우수한 전기화학 특성 및 안정성을 지닌 리튬이차전지용 올리빈 구조 양극소재 연구

Abstract

Abstracts

During two decades, lithium-ion batteries had been widely developed by many research groups for adjust to portable electronic devices. Among them, layered type cathode materials were already commercialized but there are still remaining the problem to solve, such as long-therm cycling, thermal instability issue, and cost problem. For these reasons, the demand for new type cathode materials had been increasing. In 1997, Padhi and Goodenough firstly discovered olivine type LiFePO4 cathode material which have theoretical capacity of 170 mAh g-1. Since then, the LiFePO4 considered and developed because of its several advantages, such as structural and thermal stability caused by strong polyanion bond, low prices, environmental friendliness. However, LiFePO4 has a lower working voltage than other commercialized cathode materials such as LiCoO2, LiNiCoMnO2 because of the Fe2+/Fe3+ redox reaction at 3.4 V vs. Li/Li+. Therefore, LiMnPO4 has been proposed as a candidate cathode material because its redox potential (4.1 V. Li/Li+) is higher than that of LiFePO4. However, this LiMnPO4 material have lower electrochemical properties which caused by poor electronic conductivity (< 10-10 S cm-1) than LiFePO4 (10-8 S cm-1). Therefore, it is very important to increase the electronic conductivity to enhance the properties of olivine type LiMnPO4 materials to replace the commercial layered materials. In this study, we developed the electrochemical and physico-chemical properties of olivine type LiMn1-xFexPO4 cathode materials using various synthetic method, uniform carbon coating, and substitution of the transition metal (Fe). In chapter 2, 4 V-class olivine C–LiMn1−xFexPO4 (x = 0 and 0.15) are synthesized by ultrasonic pyrolysis followed by ball milling with AB carbon to evaluate the doping effect of iron. The C–LiMn0.85Fe0.15PO4 shows excellent rate capability having discharge capacity of 150 mAh g−1 at 0.5 C-rate and 121 mAh g−1 at 2 C-rate. The capacity retention of the C-LiMn0.85Fe0.15PO4 is 91 % after 50 cycles at 55 oC whereas C–LiMnPO4 is limited to 87 %. The improved electrochemical performance of the C–LiMn0.85Fe0.15PO4 electrode is attributed to the enhanced electrical conductivity caused by tighter binding on the carbon particles with the LiMn0.85Fe0.15PO4 primary particles as well as by the surface coating of carbon on the primary particles. In chapter 3, Carbon coated LiMn0.85Fe0.15PO4 with micron-sized, nano porous, and high volumetric capacity was prepared using Mn0.85Fe0.15PO4·H2O precursor via co-precipitation method. The C-LiMn0.85Fe0.15PO4 powder has nanometer-sized pores which coated by carbon layer uniformly. The 7 ㎛ C-LiMn0.85Fe0.15PO4 exhibited an higher volumetric capacity of 369.3 mAh cm-3 at the 0.05 C-rate than nano-sized C-LiMn0.85Fe0.15PO4. And also the C-LiMn0.85Fe0.15PO4 showed excellent rate capability from 0.05 C to 3 C-rate compared to nano-sized one. These high volumetric capacity and excellent rate capability of micron-sized C-LiMn0.85Fe0.15PO4 materials comes from nano-meter sized pores in secondary micron-sized particle which have uniform carbon coating layer. In Chapter 4, Core-shell type C-LiMn0.85Fe0.15PO4-LiFePO4 olivine materials are synthesized by a co-precipitation method followed by double carbon coating with different kinds of carbon sources. Through energy-dispersive X-ray (EDX) analysis, the distribution of the carbon layer is confirmed within the pores and the surface of the particles varied with different types of carbon sources. Among them, the ascorbic acid-pitch mix coated core-shell electrode show the best discharge capacity of 154 mAh g-1 at 1/20 C-rate and 100 mAh g-1 at 5 C-rate with excellent cycle properties at various temperature. This improved electrochemical property is ascribed to the uniform carbon coating layer both inside the pores and on the outside surface using different molecular weight carbon sources and the structural and thermal stability of LiFePO4 shell.|국문 요지

