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Wi, Sungun,Park, Jungjin,Lee, Sangheon,Kang, Joonhyeon,Hwang, Taehyun,Lee, Kug-Seung,Lee, Han-Koo,Nam, Seunghoon,Kim, Chunjoong,Sung, Yung-Eun,Park, Byungwoo Elsevier 2017 Nano energy Vol.31 No.-
<P><B>Abstract</B></P> <P>The carbon-coated LiMn<SUB>0.8</SUB>Fe<SUB>0.2</SUB>PO<SUB>4</SUB> (LMFP) mesocrystal, composed of ~40-nm-sized nanocrystallites, was designed to be favorable for the fast charge transport kinetics. The carbon-coated LMFP mesocrystal exhibited good electrochemical properties (i.e., high specific capacity and superior rate capability), ensuring that the LMFP mesocrystal is a proper model system to study the reaction mechanism upon the battery cycling. In order to investigate the electronic-structure effects of each transition metal (Mn and Fe) on the electrochemical performance, we 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 LMFP electrodes during the electrochemical reaction. We believe that our comprehensive as well as complementary analyses using <I>ex situ</I> sXAS and <I>in situ</I> XAS can provide clear experimental evidence on the reaction mechanism of LiMn<SUB>0.8</SUB>Fe<SUB>0.2</SUB>PO<SUB>4</SUB> electrodes during battery operation.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Mesocrystal LiMn<SUB>0.8</SUB>Fe<SUB>0.2</SUB>PO<SUB>4</SUB> synthesized by a straightforward solvothermal method. </LI> <LI> High specific capacity and superior rate capability. </LI> <LI> Quantitative analysis of synchrotron-based soft and hard x-ray absorption spectroscopy. </LI> <LI> Clear understanding on the reaction mechanism of Mn and Fe in LiMn<SUB>0.8</SUB>Fe<SUB>0.2</SUB>PO<SUB>4</SUB>. </LI> </UL> </P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>
Wi, Sungun,Park, Jungjin,Lee, Sangheon,Kim, Jaewon,Gil, Bumjin,Yun, Alan Jiwan,Sung, Yung-Eun,Park, Byungwoo,Kim, Chunjoong Elsevier 2017 Nano energy Vol.39 No.-
<P><B>Abstract</B></P> <P>The kinetic processes during delithiation/lithiation of Li<SUB> <I>x</I> </SUB>Mn<SUB>0.8</SUB>Fe<SUB>0.2</SUB>PO<SUB>4</SUB> are thoroughly investigated through <I>operando</I> x-ray diffraction and <I>in situ</I> electrochemical impedance spectroscopy combined with galvanostatic intermittent titration technique (GITT), by which new insights on the phase propagation and sluggish kinetics of LiMn<SUB>0.8</SUB>Fe<SUB>0.2</SUB>PO<SUB>4</SUB> (LMFP) cathode materials are elaborated. <I>In situ</I> analyses on the solvothermally synthesized carbon-coated LMFP mesocrystals reveal that the phase-propagation mechanisms differ during delithiation/lithiation processes, and the sluggish kinetics of LMFP followed by the limitation of achievable (dis)charge capacities originate from the poor apparent Li<SUP>+</SUP> diffusivity, which is triggered by Mn redox reaction. Based on the in-depth characterization of the reaction kinetics in LMFP mesocrystals, our work provides fundamental understanding to design high-performance Mn-based olivine cathodes.</P> <P><B>Highlights</B></P> <P> <UL> <LI> LiMn<SUB>0.8</SUB>Fe<SUB>0.2</SUB>PO<SUB>4</SUB> mesocrystals synthesized by a straightforward solvothermal method. </LI> <LI> Superior electrochemical properties (i.e., high specific capacity and rate capability). </LI> <LI> Fundamental understanding on the reaction kinetics of LMFP by <I>in situ</I> techniques. </LI> </UL> </P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>
Wi, Sungun,Woo, Hyungsub,Lee, Sangheon,Kang, Joonhyeon,Kim, Jaewon,An, Subin,Kim, Chohui,Nam, Seunghoon,Kim, Chunjoong,Park, Byungwoo Springer US 2015 NANOSCALE RESEARCH LETTERS Vol.10 No.1
<P>The reduced graphene oxide (RGO)/carbon double-coated 3-D porous ZnO aggregates (RGO/C/ZnO) have been successfully synthesized as anode materials for Li-ion batteries with excellent cyclability and rate capability. The mesoporous ZnO aggregates prepared by a simple solvothermal method are sequentially modified through distinct carbon-based double coating. These novel architectures take unique advantages of mesopores acting as space to accommodate volume expansion during cycling, while the conformal carbon layer on each nanoparticle buffering volume changes, and conductive RGO sheets connect the aggregates to each other. Consequently, the RGO/C/ZnO exhibits superior electrochemical performance, including remarkably prolonged cycle life and excellent rate capability. Such improved performance of RGO/C/ZnO may be attributed to synergistic effects of both the 3-D porous nanostructures and RGO/C double coating.</P><P><B>Electronic supplementary material</B></P><P>The online version of this article (doi:10.1186/s11671-015-0902-7) contains supplementary material, which is available to authorized users.</P>
