금속–기체 전지 화학을 이용한 차세대 대용량 이차전지 개발에 관한 연구

학위논문 (박사)-- 서울대학교 대학원 : 공과대학 재료공학부, 2019. 2. 강기석.Nowadays, the demands for energy storage devices are explosively increasing with the market growth of energy storage applications including electric vehicles (EV) and large-scale energy storage systems (ESS). Li ion batteries are now regarded as the state-of-art energy storage chemistry owing to their high energy density and power density which satisfies requirements for the power sources of current portable electronic devices. However, current energy density of Li ion batteries is insufficient to be utilized in huge applications including EV and ESS because of the use of heavy transition metal compounds and limited storage capability of current cathode materials. Many research efforts have been devoted to discovery on next-generation energy storage chemistries (including Li–O2, Li–S, and Na ion batteries etc.) which can outperform current Li ion batteries. Among them, Li–O2 batteries have attracted enormous attentions owing to the extraordinary high theoretical energy density with the absence of heavy elements. However, poor efficiency and reversibility in Li–O2 chemistry hinders the practical realization of Li–O2 batteries to date. In this thesis, I explore novel metal–gas chemistry for devising new rechargeable batteries. Coupling various gas-phase actve materials with counter metal electrode can be an alternative to develop highly efficient and reversible secondary batteies, taking knowledge from the historical background and fundamental understnading on the Li–O2 batteries. Here, I invent a new secondary battery chemistry by revisiting primary Li–SO2 systems and develop high-performing Li–SO2 batteries based on in-depth understanding on the energy storage mechanism. In addition, I enlighten a superoxide chemistry in recent emerging Na–O2 batteries by addressing the chemical behaviors of discharge products and develop highly durable Na–O2 batteries through the introduction of advanced electrolytes to control the discharge product. Chapter 2 shed a new light on old primary Li–SO2 battery chemistry as a way to devise a new secondary battery. Although primary Li–SO2 battery has been only believed as a primary battery due to the foramtion of solid discharge prodcuts, a rechargeability of Li–SO2 battery is demonstrated based on the reversible formation and decomposition of Li2S2O4 discharge product. Novel rechargeable Li–SO2 battery with the operation voltage of ~2.8 V and capacity of 5,400 mA h g-1 exhibits higher energy efficiency and cell reversibility compared to the Li–O2 chemistry. Based on in depth mechanism studies on the critical role of electrolytes properties, conventional carbonate-based electrolytes, which have been widely used for practical Li ion batteries but not for current Li–O2 battery, are exploited in Li–SO2 rechargeable batteries. Li–SO2 battery with carbonate-based electrolytes presents superior electrochemical properties including high power and reversibility. Application of soluble catalysts into newly developed Li–SO2 battery is also demonstrated with achieving one of the most outstanding performances among previous Li–gas type batteries. Chapter 3 reveals the origin of discrepency in discharge products and underlying reaction mehcanism of Na–O2 batteries. NaO2 chemistry has recently attracted many interests owing to extremely low charge polarizations about 200 mV of Na–O2 batteries. It is unveiled that the spontaneous dissolution and ionization of primary discharge product NaO2 liberates the free O2- in the electrolyte and promotes side reactions involving the formation of Na2O2·2H2O. The chemical phase transition results in higher polarization for charge process of Na–O2 batteries with severe deterioration of cell efficiency and reversibility. On the basis of the mechanism, rational tuning of electrolyte is addressed to prevent the dissolution of NaO2. The introduction of concentrated electrolytes with little amount of free solvents is verified to suppress chemical product transition from NaO2 to Na2O2·2H2O during storage period of Na–O2 batteries. Highly durable Na–O2 batteries is developed for longer shelf-life through the stabilization of NaO2 with the use of concentrated electrolytes. I believe that this thesis can open up a new research frontier of metal–gas type secondary high-energy battery and provide insights on the fundamental understanding of the energy storage mechanism of metal–gas batteries. The revisit study on Li–SO2 battery in this thesis can also offer an avenue to devise a new rechargeable battery chemistry. In addition, successful electrolytes design and catalysts engineering in this thesis also provide guidelines how to develop high-performing metal–gas type rechargeable batteries.