolyte. Electrochemical stability window is the potential range in which the electrolyte could stay in stable without electrochemical oxidation and reduction. It is possible to block the electrolyte oxidation if the passivation layer is formed on LiNi0.5Mn1.5O4 electrode surfaces as a result of electrolyte decomposition. Unfortunately, it has been widely believed that commercial electrolyte cannot make the protective film on surface of positive electrode. To the best of our knowledge, however, there was no academic research about a reason why the film formed in oxidation condition could not work as the protective layer. In this work, the stability of surface film derived by oxidative decomposition of electrolyte was estimated on high voltage region (>4.7 V vs. Li/Li+). First, an EQCM (electrochemical quartz crystal microbalance) was used to confirm whether the surface film is formed by oxidation of commercial electrolyte or not. Because the EQCM measures the mass change of electrode, we can monitor the quantity and rate of film formation in real time. Second, chemical composition of surface film formed at specific potential was analyzed using XPS (X-ray photoelectron spectroscopy). Comparative study of compositional change of surface film formed on Pt and LiNi0.5Mn1.5O4 electrode clarified that inorganic phases, LiF and LixPFyOz, in surface film formed at 4.2 V were removed at higher potential (>4.7 V). The removal of inorganic phases, partial disappearance of surface film, could lead additional decomposition of electrolyte on newly exposed electrode surfaces and low coulombic efficiency. Because the LiF and LixPFyOz are electrochemically stable species, the removal of them should be by dissolution, not decomposition. Dissolution mechanism of inorganic compounds by hydrogen fluoride (HF) was proposed and Al2O3 as an HF scavenger was introduced to prove the dissolution mechanism. The Failure mechanisms of LiNi0.5Mn1.5O4 at elevated temperature were elucidated addition to the former work. At elevated temperatures, new degradation mechanisms of electrode ignorable at room temperature could be significant since all kinds of chemical and electrochemical reactions are accelerated due to an increase of rate constant. To clarify the failure mechanism of LiNi0.5Mn1.5O4 electrode, XRD (X-ray diffraction) and FE-SEM (field-emission scanning electron microscope) were employed. Some evidences such as the polarization increase in galvanostatic charge/discharge voltage profile, existence of charged particles in fully discharged electrode, and cross-section image of electrode implied that capacity degradation at elevated temperature is strongly related with contact loss in electrode layer. The failure, however, was suppressed by reinforcing the electric network in electrode layeradding more conducting agent and binder, and using etched Al foil or graphene oxide coated Al foil as a current collector.초록........................................................... i List of Figures.............................................................................................................................iii 1. 서 론........................................................................................................................................ 1 2. 문헌 연구 ................................................................................................................................ 5 2.1. 전기화학 전지의 원리 및 특징............................................................................... 5 2.2. 리튬 이차 전지........................................................................................................... 8 2.3. 리튬 이차전지의 구성 요소................................................................................... 11 2.3.1. 음극 물질........................................................................................................ 11 2.3.1.1. 리튬 메탈............................................................................................. 11 2.3.1.1. 탄소계................................................................................................... 12 2.3.1.2. 합금계................................................................................................... 13 2.3.1.3. 전이금속 산화물................................................................................. 15 2.3.2. 양극 물질........................................................................................................ 16 2.3.2.1. 층상 구조............................................................................................. 17 2.3.2.2. 스피넬 구조......................................................................................... 20 2.3.2.3. 올리빈 구조......................................................................................... 21 2.3.3. 전해질.............................................................................................................. 22 2.4. SEI (solid electrolyte interphase) ................................................................................. 25 2.4.1. 음극 계면........................................................................................................ 25 2.4.2 양극 계면......................................................................................................... 27 3. 실험 방법 .............................................................................................................................. 28 3.1. 전극의 제작............................................................................................................... 28 3.2. 전지의 제작............................................................................................................... 29 3.3. 전기화학 분석........................................................................................................... 30 3.4. 기기분석..................................................................................................................... 31 3.4.1. X선 광전자 분광법(X-ray photoelectron spectroscopy, XPS) ..................... 31 3.4.2 수정 진동자 미세 저울(Electrochemical quartz crystal microbalance, EQCM) ..................................................................................................................................... 32 3.4.3. 기타 기기 및 분석 방법.............................................................................. 33 4. 결과 및 고찰 ........................................................................................................................ 36 4.1. 고전압에서 형성된 피막의 조성변화가 전기화학 특성에 미치는 영향 ....... 36 4.1.1. 백금 전극에서의 전해질 산화.................................................................... 42 4.1.2. LiNi0.5Mn1.5O4 전극에서의 피막 조성 변화가 쿨롱 효율에 미치는 영향 ..................................................................................................................................... 53 4.1.3. HF scavenger 로서 Al2O3의 역할................................................................. 70 4.2. LiNi0.5Mn1.5O4 전극의 고온(60oC)에서의 열화 메커니즘.................................... 85 4.2.1. LiNi0.5Mn1.5O4의 저장 수명 특성................................................................. 85 4.2.2. LiNi0.5Mn1.5O4의 고온 충/방전 시 용량 퇴화 메커니즘.......................... 91 4.2.3. 집전체에 코팅한 Graphene oxide(GO)가 고온 성능에 미치는 영향 .. 103 5. 결론...................................................................................................................................... 114 참고문헌................................................................................................................................... 116 Abstract.................................................................................................................................... 121Docto