Woo, Hyungsub,Wi, Sungun,Kim, Jaewon,Kim, Jinhyun,Lee, Sangheon,Hwang, Taehyun,Kang, Joonhyeon,Kim, Jaewook,Park, Kimin,Gil, Bumjin,Nam, Seunghoon,Park, Byungwoo Elsevier 2018 Carbon Vol.129 No.-
<P><B>Abstract</B></P> <P>Among the efforts to apply SnO<SUB>2</SUB> as an anode, the adoption of carbonaceous materials has been considered as a decent strategy to mitigate volume expansion problem (∼300%) during cycling. Nevertheless, it still needs in-depth examinations to identify the individual role of each coating material and further improvements for practical applications. To understand the underlying correlations of various carbon coatings with electrochemical performance of active materials, disordered carbon and reduced graphene oxide (RGO) are selectively used for SnO<SUB>2</SUB> hollow spheres. The disordered carbon, which covered the surfaces of and voids between the primary particles, acts as a buffer layer for volume expansion, and the RGO, that interconnected the hollow secondary particles, provides a 2D-electronic path to the electrode. Finally, both of them are utilized on the SnO<SUB>2</SUB> hollow spheres, namely the double coating is conducted from the expectation of synergistic effects, and it successfully exhibits a moderate capacity after 100 cycles even at 1 C with a low carbon content (7.7 wt. %). The essential factors that are inherently present and thereby significantly affect the electrochemical performance of the SnO<SUB>2</SUB> electrode are successfully identified by a facile dual-carbon modification, so that this strategy will be applicable to other potential active materials.</P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>
Effective wrapping of graphene on individual Li4Ti5O12grains for high-rate Li-ion batteries
Oh, Yuhong,Nam, Seunghoon,Wi, Sungun,Kang, Joonhyeon,Hwang, Taehyun,Lee, Sangheon,Park, Helen Hejin,Cabana, Jordi,Kim, Chunjoong,Park, Byungwoo The Royal Society of Chemistry 2014 Journal of Materials Chemistry A Vol.2 No.7
Optimum Morphology of Mixed-Olivine Mesocrystals for a Li-Ion Battery
Park, Kimin,Kim, Jaewon,Wi, Sungun,Lee, Sangheon,Hwang, Taehyun,Kim, Jaewook,Kang, Joonhyeon,Choi, Joon-Phil,Nam, Seunghoon,Park, Byungwoo American Chemical Society 2018 Inorganic Chemistry Vol.57 No.10
<P>In this present work, we report on the synthesis of micron-sized LiMn<SUB>0.8</SUB>Fe<SUB>0.2</SUB>PO<SUB>4</SUB> (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.</P><P>Micron-sized LiMn<SUB>0.8</SUB>Fe<SUB>0.2</SUB>PO<SUB>4</SUB> mesocrystals having various morphologies were facilely synthesized via a solvothermal method. The origin of the different electrochemical performances of the mesocrystals was investigated in terms of primary particle size, pore structure, antisite-defect concentration, and secondary particle shape. We believe that this work can provide a rule of design for the electrochemically favorable meso/nanostructures, which is of great potential for improving battery performance.</P> [FIG OMISSION]</BR>
Nanoscale Interface Control for High-Performance Li-Ion Batteries
Yuhong Oh,Seunghoon Nam,Sungun Wi,Saeromi Hong,Byungwoo Park 대한금속·재료학회 2012 ELECTRONIC MATERIALS LETTERS Vol.8 No.2
Li-ion batteries have attracted great interest for the past decades, and now are one of the most important power sources for portable electronic devices, store electricity, hybrid electric vehicles (HEV), etc. However,Li-ion-battery technologies still have several problems related to the electrochemical performance (cycle-life performance and power density) or safety of the active electrode materials. There have been numerous break-through challenges to overcome these problems by extensive research. Among the various methods to improve the battery’s electrochemical properties, nanoscale coating on active materials and control of the nanostructured morphology were proven as effective approaches over the last decade. In this review paper, enhanced elec-trochemical properties of the cathode and anode materials via nanoscale interface modification and the respective enhancing mechanisms will be discussed.
