Utilization of various oxidation state of vanadium for building versatile electrode materials for rechargeable batteries. = 다양한 산화수를 갖는 바나듐을 기반으로 하는 이온 전지의 개발

국문 초록 (Abstract)

현재 에너지저장장치 시장은 LiCoO2를 필두로 LiFePO4, Li[Ni1-x-yCoxMny]O2 등 다양한 물질을 기반으로 급속도로 커져가고 있다. 이러한 상황에 더하여 기존 노트북, 핸드폰 등의 전원 공급 장치로서의 소형 디바이스의 에너지 저장장치로서뿐만 아니라 전기자동차의 전원 공급 장치, 발전소용 대용량 저장장치 등 대형 에너지 저장장치 분야에 있어서도 지속적인 시장의 성장이 진행 중에 있다. 하지만, 이러한 시장 성장에 비해 리튬이온전지 시장은 많은 문제점에 시달리고 있다. 먼저, 가장 시장을 넓게 장악하고 있는 LiCoO2의 경우 원재료가 되는 코발트의 가격이 매우 비싸고, 독성물질의 발생 등 많은 문제점을 안고 있다. 또한, 리튬이온전지의 기본인 리튬 또한 시장의 급격한 확산에 따른 가격상승과 중국, 남미와 같은 특정지역에 매장량이 집중되어 있어 원자재의 무기화 가능성도 간과할 수 없다. 이러한 현상을 극복하기 위하여 미국, 일본 등 세계 여러 강대국을 필두로 다양한 물질에 대한 연구가 진행되고 있다.
이러한 연구는 크게 두가지 방향으로 진행되고 있다. 첫 번째로는 기존 리튬이온전지 시스템을 개발, 발전시키는 방안이 있으며, 세부적으로는 기존 상용화 되어있는 음극활물질인 하드 카본보다 우월한 용량 성능과 고율특성을 갖는 물질을 개발하며, 양극활물질의 경우 낮은 에너지 밀도를 개선하기 위하여 코발트를 사용하지 않는 고에너지밀도의 신규 양극활물질을 개발하는 방안이 제시되고 있다. 두 번째 방안으로는 기존의 리튬이온전지 시스템을 벗어난 새로운 이온전지 시스템을 개발하는 방안으로 제시되는 신규 이온으로는 대표적으로 나트륨과 포타슘, 아연 등이 있다.
본 연구에서는 0에서 5+에 이르기까지 다양한 산화수를 가져 음극 및 양극 모두 사용이 가능한 바나듐을 기반으로 리튬 음극, 리튬 양극, 나트륨 양극, 아연이온전지의 4가지 파트로 나누어 연구를 진행하였다. 네 개의 파트는, 첫 번째 리튬이온전지용 음극물질 개발, 두 번째는 리튬이온전지용 양극물질 개발, 세 번째는 나트륨 이온전지용 전극물질 개발, 마지막 네 번째로는 아연이온전지용 전극물질 개발로 나누어 연구를 진행하였다. 첫 번째 파트인 리튬이온전지용 음극물질 개발에서는 음극물질로 활용이 가능한 LiVOPO4를 water-in-oil 타입으로 합성하여 다양한 온도에서 열처리하여 최적의 단일상을 갖는 조건을 확보하였다. 단일상의 전극물질은 CR-2032 코인셀로 제작하여 전기화학적 성능을 평가하였다. 충, 방전 과정에서의 구조변화를 다양한 측정방법을 통해 확인하였으며, 이를 통해 고율특성을 갖는 원인을 파악하였다.
두 번째로 리튬이온전지용 양극물질 또한 연구가 진행되었는데, 초음파 열분무 분해법(spray-pyrolysis method)을 활용하여 합성한 Li3V2(PO4)3에 다양한 조성의 Mn을 도핑하여 최적 조성을 확인하였다. 이렇게 합성된 전극물질의 전기화학적 성능을 평가하였으며, Mn 도핑이 전극물질에 어떤 영향을 미쳐 전기화학적 성능을 향상시키는지 확인하였다. 이를 통해 다른 물질에도 이를 적용할 수 있음을 확인하였다.
세 번째 파트인 나트륨 이온전지용 양극 활물질의 경우 앞에서 연구된 리튬이온전지용 양극 활물질인 Li3V2(PO4)3를 응용하여 Na3V2(PO4)3를 연소법(combustion method)을 통해 합성하였다. 상기 양극활물질은 이전 연구를 응용하여 Mn 도핑과 카본계 물질을 활용한 표면개질을 적용하였으며, 전기화학 성능을 향상시켰다. Mn도핑이 물질에 미치는 영향과 전기화학 반응에서의 참여 여부 등을 확인하기 위하여 다양한 분석법과 제일계산을 이용하여 연구를 진행하였다. 또한, 상용화 가능성을 확인하기 위하여 하드카본을 음극으로 하는 완전지를 제작하여 성능을 평가하였다.
마지막으로 아연이온전지의 경우 기존 리튬, 나트륨 이온전지와 다르게 수계 전해질을 기반으로 하여 고전압에서의 전기화학 반응이 어려우므로 저전압에서 고용량을 낼 수 있는 물질인 VO2(B)를 활용하여 연구를 진행하였다. 아연이온전지의 활물질로 활용한 VO2(B)는 용매열 합성법 (solvothermal method)을 활용하여 합성하였다. 합성된 물질은 세척과 건조과정을 거쳐 전극물질로 활용하였으며, 아연 메탈을 상대전극으로 하는 코인셀로 제작되어 전기화학 성능을 평가하였다. 또, 이전에 연구된 적이 없는 VO2(B)로의 아연 이온의 삽입위치를 제일계산을 통해 확인하고 이를 다양한 분석방법을 통해 증명하였다. 또한 rGO와의 복합체를 합성하여 전기화학적 성능을 향상시켰고, 그 중에서도 수명 성능을 크게 향상시켰으며, 충, 방전과정에서 발생하는 전해질 분해반응과 이를 통해 형성된 부반응 물질이 물질에 미치는 영향을 ex-situ XRD, XPS 등을 활용하여 확인하였다.
본 연구결과 개발된 다양한 이온전지용 전극물질은 기존의 상용화되어진 물질과 동등 혹은 우세한 전기화학적 성능을 보여주었다. 상기 연구들을 통해 개발된 여러 물질들은 기존 이온전지에서 문제점으로 지적되어지던 고비용의 코발트를 사용하지 않는 전극물질이며, 신규 이온전지용 전극물질로서 에너지 저장장치 시장에 있어 새로운 방향을 제시 할 수 있는 연구결과이다.