광원의 종류에 따른 콤포짓트레진의 중합도

Thesis (master`s)--서울대학교 대학원 :치의학과 치과생체재료학전공,2001.Maste

A Study on static inelastic analysis and limit analysis of steel structures

학위논문(박사)--서울大學校 大學院 :建築學科,1996.Docto

우리나라의 사회계층간 건강행태 차이

학위논문(석사)--서울대학교 보건대학원 :보건학과 보건정책학전공,2000.Maste

(An) economic analysis of transport projects using probabilistic simulation model

학위논문 (박사)-- 서울대학교 대학원 : 건설환경공학부, 2011.2. 전경수.Docto

Effect of Surface Modification of Zinc Powders with Organosilanes on the Corrosion Resistance of Thin Zinc Pigmented Organic Coated Steel Sheet

MasterConventionally, zinc (Zn) pigmented coating has been used in heavy industry, such as bridges, offshore construction and ships, to protect steel from corrosion. Zn particles can provide both galvanic protection and a barrier protection to coatings on steel panels. More recently, Zn pigmented coating has been used in the automotive industry to impart weldability to pre-coated steel sheetsin this context, Zn pigmented coating is called “Pre-sealed coating”, or “Weldable primer”. Pre-coated metal sheets have many advantages, such as good corrosion resistance, reduced production cost and simple manufacturing procedures. However, in the automotive industry, the main function of Zn particles in the coating is to provide an electrical pathway between the welding electrode and the steel substrate. For that reason, the pre-sealed coating should be thin enough (< 10 m) to guarantee weldability. Due to the limited coating thickness, the demands for higher increasingly corrosion resistance are difficult to meet. Efforts have been made to increase the corrosion resistance of Zn pigmented coating, reducing the coating thickness to maintain or improve the electrical resistance spot-weldabilitythese efforts include optimizing the size and shape of Zn particles, optimizing the pigment/binder ratio, and incorporating corrosion-inhibitive pigments or conductive pigments to improve corrosion resistance and weldability. However, incorporating these pigments can degrade the mechanical properties of the organic coating, particularly if the coating is thin. Hence, improvement of the current systems without addition of other functional pigments would be a promising strategy.Therefore, in this work, surface modification of Zn powders using aqueous organosilane (OS) solutions was used to improve the corrosion resistance of thin ZRCs. To investigate the effects of OS properties on the surface modification, six different OSs were tested as surface modifying materials: N-propyltriethoxy silane (PES), which has a short propyl chainbis-1,2-triethoxysilyl ethane (BTSE), which has six hydroxyl groups and ethyl side chain between siloxane groups3-aminopropyletriethoxysilane (APS), which has an amine side chain at the end of the middle of moleculebis-trimethoxysilylpropyl amine (BTSPA), which has an amine side chain in the middle of molecule3-glycidyloxypropyl trimethoxysilane (GPTMS), which includes an alkyl epoxy chainand bis-triethoxysilylpropyl tetrasulfide (BTSPS), which contains tetrasulfide groups in the middle of molecule.Scanning electron microscopy (SEM), the scanning vibrating electrode technique (SVET), cross sectional analysis, Fourier transform infrared spectroscopy (FT-IR) and X-ray photoelectron spectroscopy (XPS) were conducted to investigate the surface characteristics of modified