지난 20년 동안 리튬이온전지는 휴대용 전자기기에 사용되기 위해 많은 연구 그룹에 의해 개발되었다. 그중, 층상계 구조의 양극 소재가 가장 먼저 사용화가 되어 널리 쓰여 지고 있지만 장기 수명, 열 안정성 그리고 가격 적인 측면의 문제점이 여전히 존재하고 있다. 이러한 문제를 해결하기 위해 새로운 종류의 양극 소재에 대한 요구가 증대되어 왔다. 1997년에 Padhi와 Goodenough교수에 의해 처음 발견된 올리빈 구조의 LiFePO4 양극 소재는 170 mAh g-1의 이론용량을 지니고 있으며, 강한 polyanion 결합에 의한 열적, 구조적 안정성, 저가격, 친환경적인 측면의 장점을 가지고 있어 많은 연구가 진행되어 왔다. 하지만, LiCoO2나 LiNiCoMnO2와 같은 다른 상용화된 양극소재와 비교하였을 때, 3.4 V의 낮은 구동 전압이 문제가 되었다. 따라서, 기존 LiFePO4 물질 보다 높은 구동전압을 가진 LiMnPO4 (4.1 V) 소재에 대한 연구의 중요성이 증대되었다. 그러나 LiMnPO4 (< 10-10 S cm-1) 소재는 LiFePO4 (10-8 S cm-1)소재보다 훨씬 더 낮은 전도도를 지니고 있어 이로 인한 낮은 전기화학 특성이 가장 큰 문제가 되고 있다. 현재 상용화 되고 있는 층상계 구조의 양극 소재를 대체하기 위해서는 전도도를 향상시키고 고전압의 소재를 찾는 것이 가장 중요한 방법이다. 이번 연구를 통해 다양한 합성방법, 균일한 카본 코팅 그리고 전이금속 치환을 통해 올리빈 구조 양극 소재의 전기화학적 특성 및 물리화학적 특성을 향상시키고자 하였다. 제 2장에서는 초음파 분무 열분해법, 아세틸렌 블랙 카본과의 볼 분쇄법 및 전이금속 (Fe) 치환법을 사용하여 4 V 에서 구동되는 C–LiMn1−xFexPO4 (x = 0 and 0.15) 양극 소재를 합성하였다. 특히 Fe가 치환된 C–LiMn0.85Fe0.15PO4 소재는 0.5 C-rate에서 150 mAh g−1 그리고 심지어 2 C-rate에서도 121 mAh g−1 의 우수한 방전용량 특성을 보였다. 55 oC 고온 수명특성 테스트에서도 Fe가 치환되지 않은 C-LiMnPO4 소재 대비 우수한 91 %의 방전용량 유지율을 50 싸이클 동안 나타내었다. 이러한 결과를 통해, LiMn0.85Fe0.15PO4 표면과 일차입자에 균일하게 코팅된 카본과 전이금속 치환을 통해 4V급 올리빈 구조 양극 소재의 전기화학 특성을 향상시킬 수 있다는 것을 알 수 있었다. 제 3장에서는 공침반응기 합성법으로 합성된 Mn0.85Fe0.15PO4·H2O 전구체를 사용하여 나노 기공을 가지는 고밀도 마이크론 크기의 C-LiMn0.85Fe0.15PO4를 제조하고 그 전기화학적 특성 및 물리화학적 특성을 평가하였다. 제조된 C-LiMn0.85Fe0.15PO4는 균일하게 카본이 코팅된 나노크기의 기공을 가지고 있는 것을 확인하였다. 또한, 나노크기의 C-LiMn0.85Fe0.15PO4전극 대비, 7 마이크론 크기의 C-LiMn0.85Fe0.15PO4를 통해 제조된 전극을 테스트 하였을 때, 0.05 C-rate에서 369.3 mAh cm-3의 높은 에너지 밀도를 가지는 것을 확인할 수 있었다. 또한, 마이크론 사이즈의 소재가 0.05 C-rate에서 3 C-rate까지 측정된 출력특성 결과에서도 나노 사이즈의 소재보다 훨씬 뛰어난 특성을 보이는 것을 확인하였다. 마이크론 사이즈의 C-LiMn0.85Fe0.15PO4 의 이러한 우수한 전기화학적 특성은 나노크기의 기공에 카본이 균일하게 코팅되었기 때문이라는 것을 확인할 수 있었다. 제 4장에서는 앞서 합성된 마이크론 크기의 C-LiMn0.85Fe0.15PO4소재의 단점을 극복하기 위한 방안으로 고안된 코어-쉘 구조의 양극 소재에 대해 다루었다. 코어-쉘 구조의 C-LiMn0.85Fe0.15PO4-LiFePO4 양극 소재는 공침반응기를 통해 합성되었으며, 다양한 카본의 종류와 양의 조합을 통해 최적화된 소재를 얻을 수 있었다. Energy-disperse X-ray 분석법을 통해 내부 기공과 외부 표면에 균일하게 카본층이 형성된 것을 확인할 수 있었다. 최적화된 코어-쉘 소재는 0.05 C-rate에서 154 mAh g-1, 5 C-rate에서 100 mAh g-1의 우수한 출력특성을 가지면서 상온 및 고온에서도 우수한 수명특성을 가지는 것을 확인할 수 있었다. 이러한 우수한 전기화학적 특성은 내부 기공과 외부표면에 균일하게 코팅된 카본과 열적, 구조적으로 안정성 LiFePO4 쉘을 통해 얻을 수 있었다.; Abstracts