다국어 초록 (Multilingual Abstract)

These days, the energy storage market is rapidly growing based on various materials such as LiFePO4 and Li[Ni1-x-yCoxMny]O2, starting with LiCoO2. In addition to small storage devices for power supplies for existing notebooks and mobile phones, the market is expanding into various fields such as medium-sized storage devices for electric vehicles and storage devices, and mass storage devices for power plants. However, compared to this market growth, the lithium ion battery market is suffering from many problems. First, in the case of LiCoO2, which has the largest market dominance, cobalt prices are continuously rising, and many toxic substances are generated when batteries are produced. In addition, lithium, which is the base of lithium-ion batteries, is also concentrated in certain regions such as China and South America, which could lead to the weaponization of raw materials and the sharp increase in the price due to the rapid spread of the market. To overcome this phenomenon, we are researching various materials in various countries such as USA and Japan.
These studies are proceeding in two major directions. First, there is a plan to improve and develop existing lithium ion battery systems. In detail, it is to develop a material having superior capacity performance and high rate characteristics than hard carbon, which is a commercially available negative electrode active material and the case of the cathode active material, a method of developing a new cathode positive material having a high energy density without using cobalt is proposed in order to improve the low energy density. Second, new ions suggested as a way to develop a new ion battery system out of the existing lithium ion battery system are sodium, potassium, and zinc.
In this study, the research was divided into four parts, lithium anode, lithium cathode, sodium cathode, and zinc ion batteries, based on vanadium, which has various oxidation states ranging from 0 to 5+ and can be used both as cathode and anode. The three parts were divided into the development of electrode materials for lithium ion batteries, the development of electrode materials for sodium ion batteries, and the development of electrode materials for zinc ion batteries. In the first part, LiVOPO4, which can be used as an anode material, was synthesized as a water-in-oil type and heat treatment was performed at various temperatures to obtain an optimum single-phase condition. The single phase electrode material was fabricated from a CR-2032 coin cell and its electrochemical performance was evaluated. The structural changes in charge and discharge processes were confirmed through various measurement methods, and the cause of high rate characteristics was identified.
In the second part, the cathode material was also studied. The Li3V2(PO4)3 was synthesized by using ultrasonic spray pyrolysis method, and optimal composition was confirmed by doping various compositions of Mn. The electrochemical performance of the synthesized electrode material was evaluated, and it was confirmed that how the Mn doping improves the electrochemical performance by affecting the electrode material. It was confirmed that Mn doping can be applied to other materials.
In the third part, Na3V2(PO4)3 was synthesized by the combustion method using Li3V2(PO4)3, a cathode active material for lithium ion battery, which was studied in the case of a cathode active material for a sodium ion battery. The cathode active material was subjected to surface modification using a carbon based material and Mn doping was applied to improve the electrochemical performance by applying the previous research. In addition, to investigate the effect of Mn doping on the material and its participation in the electrochemical reaction, various analytical methods and the first calculation were used. Moreover, to confirm the feasibility of commercialization, a full-cell with hard carbon as a anode was fabricated and its performance was evaluated.
Finally, zinc ion battery is different from conventional lithium and sodium ion batteries because it is difficult to electrochemically react at high voltage based on water electrolyte. Therefore, research was conducted using VO2(B), which is capable of producing high capacity at low voltage. VO2(B), which was used as an active material of zinc ion battery, was synthesized by solvothermal method. The synthesized material was used as an electrode material after washing and drying, and the electrochemical performance was evaluated by using a coin cell using zinc metal as a counter electrode. In addition, the location of zinc ion insertion into VO2(B), which has not been studied previously, is confirmed through the first calculation and proved through various analysis methods. In addition, a composite with rGO was synthesized to improve the electrochemical performance, and the lifetime performance was remarkably improved. The effect of electrolyte decomposition reaction and side reaction materials formed on the electrolyte during charge and discharge processes was confirmed by ex-situ XRD and XPS.
From above result of the study, various electrode materials for ion batteries showed the same or superior electrochemical performance as those of existing commercialized materials. The various materials developed through the above studies are high cost cobalt-free electrode materials that have been pointed out as a problem in existing ion cells. Researches that can suggest new directions in the energy storage device market as electrode materials for new ion batteries to be.