최근 전기자동차 등의 에너지 저장 기술 기반 시장의 성장과 함께 에너지 저장 장치에 대한 수요가 폭발적으로 증가하고 있다. 현재 최첨단 에너지 저장 기술로 여겨지는 리튬이온전지는 높은 에너지 밀도와 출력을 바탕으로 다양한 소형 전자기기의 전력 공급원으로 사용되고 있다. 하지만 전지의 양극 소재 내 무거운 전이 금속과 제한된 저장 용량으로 인해, 현재 리튬이온전지의 에너지 밀도는 전기자동차와 같은 대형 에너지 저장 기술에 적용되기에 부족하다. 이에 리튬이온전지의 성능을 뛰어 넘는 차세대 전지 화학(리튬-공기, 리튬-황, 소듐 전지 등)을 개발하기 위한 많은 연구들이 진행되어 왔다. 그 중 리튬-공기 전지는 무거운 원소를 활용하지 않으며 매우 높은 이론 에너지 밀도를 갖고 있어 차세대 전지로 상당한 주목을 받고 있다. 하지만, 리튬과 산소의 화학 반응의 낮은 효율과 가역성으로 인해 실제 상용 전지로서 차세대 리튬-공기 전지의 개발에 어려움을 겪고 있다. 본 논문에서는, 새로운 금속과 기체의 전지 화학을 탐구하여 차세대 대용량 이차 전지를 개발하는 방법을 제시한다. 기존 리튬-공기 전지의 개발 배경과 원론적인 이해를 바탕으로, 전기화학 활성을 나타내는 다양한 기체 활물질을 알칼리 금속과 조합하여 전기화학 전지를 구성하면 높은 효율과 가역성의 전지를 개발하는 한 방법이 될 수 있다. 본 논문에서 기존 일차 리튬-이산화황 일차전지를 재조명하여 새로운 이차 전지를 제안하고, 에너지 저장 기작 규명과 함께 고성능의 리튬-이산화황 전지를 개발했다. 또한, 최근 각광 받고 있는 나트륨-공기 전지의 초과산화물 반응 기작을 밝혀내고, 반응 생성물을 제어할 수 있는 전해질을 도입하여 화학적으로 안정성이 높은 나트륨-공기 전지 시스템을 개발했다. 2장에서는, 새로운 이차 전지를 개발하기 위해 기존 리튬-이산화황 일차전지를 재조명한다. 리튬-이산화황 전지는 고체 방전 산물의 생성으로 인해 재충전이 불가능하다고 여겨졌으나 본 연구에서 아이티온산리튬의 가역적인 생성과 분해를 통한 리튬-이산화황 전지의 충전 가능성을 증명했다. 리튬-이산화황 이차 전지는 약 2.8 V의 반응 전압과 5,400 mA h g-1의 용량을 발현하며, 기존 리튬-공기 전지 보다 높은 에너지 효율과 전지 가역성을 나타냈다. 메커니즘에 대한 심도 있는 연구를 바탕으로 리튬이온전지에 사용되는 상용 탄산염계 전해질을 리튬-이산화황 전지에 도입해냈다. 탄산염계 전해질을 사용한 전지는 출력과 가역성 등에서 우수한 성능을 나타냈다. 액상 촉매 또한 적용되어 리튬-기체 형태 전지들 중 가장 우수한 수준의 전지 성능을 보이는 리튬-이산화황 전지를 개발했다. 3장에서는, 나트륨-공기 전지에서 발견되는 이종 반응 산물의 원인과 그 메커니즘에 대해 밝혀냈다. 매우 낮은 충전 과전압으로 충전이 가능하기 때문에 나트륨초과산화물 기반 전지는 최근 많은 관심을 받고 있다. 본 연구에서 나트륨초과산화물이 전해질 내 용해 및 해리되어 자유 산소 라디칼을 발생시키고 수화된 나트륨과산화물을 형성하는 부반응을 촉진한다는 것을 밝혀냈다. 이러한 부반응은 충전 시 높은 과전압을 요하며 전지 성능의 급격한 열화를 초래한다. 이에 메커니즘에 대한 이해를 바탕으로 나트륨초과산화물의 용해를 막기 위한 전해질을 탐색했다. 고농도의 전해질을 도입하여 나트륨초과산화물의 부반응을 억제할 수 있음을 규명해냈고, 높은 저장 수명을 나타내는 높은 화학 안정성의 나트륨-공기 전지를 개발해냈다. 본 논문은 금속-기체 타입의 새로운 이차전지라는 연구 방향을 제시해 줄 뿐만 아니라 에너지 저장 메커니즘을 이해하는데 영감을 줄 수 있을 것으로 기대된다. 또한 본 연구에서 기존 전지에 대해 재조명한 연구 방식은 새로운 이차 전지를 개발하는 하나의 방안을 제시해주며, 전해질 및 촉매 개발 등의 연구는 금속-기체 전지 성능 향상을 위한 연구 방향을 제시해 줄 것으로 기대된다.List of Tables List of Figures Chapter 1. Introduction 1.1. Motivation and objectives 1.2. Introduction to metal-air battery 1.3. References Chapter 2. Evolution of Li-SO2 secondary battery 2.1 Revisiting Li-SO2 primary batteries for rechargeable systems 2.1.1 Research background 2.1.2 Experimental method 2.1.2.1 Preparation of Li-SO2 cells 2.1.2.2 Electrochemical characterization and analyses 2.1.3 Results and discussions 2.1.4. Concluding remarks 2.1.5. References 2.2 High-efficiency and high-power rechargeable Li–SO2 batteries exploiting conventional carbonate-based electrolytes 2.2.1 Research background 2.2.2 Experimental method 2.2.2.1 Computational details 2.2.2.2 Preparation and assembly of Li–SO2 cells 2.2.2.3 Characterization of Li–SO2 cells 2.2.3 Results and discussions 2.2.3.1 Theoretical investigation of Li–SO2 chemistry 2.2.3.2 Feasibility of Li–SO2 chemistry in carbonate electrolytes 2.2.3.3 Performance of Li–SO2 cells using carbonate electrolytes 2.2.3.4 Discussion 2.2.4. Concluding remarks 2.2.5. References Chapter 3. Developing highly durable Na-O2 battery 3.1 Chemical and electrochemical behaviors of NaO2 in Na–O2 batteries 3.1.1 Research background 3.1.2 Experimental method 3.1.2.1 Cell assembly and galvanostatic cycling of Na–O2 cells 3.1.2.2 Characterization of Na–O2 cells 3.1.2.3 Theoretical calculations of solvation energy 3.1.3 Results and discussions 3.1.3.1 Electrochemical profile of Na–O2 batteries 3.1.3.2 Time-resolved characterization of discharge products 3.1.3.3 Morphological change of discharge products over time 3.1.3.4 Dissolution and ionization of NaO2 3.1.3.5 Proposed mechanism of Na–O2 batteries 3.1.4. Concluding remarks 3.1.5. References 3.2 Highly durable and stable NaO2 in concentrated electrolytes for Na–O2 batteries 3.2.1 Research background 3.2.2 Experimental method 3.2.2.1 Materials and cell assembly 3.2.2.2 Characterization of Na–O2 cells 3.2.3 Results and discussions 3.2.3.1 Chemical instability of NaO2 on electrochemistry 3.2.3.2 Physicochemical properties of concentrated electrolytes 3.2.3.3 Prolonged lifetimes of NaO2 in concentrated electrolytes 3.2.3.4 Reversibility of Na–O2 batteries 3.2.4. Concluding remarks 3.2.5. References Chapter 4. Conclusion Chapter 5. Abstract in Korean Curriculum VitaeDocto