Kang, Joonhyeon,Kim, Jinyoung,Lee, Sangheon,Wi, Sungun,Kim, Chunjoong,Hyun, Seungmin,Nam, Seunghoon,Park, Yongjoon,Park, Byungwoo Wiley 2017 ADVANCED ENERGY MATERIALS Vol.7 No.19
<P>This paper introduces oxygen-deficient black TiO2 with hierarchically ordered porous structure fabricated by a simple hydrogen reduction as a carbon- and binder-free cathode, demonstrating superior energy density and stability. With the high electrical conductivity derived from oxygen vacancies or Ti3+ ions, this unique electrode features micrometer-sized voids with mesoporous walls for the effective accommodation of Li2O2 toroid and for the rapid transport of reaction molecules without the electrode being clogged. In the highly ordered architecture, toroidal Li2O2 particles are guided to form with a regular size and separation, which induces the most of Li2O2 external surface to be directly exposed to the electrolyte. Therefore, large Li2O2 toroids (approximate to 300 nm) grown from solution can be effectively charged by incorporating a soluble catalyst, resulting in a very small polarization (approximate to 0.37 V). Furthermore, disordered nanoshell in black TiO2 is suggested to protect the oxygen-deficient crystalline core, by which oxidation of Ti3+ is kinetically impeded during battery operation, leading to the enhanced electrode stability even in a highly oxidizing environment under high voltage (approximate to 4 V).</P>
Kim, Jaewon,Lee, Kyung Eun,Kim, Kyung Hwan,Wi, Sungun,Lee, Sangheon,Nam, Seunghoon,Kim, Chunjoong,Kim, Sang Ouk,Park, Byungwoo Elsevier 2017 Carbon Vol.114 No.-
<P>Graphene has been intensively adopted into boosting the electrochemical performances of battery electrode materials due to its superior nature. In the case of Li4Ti5O12 (LTO), the application of graphene has been specifically focused on ameliorating the low electronic conductivity of LTO. So far, these attempts aiming to increase the composite's electronic conductivity involved thick graphene layers, which inevitably hindered Li-ion diffusion and eventually harmed the electrochemical kinetics in LTO's surface. In this work, high quality minimum-impurity graphene oxide was prepared by means of thorough cleaning and dialysis, which enabled each single-layered graphene to successfully wrap individual LTO particles. The resulting single-layer graphene-wrapped LTO exhibits an excellent specific capacity of 130 mAh g(-1) even at a lithiation/delithiation of 30 degrees C. Such a high rate capability is one of the highest values among the reported LTO with comparable sizes (similar to 200 nm). To uncover the reasons for such high performance, electrochemical properties from varied graphene contents were juxtaposed for comparison, and as a result, number of graphene layers and the corresponding kinetic parameters were found correlated. With adequate validity, single graphene layer was revealed to be the uttermost optimum for both Li+ diffusion and electronic conduction. (C) 2016 Elsevier Ltd. All rights reserved.</P>