Zn particles. Corrosion resistance of Zn pigmented coatings containing OS modified Zn particles was evaluated using a salt sprayed test (SST). Electrochemical impedance spectroscopy (EIS) was used to describe the corrosion protection mechanism of the coating and to determine the key factors that cause corrosion. In the general corrosion test, Zn particles with OS-modified surfaces showed higher resistance to corrosion than did non-treated Zn particles. The electrochemical activity of Zn particles was diminished by surface modification with OSs. The passivation layer formed by OSs on the surface of Zn particles was indirectly characterized using XPS depth profile analysis of surface-modified Zn electrodesthe thickness of modified surface layer depended mainly on the chemical structure of the OS. FT-IR analysis showed that the intensities of specific analysis peaks were proportional to the thickness of the passivation layerthis suggests that covalent bonding is formed between not only OS molecules but also the OS and the Zn.In addition, the activity of the passivation layer on the Zn particle was indirectly evaluated using SVET. The current-density of Zn electrodes treated with OSs having unreacted functional groups was higher than that of non-treated bare Zn electrodes. Current-density signified electrochemical activity in the samples. Unreacted functional groups of OSs are polar and electronegative in solution, so when they were exposed to the sodium chloride solution they attracted ions dissolved in it. A higher potential deviation of OS treated Zn electrodes , measured by SVET, was induced by formation of ion rich layer on the electrodes. This large potential drop was converted to high current-density and indicated that the OSs are reactive or electrochemically active. The activity results revealed the reactive characteristics of OSs.Coatings that included Zn particles treated using OSs that have functional groups resisted corrosion better than those treated using OSs that lack them. In contrast, corrosion resistance of Zn-pigmented coating was not significantly improved. For example, using alkylsilane (PES and BTSE) as modification materials resulted in no significant improvement of corrosion resistance. Electrochemical impedance spectroscopy was used to characterize the corrosion protection mechanism. Coatings containing Zn particles treated with APS, BTSPA, GPMS and BTSPS showed superior anti-corrosion properties. For instance, water uptake by the coating during the early corrosion stages was hampered owing to enhancement of the coating’s barrier property. This barrier property improvement was related to the presence of unreacted OS functional groups, which could react with the resin matrix and form an interfacial bond that increased the barrier property of the coating. Moreover, corrosion activity of the interface between substrate and coating was promoted by surface modification of Zn particles. Nonetheless, surface modification of Zn particles did not affect the disbonded area between coating and substrate in later stages.The coating that contained Zn particles treated with BTSPA had the greatest resistance to corrosion

버스 노선 최적화 방안에 관한 연구 : 서울시 굴곡노선 사례를 중심으로

학위논문(석사)--서울대학교 대학원 :토목공학과 도시공학전공,1998.Maste

Effect of Er:YAG laser-irradiated dentin on the shear bond strength of composite resin