Magnetic order and frustrated dynamics in Li(Ni0.8Co0.1Mn0.1)O2: a study by {\mu}+SR and SQUID magnetometry

Recently, the mixed transition metal oxides of the form Li(Ni1-y-zCoyMnz)O2, have become the center of attention as promising candidates for novel battery material. These materials have also revealed very interesting magnetic properties due to the alternate stacking of planes of metal oxides on a 2D triangular lattice and the Li-layers. The title compound, Li(Ni0.8Co0.1Mn0.1)O2, has been investigated by both magnetometry and measurements and {\mu}+SR. We find the evolution of localized magnetic moments with decreasing temperature below 70 K. The magnetic ground state (T = 2 K) is, however, shown to be a frustrated system in 3D, followed by a transition into a possible 2D spinglass above 22 K. With further increasing temperature the compound show the presence of remaining correlations with increasing effective dimensionality all the way up to the ferrimagnetic transition at TC = 70 K.Comment: Accepted for publication in Physics Procedia (muSR2011 Conference

Capillary based Li-air batteries for <i>in situ</i> synchrotron X-ray powder diffraction studies

A novel design for in situ X-ray diffraction Li–O2 battery reveals the crystallographic details for the precipitation and decomposition of Li2O2 for the 1st and 2nd cycles of the battery.</p

Tailoring asymmetric discharge-charge rates and capacity limits to extend Li-O2 battery cycle life

Widespread issues with the fundamental operation and stability of Li-O2 cells impact cycle life and efficiency. While the community continues to research ways of mitigating side reactions and improving stability to realize Li-O2 battery prospects, we show that limiting the depth-of-discharge while unbalancing discharge/charge rate symmetry can extend Li-O2 battery cycle life by ensuring efficient reversible Li2O2 formation, markedly improving cycle life. Systematic variation of the discharge/charge currents shows that clogging from discharging the Li-O2 cell at high current (250 μA) can be somewhat negated by recharging with a lower applied current (50 μA), with a marked improvement in cycle life achievable. Our measurements determined that specific reduction of the depth of discharge in decrements from equivalent capacities of 1000 mAhg-1 to 50 mAhg-1 under symmetric discharge/charge currents of 50 μA strongly affected the cumulative discharge capacity of each cell. A maximum cumulative discharge capacity was found to occur at ~10 % depth of discharge (500 mAhg-1) and the cumulative discharge capacity of 39,500 mAhg-1 was significantly greater than cells operated at higher and lower depths of discharge. The results emphasize the importance of appropriate discharge/charge rate and depth of discharge selection for other cathode/electrolyte combinations for directly improving cycle life performances of Li-O2 batteries

