A Study on Morphological, Electrochemical Properties and the Reaction Mechanism of Hollow NiCo2O4 Anode Materials with Porous Shell for Lithium Ion Batteries

국문 초록 (Abstract)

이원 금속 산화물은 리튬 이온 배터리에서 각광받는 음극 소재이다. 특히, NiCo2O4는 낮은 율 특성 및 수명 특성과 같이 현재 사용되는 음극 소재의 단점을 극복하기 위한 물질 중 하나이다. Co3O4에서 Co 성분에 의한 비싼 가격과 독성 때문에 Co를 다른 전이금속으로 치환하는 것은 리튬 이온 배터리에서 중요한 요소이다. 또한, Hollow structure는 높은 비표면적을 가지고 있기 때문에 각광받고 있는 Morphology이다.
이에 본 연구는 3차원 Hollow structure의 NiCo2O4를 합성하기 위하여 Co-precipitation의 손쉬운 합성 방법을 사용하였다. 이러한 방법은 그램 단위의 많은 양을 균일하게 합성할 수 있다. N2 adsorption-desorption isotherms, Scanning electron microscopy (SEM) 및 Transmission electron microscopy (TEM) 등을 통하여 Hollow NiCo2O4의 Morphology를 분석하였고, 50 nm의 Nanoparticles/flakes shell에 의하여 3~4 nm의 Pore size를 가지고 134 m2 g-1의 높은 비표면적을 가진 Porous hollow structure를 확인할 수 있었다. 전기화학적 특성에서는 높은 용량, 우수한 율 특성 및 수명 특성을 나타내었다. 또한, 방사광 X선을 사용한 ex situ X-ray diffraction, ex situ X-ray absorption spectroscopy 및 TEM 분석을 통하여 이 물질의 초기 두 사이클에서의 반응 메커니즘을 깊이 있게 알아보았다. Cyclic voltammetry 또한 수 사이클 동안의 반응 메커니즘을 연구하기 위하여 사용하였다.

다국어 초록 (Multilingual Abstract)

Binary metal oxides are regarded as one of promising anode materials for lithium ion batteries. Especially, NiCo2O4 is a candidate to overcome the currently existing disadvantages of anode materials such as rate capability and cycle life. The substitution of Co content to other transition metal due to high cost and toxicity compared with pure Co3O4 makes it more significant for lithium ion batteries. Also, hollow structures have been attracting due to the high surface area.
In this thesis, a facile synthesis method is reported following by co-precipitation and calcination in the air to achieve three-dimensional hollow structured NiCo2O4. This method helps to produce uniformly synthesized structure with gram quantities. N2 sorption analysis, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) observations show that the surface area of the hollow NiCo2O4 is determined to be 134 m2 g-1 with a unique porous morphology of narrow pore size distribution of 3 to 4 nm including nanoparticles/flakes in a size range of about 5 to 50 on the surface of each particles. It is evidently exhibited by hollow structures NiCo2O4 electrode which has the increased capacity, excellent rate capability and cycling stability. Also, we have investigated the advanced reaction mechanism of this electrode material by ex situ X-ray diffraction, ex situ X-ray absorption spectroscopy using synchrotron X-ray beam, and TEM observations during initial two cycles tested at low scan rate of 100 mA g−1. Cyclic voltammetry is also used for the study of reaction mechanism after subsequent cycles.

목차 (Table of Contents)

• Table of Contents 1
• List of Figures 3
• Abstract 6
• I. Introduction 8
• II. Background 10
• 1. Rechargeable Lithium Ion Batteries 10
• 1.1. Lithium Ion Battery 10
• 1.2. Anode materials in LIBs 12
• 1.3. Nickel cobalt oxides 15
• 1.4. Hollow structure 17
• 2. X-ray analysis 18
• 2.1. X-ray Diffraction 18
• 2.2. Synchrotron Based X-ray Diffraction 20
• 2.3. X-ray Absorption Spectroscopy 21
• III. Experimental 23
• 1. Synthesis of hollow NiCo2O4 23
• 2. Characterizations 24
• 2.1. N2 sorption analysis 24
• 2.2. Scanning electron microscopy 24
• 2.3. Transmission electron microscopy 24
• 2.4. Electrochemical analysis 25
• 2.4.1. Preparation of the electrode and coin cells 25
• 2.4.2. Electrochemical performance 25
• 2.5. Structural analysis 26
• 2.5.1. X-ray diffraction 26
• 2.5.2. X-ray analysis based on synchrotron radiation 26
• 2.5.2.1. Ex situ X-ray diffraction 26
• 2.5.2.2. Ex situ X-ray absorption spectroscopy 27
• IV. Results and discussion 28
• V. Conclusion 63
• References 65
• Abstract in Korean 69