목차 (Table of Contents)

• Part 1. Vanadium anode materials in lithium-ion batteries
• Chapter I. Introduction.............................................................................. 1
• 1. Theoretical background................................................................................... 1
• a. History and necessity of secondary batteries............................................. 1
• b. Principle of secondary batteries................................................................. 5
• 2. Summary of active materials........................................................................... 8
• a. Positive electrode materials....................................................................... 8
• b. Negative electrode materials..................................................................... 10
• 3. Vanadium as electrode materials..................................................................... 13
• a. Lithium anode materials. .......................................................................... 13
• b. Lithium cathode materials. ....................................................................... 14
• c. Sodium cathode materials. ........................................................................ 17
• d. Zinc electrode materials. ........................................................................... 21
• Chapter II. Experimental........................................................................... 25
• 1. Synthesis of active material powders............................................................... 25
• 2. Material characterization................................................................................. 26
• 3. Cell assembly and electrochemical characterization....................................... 27
• 4. Investigate the structural change during charge and discharge...................... 28
• Chapter III. Result and Discussion........................................................... 29
• 1. Structural analysis of synthesized powders...................................................... 29
• 2. Electrochemical properties of synthesized powders........................................ 37
• 3. Structural change during charge and discharge................................................ 42
• Chapter IV. Conclusion............................................................................. 51
• Chapter V. References............................................................................... 52
• Part 2. Vanadium cathode materials in lithium-ion batteries
• Chapter I. Experimental............................................................................ 61
• 1. Synthesis of active material powders............................................................... 61
• 2. Material characterization................................................................................. 62
• 3. Cell assembly and electrochemical characterization....................................... 63
• 4. Computational details...................................................................................... 64
• Chapter II. Result and Discussion............................................................. 65
• 1. Synthesis of active materials and material characterization............................. 65
• 2. First principle calculation of active materials.................................................. 71
• 3. Electrochemical properties of synthesized powder.......................................... 75
• 4. Reversibility of materials................................................................................. 81
• 5. Thermal properties of active materials............................................................. 92
• Chapter III. Conclusion............................................................................. 96
• Chapter IV. References.............................................................................. 98
• Part 3. Vanadium cathode material in sodium-ion batteries
• Chapter I. Experimental.......................................................................... 100
• 1. Materia preparation....................................................................................... 100
• 2. Material characterization.............................................................................. 102
• 3. Electrochemical properties........................................................................... 103
• 4. Post-cycled electrodes................................................................................... 104
• 5. Computational details................................................................................... 105
• Chapter II. Result and Discussion.......................................................... 106
• 1. Synthesis of active materials.......................................................................... 106
• 2. Electrochemical properties in Na cells.......................................................... 114
• 3. Structural investigation during Na+ deintercalation/intercalation.................. 119
• Chapter III. Conclusion........................................................................... 130
• Chapter IV. References............................................................................ 131
• Part 4. Vanadium electrode material in zinc-ion batteries
• Chapter I. Experimental............................................................................ 133
• 1. Material prepration........................................................................................... 133
• 2. Material characterization................................................................................. 134
• 3. Electrochemical properties.............................................................................. 135
• 4. Post-cycled electrodes..................................................................................... 136
• 5. Computational details...................................................................................... 137
• 6. Potentiodynamic electrochemical impedance spectroscopy........................... 138
• Chapter II. Result and Discussion............................................................. 139
• 1. Structural analysis of synthesized powders...................................................... 139
• 2. Structural investigation with first principle calculation................................... 147
• 3. Electrochemical properties in Zn cells............................................................. 151
• 4. Potentiodynamic electrochemical impedance spectroscopy............................ 155
• 5. Structural change during charge and discharge................................................ 160
• 6. Post-cycled electrodes...................................................................................... 163
• Chapter III. Conclusion............................................................................. 169
• Chapter IV. References.............................................................................. 171
• 국문초록...................................................................................................... 174
• Acknowledgements..................................................................................... 179