Experimental and Numerical Investigations of Wettability of Positive Electrodes for Li−O2 Batteries

The objective of this dissertation is to characterize the positive electrode wettability and its effects on the performance (e.g., discharge capacity) of Li−O2 batteries. The investigations include an experimental study of discharging electrodes with various wettabilities, proposing and examining the intermittent discharge strategy, and the numerical simulation of the distribution of the electrolyte at various saturations and of the discharge performance of Li−O2 batteries at the pore scale. Future work will measure the structure of positive electrodes using advanced imaging technology such as transmission X-ray microscopy. First, I fabricated the electrodes and adjusted their wettability by mixing acetylene black carbon particles with various binders. The wettability was quantitatively characterized by the contact angle and ionic resistance. The customized electrodes were then discharged in Li−O2 batteries at 0.1 mA/cm2 through which the relationship between electrode wettability and discharge capacity was obtained. The discharge capacity of the electrode with 15% PVDF (36.5°) binder was 1665.8 mAh/g while the customized electrode with 15% PTFE (128.4°) binder had a discharge capacity of 4160.8 mAh/g. The effects of lyophobicity on O2 transfer in the porous electrode have been proved. A positive electrode with mixed wettability was designed and tested, which acquired the highest specific discharge capacity of 5149.5 mAh/g. The structure of this electrode included two lyophobic carbon coatings on top and bottom and one lyophilic carbon coating in the middle. Further design may focus on appropriately configuring the wettability to balance the gas paths for O2 diffusion and wetted area for reaction sites. A novel strategy for discharging Li−O2 batteries was then proposed and identified. The battery was periodically discharged and rested, which can enhance O2 availability and increase the discharge capacity. Periodically resting the battery increased the specific discharge capacity by at least 50% at various current densities (0.1 - 1.5 mA/cm2). Afterward, the investigation combined the electrode wettability and the intermittent strategy. Compared with the continuous strategy, the capacity of lyophobic electrodes increased by over 100% when the intermittent strategy was applied. Besides, a multi-step discharge strategy can provide greater capacity when the battery is discharged at decreasing current rates (2.0, 1.5, and 1.0 mA/cm2). The importance of O2 diffusion is emphasized and provide practical strategies are proposed to improve the deep discharge capacity of Li-O2 batteries, especially at high current rates (> 1.0 mA/cm2). Finally, a numerical study was conducted to investigate the electrode with different saturations of the electrolyte. The effects of electrolyte saturation levels and the distribution of electrolyte have been demonstrated by comparing the corresponding discharge performance of Li-O2 batteries. It was found that fully saturated electrodes (100% saturation) have high oxygen transfer resistance, which will result in the lowest discharge capacity of 7.41 Ah/g. On the contrary, over-dried battery (with 1.0 mA/cm2). Finally, a numerical study was conducted to investigate the electrode with different saturations of the electrolyte. The effects of electrolyte saturation levels and the distribution of electrolyte have been demonstrated by comparing the corresponding discharge performance of Li-O2 batteries. It was found that fully saturated electrodes (100% saturation) have high oxygen transfer resistance, which will result in the lowest discharge capacity of 7.41 Ah/g. On the contrary, over-dried battery (with 7 Ah/g) at high current (20 A/m2) similar to hydrophilic electrodes which are fully saturated by the electrolyte at low current (1 A/m2). The modeling study found that designing the electrode with a mixture of lyophilic and lyophobic pores is critical to significantly increasing (by orders of magnitude) the operating current and power of the Li–O2 battery. In the future, plans are to characterize the geometry of the positive electrode using the imaging techniques (e.g., transmission X-ray microscopy) and gas sorption method. Based on the characterization of the porous structure, the relationship between the porous structure and the mass transport phenomena will be clarified

Toward Reversible and Moisture-Tolerant Aprotic Lithium-Air Batteries

The development of moisture-tolerant, LiOH-based non-aqueous Li-O2 batteries is a promising route to bypass the inherent limitations caused by the instability of their typical discharge products, LiO2 and Li2O2. The use of the I−/I3− redox couple to mediate the LiOH-based oxygen reduction and oxidation reactions has proven challenging due to the multiple reaction paths induced by the oxidation of I− on cell charging. In this work, we introduce an ionic liquid to a glyme-based electrolyte containing LiI and water and demonstrate a reversible LiOH-based Li-O2 battery cycling that operates via a 4 e−/O2 process with a low charging overpotential (below 3.5 V versus Li/Li+). The addition of the ionic liquid increases the oxidizing power of I3−, shifting the charging mechanism from IO−/IO3− formation to O2 evolution