학위논문(박사)--서울대학교 대학원 :치의학과 치과재료과학전공,2004.Docto

상세 모델링을 고려한 적층 복합재의 전자기 리벳팅에 관한 병렬 수치해석 연구

학위논문 (박사)-- 서울대학교 대학원 : 기계항공공학부, 2011.8. 김승조.Docto

중공 타이타니아 입자의 상 분율효과가 차열 도료에 미치는 영향

DoctorIn recent years, since polymer films containing thermal blocking and near-infrared (NIR)-reflective pigments have received much attention for their potential applications in energy-saving fields, it serves energy saving efficiently as not only blocks the transmission of solar heat rays from the outside, but also reduces the consumption of cooling and heating energy by the insulation effect that minimizes indoor heat loss. However, in practical environments, dust present in the air is easily adsorbed and adheres to the surface of these films, thus gradually reducing their NIR reflectance. To overcome drawbacks, there is an increasing need to develop a thermal blocking system along with long-term periods. Thermal energy control can be roughly achieved by two methods: thermal insulation and thermal reflection. Insulation means a function of preventing heat from being transferred to the inside by using a heat insulating material having a low thermal conductivity. Hence, it is obvious that the thermal conductivity coefficient of the material is a major factor indicating the adiabatic effect. On the other hand, IR reflection means to selectively and efficiently reflect infrared rays, which account for more than 50% of sunlight, to prevent the temperature rise of the roof or exterior walls, which is the cause of the increase in the internal temperature of the building. It can be seen more appropriate that reduction of thermal factors through light reflection. In this present work, it is intend to research and develop coated steel sheets with thermal control characteristics such as heat insulation and heat reflection together without surface contamination. In addition, it is also intended to develop a single/thermal coated steel sheets which can be applied to high-speed thin film coating for commercialization and high productivity. In general, since the rate of heat transfer by conduction and convection in the closed micro-sized void is very low, in the case of micro hollow spheres having a high porosity of 80% or more, the effective thermal conductivity is expected to be considerably low. Therefore, hollow silica spheres having a lower thermal conductivity have already been attempted to develop with improved mechanical properties by others. In this present work, nanocrystalline titania have a role as a photocatalyst when absorbing light. It can absorb sunlight and remove contaminants from the particle surface. In addition, it has the advantage of higher reflectivity than silica. So, it was designed to maximize the effect after applying it to the reflective coating by combining the advantages of the hollow structure and the advantages of the titania photocatalyst. Hollow structured titania was prepared by direct chemical deposition method as follows: (1) preparation of hard templates; (2) functionalization/modification of template surface to achieve favorable surface properties; (3) coating the templates with designed materials or their precursors by various approaches, possibly with post-treatment to form compact shells; and (4) selective removal of the templates to obtain hollow structures. Anionic polymer seed were prepared by multi stage emulsion polymerization to assign electronegative characteristics. With help of electrostatic attraction and controlling hydrolysis and condensation reaction, 200-300 nm coreshell titania was successfully produced. The, sintering process at high temperature was carried out to remove seed and form hollow titania. The titania has high porosity, high surface area by controlling shell thickness. Moreover, the hollow spheres showed excellent photocatalytic performance compared with commercial titania nanopowders. It was observed that the photocatalytic powder correlated with sintering temperature. Optimized manufacturing hollow titania that has photocalytic acitivity has been deduced by calicination processing control. IR reflective coating was applied on steel sheets with the synthesized hollow titania to confirm their photocatalytic and reflection performance. Interestingly, most of the particles located near surface with help of hollow structure in a coating. It is important to the position of particles in coating especially for photocatalytic ability implementation. When the photocatalayst was immobilized deeply in coating, it was hard to observed sun light and act organic degradation. In my research, it was observed that the hollow particles were induced to float to the surface by the hollow structure. No more additional coating is needed using this technology that can decrease production cost. Moreover, hollow titania provides enhanced reflection to coating due to both high reflectance nature of titania and existence of void which is able to give more light scattering probability. In the ends, accomplished performance was achieved in terms of thermal blocking (due to increase in IR reflection) and surface cleaning (due to photocatlaytic characteristic) within one coating