고성능 리튬 이온 전지를 위한 망간계 올리빈 양극물질 연구

학위논문 (박사)-- 서울대학교 대학원 : 재료공학부, 2017. 2. 박병우.Energy sources are important for the way of life in modern society, but most of the energy demand now depends on the power of nuclear and fossil fuels. This will eventually accelerate global warming and seriously deplete natural resources. As a result, it is important to develop efficient, environmentally friendly, and safe energy sources such as fuel cells and solar cells, and the development of efficient energy storage systems for storing these eco-friendly energy sources is also becoming an important issue. Among the various energy storage systems, lithium-ion batteries are attracting attention as the most realistic energy source because they have the charm of high energy density and durability. Because the performance of a battery is usually determined by electrode materials, people have been looking for a breakthrough challenge to overcome the limitations of the known materials. Conventional cathode materials such as lithium transition metal oxides (LiMO2, M = transition metals) possess intrinsic chemical instability at overcharged state. They release oxygen from the crystal structure or experience irreversible phase transformation at elevated temperature, which consequently raises safety concerns during operation. In this respect, numerous studies have been carried out in order to find a safe and stable cathode material. Among many candidates, phosphate materials have been considered as the best candidate of energy storage system for large-scale applications due to its high structural stability and safety by strong P-O covalent bonding, potentially low production cost, high energy density, and excellent cyclability. Olivine structured lithium iron phosphate (LiFePO4) has been extensively studied as a promising candidate for cathode materials of lithium-ion batteries due to its high theoretical capacity, superior structural stability, environmental benignity, and low cost. However, the LiFePO4 has relatively low redox potential (3.4 V vs. Li+/Li), which results in low energy density limiting its wider application to the market. For this reason, isostructural LiMnPO4 with higher redox potential (4.1 V vs. Li+/Li) has emerged as an alternative material for LiFePO4. Therefore, in my thesis, I focused on design of novel Mn-based olivine cathode materials (LiMn0.8Fe0.2PO4) and comprehensive analysis of the reaction mechanism of Mn and Fe in LiMn0.8Fe0.2PO4 electrodes during battery operation. In Chapter 1, the issues to overcome the limitation of olivine cathode materials for practical application are briefly introduced, mainly dealing with the development of Mn-based olivine cathode materials. In Chapter 2, electrochemically efficient micro/nano-structured LiMn0.8Fe0.2PO4 electrodes were designed by controlling synthesis parameters. I demonstrated that control of the size and shape of the LiMn0.8Fe0.2PO4 crystals as well as of the particles tendency toward oriented agglomeration (mesocrystal) is possible by applying synthesis route. Furthermore, performance enhancement of LiMn0.8Fe0.2PO4 has been realized by a morphology tailoring from ellipsoidal-shaped mesocrystals into flake-shaped mesocrystals. The origin of the enhanced electrochemical performance is investigated in terms of the primary particle size, porosity, anti-site defect concentration, and secondary particle shape. I believe that this work provides one of the routes to design electrochemically-favorable meso/nano-structures, which is of great potential for improving the battery performance by tuning the morphology of particles at the multi-length scale. A thorough understanding on the electronic structure of LiMn0.8Fe0.2PO4 can provide a guide to design high performance multi-transition-metal olivine materials, since the electronic structure comprises the electrochemical potential and structural stability of cathodes during battery operation. Thus, in Chapter 3, in order to investigate the electronic-structure effects of each transition metal (Mn and Fe) on the electrochemical performance, I performed synchrotron-based soft and hard x-ray absorption spectroscopy (sXAS and XAS), and quantitatively analyzed the changes of the transition-metal redox states in the carbon-coated LiMn0.8Fe0.2PO4 electrodes during the electrochemical reaction. I believe that our comprehensive as well as complementary analyses using ex situ sXAS and in situ XAS can provide clear experimental evidence on the reaction mechanism of LiMn0.8Fe0.2PO4 electrodes during battery operation. In chapter 4, the kinetic processes during lithiation/delithiation reaction of LixMn0.8Fe0.2PO4 were investigated through in situ x-ray diffraction (XRD) and in situ electrochemical impedance spectroscopy (EIS) combined with galvanostatic intermittent titration technique (GITT), by which unprecedented insights on the phase propagation and sluggish kinetics of LiMn0.8Fe0.2PO4 (LMFP) cathode materials are delivered. In situ analyses on the carbon-coated LMFP mesocrystal disclosed that the phase-propagation mechanism of LMFP differs during lithiation/delithiation process, and the sluggish kinetics of LMFP mesocrystal and resultant limitation of obtainable discharge capacity is featured from significant reduction of apparent Li+ diffusivity during cycling through the region governed by Mn redox reaction. Being an in-depth characterization on the in operando kinetics of LMFP mesocrystal, I believe that this work provides fundamental understandings needed for proceeding to high-performance Mn-based olivine cathodes. Finally, in Appendix 1, the graphene-wrapped LiFePO4 (LiFePO4/G) was introduced as a cathode material for Li-ion battery with an excellent rate capability. A straightforward solid-state reaction between graphene oxide-wrapped FePO4 and a lithium precursor resulted in highly conducting LiFePO4/G composites, which are featured by ~70-nm sized LiFePO4 crystallites with robust connection to external graphene network. This unique morphology enables all LiFePO4 particles to be readily accessed by electrons during battery operation, leading to remarkably enhanced rate capability. The in situ electrochemical impedance spectra were studied in detail throughout charge and discharge processes, by which enhanced electronic conductance and thereby reduced charge transfer resistance was confirmed as the origin of the superior performance in the novel LiFePO4/G.Chapter 1. Mn-Based Olivine Materials as a Cathode Material 1 1.1. Olivine Structured LiMnPO4 Cathode Materials 4 1.2. Nanosized LiMnPO4 8 1.3. Coating with Electron-Conductive Materials 11 1.4. Doping or Alloying with Other Transition Metals 13 1.5. References 17 Chapter 2. Morphology Evolution of LiMn0.8Fe0.2PO4 Mesocrystal for Advanced Cathode Materials in Lithium-Ion Batteries 21 2.1. Introduction 21 2.2. Experimental Section 24 2.3. Results and Discussion 27 2.4. Conclusions 66 2.5. References 67 Chapter 3. Synchrotron-Based X-Ray Absorption Spectroscopy for the Electronic Structure of LixMn0.8Fe0.2PO4 Mesocrystal in Li+ Batteries 73 3.1. Introduction 73 3.2. Experimental Section 76 3.3. Results and Discussion 79 3.4. Conclusions 113 3.5. References 114 Chapter 4. Insights on the Delithiation/Lithiation Reactions of LixMn0.8Fe0.2PO4 Mesocrystal in Li-Ion Batteries by In Situ Techniques 124 4.1. Introduction 124 4.2. Experimental Section 127 4.3. Results and Discussion 130 4.4. Conclusions 161 4.5. References 162 Appendix 168 국문 초록 215Docto