During two decades, lithium-ion batteries had been widely developed by many research groups for adjust to portable electronic devices. Among them, layered type cathode materials were already commercialized but there are still remaining the problem to solve, such as long-therm cycling, thermal instability issue, and cost problem. For these reasons, the demand for new type cathode materials had been increasing. In 1997, Padhi and Goodenough firstly discovered olivine type LiFePO4 cathode material which have theoretical capacity of 170 mAh g-1. Since then, the LiFePO4 considered and developed because of its several advantages, such as structural and thermal stability caused by strong polyanion bond, low prices, environmental friendliness. However, LiFePO4 has a lower working voltage than other commercialized cathode materials such as LiCoO2, LiNiCoMnO2 because of the Fe2+/Fe3+ redox reaction at 3.4 V vs. Li/Li+. Therefore, LiMnPO4 has been proposed as a candidate cathode material because its redox potential (4.1 V. Li/Li+) is higher than that of LiFePO4. However, this LiMnPO4 material have lower electrochemical properties which caused by poor electronic conductivity (< 10-10 S cm-1) than LiFePO4 (10-8 S cm-1). Therefore, it is very important to increase the electronic conductivity to enhance the properties of olivine type LiMnPO4 materials to replace the commercial layered materials. In this study, we developed the electrochemical and physico-chemical properties of olivine type LiMn1-xFexPO4 cathode materials using various synthetic method, uniform carbon coating, and substitution of the transition metal (Fe). In chapter 2, 4 V-class olivine C–LiMn1−xFexPO4 (x = 0 and 0.15) are synthesized by ultrasonic pyrolysis followed by ball milling with AB carbon to evaluate the doping effect of iron. The C–LiMn0.85Fe0.15PO4 shows excellent rate capability having discharge capacity of 150 mAh g−1 at 0.5 C-rate and 121 mAh g−1 at 2 C-rate. The capacity retention of the C-LiMn0.85Fe0.15PO4 is 91 % after 50 cycles at 55 oC whereas C–LiMnPO4 is limited to 87 %. The improved electrochemical performance of the C–LiMn0.85Fe0.15PO4 electrode is attributed to the enhanced electrical conductivity caused by tighter binding on the carbon particles with the LiMn0.85Fe0.15PO4 primary particles as well as by the surface coating of carbon on the primary particles. In chapter 3, Carbon coated LiMn0.85Fe0.15PO4 with micron-sized, nano porous, and high volumetric capacity was prepared using Mn0.85Fe0.15PO4·H2O precursor via co-precipitation method. The C-LiMn0.85Fe0.15PO4 powder has nanometer-sized pores which coated by carbon layer uniformly. The 7 ㎛ C-LiMn0.85Fe0.15PO4 exhibited an higher volumetric capacity of 369.3 mAh cm-3 at the 0.05 C-rate than nano-sized C-LiMn0.85Fe0.15PO4. And also the C-LiMn0.85Fe0.15PO4 showed excellent rate capability from 0.05 C to 3 C-rate compared to nano-sized one. These high volumetric capacity and excellent rate capability of micron-sized C-LiMn0.85Fe0.15PO4 materials comes from nano-meter sized pores in secondary micron-sized particle which have uniform carbon coating layer. In Chapter 4, Core-shell type C-LiMn0.85Fe0.15PO4-LiFePO4 olivine materials are synthesized by a co-precipitation method followed by double carbon coating with different kinds of carbon sources. Through energy-dispersive X-ray (EDX) analysis, the distribution of the carbon layer is confirmed within the pores and the surface of the particles varied with different types of carbon sources. Among them, the ascorbic acid-pitch mix coated core-shell electrode show the best discharge capacity of 154 mAh g-1 at 1/20 C-rate and 100 mAh g-1 at 5 C-rate with excellent cycle properties at various temperature. This improved electrochemical property is ascribed to the uniform carbon coating layer both inside the pores and on the outside surface using different molecular weight carbon sources and the structural and thermal stability of LiFePO4 shell.