The role of carbon incorporation in SnO2 nanoparticles for Li rechargeable batteries

Since carbothermal reduction of SnO2 occurs above 600 degrees C, carbon-coating experiments using various polymer precursors have been carried out at relatively low temperatures (similar to 500 degrees C). It is not likely, however, that the carbon synthesized at similar to 500 degrees C much enhances the conductivity of SnO2 anodes, because polymer precursors have undergone insufficient carbonization. This article confirms that the main role of carbon coating is sustaining the domain of each Sn nanoparticle by preventing its aggregation, and thereby improving the cycling performance of SnO2 nanoparticles. The transmission electron microscopy after cycling showing well dispersed Sn nanoparticles and electrochemical impedance spectroscopy revealing larger charge-transfer resistances with increasing carbon contents are in line with these interpretations.close201

Stable Solid Electrolyte Interphase Layer Formed by Electrochemical Pretreatment of Gel Polymer Coating on Li Metal Anode for Lithium???Oxygen Batteries

Lithium-oxygen (Li-O-2) batteries exhibit the highest theoretical specific energy density among candidates for next-generation energy storage systems, but the instabilities of Li metal anode (LMA), air electrode, and electrolyte largely limit the practical applications of these batteries. Herein, we report an effective method to protect the LMA against side reactions between the LMA and the crossover contaminants such as highly reactive oxygen moieties. A solid electrolyte interphase (SEI) layer rich in inorganic components is formed on the LMA coated with poly(ethylene oxide) thin film through an in situ electrochemical precharging step under oxygen atmosphere. This uniformly distributed SEI layer interacts with the flexible polymer matrix and forms a submicrometer-sized gel-like polymer layer. This polymer-supported SEI layer leads to much longer cycle life (130 vs 65 cycles) as compared to that of pristine cells under the same testing conditions. It is also very effective to stabilize the LMA/electrolyte interphase with a redox mediator

Electronic Effect in Methanol Dehydrogenation on Pt Surfaces: Potential Control during Methanol Electrooxidation

Establishing a relationship between the catalytic activity and electronic structure of a transition-metal surface is important in the prediction and design of a new catalyst in fuel cell technology. Herein, we introduce a novel approach for identifying the methanol oxidation reactions, especially focusing on the effect of the Pt electronic structure on methanol dehydrogenation. By systematically controlling the electrode potential, we simplified the reaction paths, excluding other unfavorable effects, and thereby obtained only the methanol dehydrogenation activity in terms of the electronic structure of the Pt surface. We observed that the methanol dehydrogenation activity of Pt decreases when the position of the d-band center relative to the Fermi level is lower, and this fundamental relation provides advanced insight into the design of an optimal catalyst as the anode for direct methanol fuel cells

Optimum Morphology of Mixed-Olivine Mesocrystals for a Li-Ion Battery

In this present work, we report on the synthesis of micron-sized LiMn_0.8Fe_0.2PO₄ (LMFP) mesocrystals via a solvothermal method with varying pH and precursor ratios. The morphologies of resultant LMFP secondary particles are classified into two major classes, flakes and ellipsoids, both of which are featured by the mesocrystalline aggregates where the primary particles constituting LMFP secondary particles are crystallographically aligned. Assessment of the battery performance reveals that the flake-shaped LMFP mesocrystals exhibit a specific capacity and rate capability superior to those of other mesocrystals. The origin of the enhanced electrochemical performance is investigated in terms of primary particle size, pore structure, antisite-defect concentration, and secondary particle shape. It is shown that the shape of the secondary particle has just as much of a significant effect on the battery performance as the crystallite size and antisite defects do. We believe that this work provides a rule of design for electrochemically favorable meso/nanostructures, which is of great potential for improving battery performance by tuning the morphology of particles on multilength scales

Preparation and Exceptional Lithium Anodic Performance of Porous Carbon-Coated ZnO Quantum Dots Derived from a Metal–Organic Framework

Hierarchically porous carbon-coated ZnO quantum dots (QDs) (∼3.5 nm) were synthesized by a one-step controlled pyrolysis of the metal–organic framework IRMOF-1. We have demonstrated a scalable and facile synthesis of carbon-coated ZnO QDs without agglomeration by structural reorganization. This unique microstructure exhibits outstanding electrochemical performance (capacity, cyclability, and rate capability) when evaluated as an anode material for lithium ion batteries

Coordination-Dependent Chemical Reactivity of TFSI Anions at a Mg Metal Interface

Charge transfer across the electrode–electrolyte interface is a highly complex and convoluted process involving diverse solvated species with varying structures and compositions. Despite recent advances in in situ and operando interfacial analysis, molecular specific reactivity of solvated species is inaccessible due to a lack of precise control over the interfacial constituents and/or an unclear understanding of their spectroscopic fingerprints. However, such molecular-specific understanding is critical to the rational design of energy-efficient solid–electrolyte interphase layers. We have employed ion soft landing, a versatile and highly controlled method, to prepare well-defined interfaces assembled with selected ions, either as solvated species or as bare ions, with distinguishing molecular precision. Equipped with precise control over interfacial composition, we employed in situ multimodal spectroscopic characterization to unravel the molecular specific reactivity of Mg solvated species comprising (i.e., bis(trifluoromethanesulfonyl)imide, TFSI–) anions and solvent molecules (i.e., dimethoxyethane, DME/G1) on a Mg metal surface relevant to multivalent Mg batteries. In situ multimodal spectroscopic characterization revealed higher reactivity of the undercoordinated solvated species [Mg-TFSI-G1]+ compared to the fully coordinated [Mg-TFSI-(G1)2]+ species or even the bare TFSI–. These results were corroborated by the computed reaction pathways and energy barriers for decomposition of the TFSI– within Mg solvated species relative to bare TFSI–. Finally, we evaluated the TFSI reactivity under electrochemical conditions using Mg(TFSI)2–DME-based phase-separated electrolytes representing different solvated constituents. Based on our multimodal study, we report a detailed understanding of TFSI– decomposition processes as part of coordinated solvated species at a Mg-metal anode that will aid the rational design of improved sustainable electrochemical energy technologies