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Choi, Daehyeon,Kang, Joonhee,Han, Byungchan Elsevier 2019 ELECTROCHIMICA ACTA Vol.294 No.-
<P><B>Abstract</B></P> <P>Conventionally neglected mechanism of reversible redox reactions by oxygen ions in lithium-nickel oxide materials (LNO; Li<SUB>2<I>x</I> </SUB>Ni<SUB>2-2<I>x</I> </SUB>O<SUB>2</SUB>, 0 < <I>x</I> < 1) is proposed as a primary cause of unexpectedly high energy density of Li-ion battery. Using first-principles density functional theory calculations, cluster expansion theory, and Monte Carlo simulations, we unveil the underlying mechanism that is ascribed to the phase transition between layered and rocksalt structures initiated by cation disordering of Li and Ni at certain Li composition. At <I>x</I> = 0.5, the oxygen ions are put under specific chemical bondings of straight-linear type LiOLi configuration. They enable an active oxygen redox reaction involving peroxo (O<SUB>2</SUB> <SUP>2−</SUP>) and superoxo (O<SUB>2</SUB> <SUP>−</SUP>) ions, shown to dramatically increase the energy density of the LNO cathode. Using Monte Carlo simulations, we identify the proportional information to find the LiOLi configurations around the synthetic temperature of LNO materials. Our results indicate that the cation-disorder is a driving force for the oxygen redox via formation of the specific bondings. On the basis of our study, it is expected to provide useful guidelines for the design of Li-ion batteries with high energy densities beyond conventional ones.</P> <P><B>Graphical abstract</B></P> <P>Using first-principles calculations, we unveil the underlying mechanism that is ascribed to the high energy density of LiNiO<SUB>2</SUB> initiated by cation disordering of Li and Ni at certain Li composition.</P> <P>[DISPLAY OMISSION]</P>
Choi, Daehyeon,Kang, Joonhee,Park, Jinwoo,Han, Byungchan The Royal Society of Chemistry 2018 Physical chemistry chemical physics Vol.20 No.17
<P>The solid electrolyte interphase (SEI) of Li-ion batteries (LIBs) has been extensively studied, with most research focused on the anode, because of its significant impact on the prolonged cycle life, initial capacity loss, and safety issues. Using first-principles density functional theory (DFT) calculations and <I>ab initio</I> molecular dynamics (AIMD) simulations with the Hubbard correction, we examine the thermodynamic structure prediction and electrochemical stability of a spinel LiMn2O4 cathode interfaced with a carbonate electrolyte. The electronic energy levels of frontier orbitals of the electrolyte and the work function of the cathode offer clear characterization of the interfacial reactions. Our results based on both DFT calculations and AIMD simulations propose that the proton transfer mechanism at the hybrid interface is essential for initiating the SEI layer formation on the LiMn2O4 surface. Our results can be useful for identifying design concepts in the development of stable and high capacity LIBs with optimized electrodes and high-performance electrolytes.</P>
Taejin Choi,Daehyeon Kim,Bo-An Jang,Seong-Seung Kang 한국지질과학협의회 2016 Geosciences Journal Vol.20 No.6
The influence of microorganisms on the physical properties and strength of ilmenite and magnetite ores was determined in culture studies and laboratory tests of rock parameters such as absorption, porosity, elastic wave velocity, and uniaxial compressive strength. Initial pH condition was determined in the culture experiments, and then abiotic and biotic oxidation tests were performed to determine variations in pH, physical properties, and the strengths of the two ferruginous ore rocks. For ilmenite and magnetite ores, average initial values were, respectively, 0.185% and 0.496% for porosity, 0.046% and 0.123% for absorption, 1919 m/s and 1467 m/s for elastic wave velocity, and 432 MPa and 168 MPa for uniaxial compressive strength. The pH of ilmenite ore at the end of the abiotic and biotic oxidation tests was in the range of 3.82–4.26 and 2.20–2.57, respectively, while that of magnetite ore was 4.02–5.16 and 1.50–1.90, respectively. In magnetite ore, the changes in porosity and absorption due to the biotic oxidation test were larger than those observed in the abiotic oxidation test, while the variations in ilmenite ore due to both tests (abiotic and biotic oxidation) were small. The differences in porosity and absorption between the abiotic and biotic oxidation tests were larger in magnetite ore than in ilmenite ore, while the elastic wave velocity was smaller in magnetite ore than in ilmenite ore. In addition, the variations in uniaxial compressive strength due to the abiotic oxidation test were not significantly different than initial values in both ilmenite and magnetite ores, while variations caused by the biotic oxidation test were relatively large in both ferruginous ore rocks. Our results suggest that variations in the physical properties and strength of ilmenite and magnetite ores are largely dependent on microbial activity.
계면 열전달 매칭 기반 온도감응 열메타물질 열기능 제어
경대현(Daehyeon Kyeong),이현민(Haunmin Lee),최원준(Wonjoon Choi) 대한기계학회 2020 대한기계학회 춘추학술대회 Vol.2020 No.12
We propose the concept of phase change thermal metamaterial which is appropriate to manipulate the heat flux based on a specific temperature by controlling the interfacial thermal resistance. The phase change thermal metamaterial is a material capable of controlling the heat path in response to a specific temperature through a combination of a metal and a phase change material. Therefore, when the phase of materials is changed from solid to liquid or vice versa, the characteristics of the interface between the materials change continuously so that the interfacial thermal resistance acts as a more important factor. Because interfacial thermal resistance is mainly determined by the contact area and the degree of phonon scattering between the materials, it is conclusively related with the effective thermal conductivity in phase change material. Here, we demonstrated the impacts of the graphene nanofiller in phase change material and Layer-by-Layer coatings (MWCNT-PEI, MWCNT-PEI-Silane) via simulation tool and experimental setups using thermal unit-cell shifter which is proper to manipulate the interfacial thermal resistance, thereby optimally designing temperature responsive thermal metamaterals.
실내실험 및 현장실험을 통한 고밀도 폴리 우레탄 공법의 물리 · 역학적 특성 분석
최준영 ( Junyoung Choi ),김대현 ( Daehyeon Kim ) 대한지질공학회 2021 지질공학 Vol.31 No.1
고밀도 폴리 우레탄 공법은 주입 물질의 순간 팽창압을 이용하여 연약지반의 불안정한 지반의 안정화를 위해 적용되며 내구성이 매우 탁월한 친환경 공법이다. 순간 팽창압에 의해 연약지반 보강 및 복원, 차수 공사가 가능하다. 본 주입공법의 경우 작업시간이 매우 짧고 신속하여 열악한 환경에서도 타 공법보다 접급성과 작업성이 매우 탁월하다. 따라서 본 연구에서는 고밀도 폴리 우레탄이 지반 내에서 작용하는 물리 · 역학적 특성을 검증하고자 하였다. 먼저, 주입재료의 강도정수 및 물성값을 확인하기 위하여 일축 압축강도시험, 직접전단시험, 토압시험 등을 수행하였고 환경적인 문제에 대한 부분을 확인하고자 투수 시험 및 토양환경안정성 시험을 수행하였다. 실험결과, 조밀한 사질토와 단단한 점성토를 대상으로 비교하였을 때, 내부마찰각은 약 2배, 점착력은 약 2.5~3.5배 이상 높은 것으로 확인되었고, 투수계수 또한 1.0 × 10<sup>-5</sup>의 범위 이내를 만족하며, 기존 그라우팅 공법과 비교하였을 때 현저히 낮은 투수계수를 확인할 수 있었다. The high-density polyurethane method uses the instantaneous expansion pressure of injected material to stabilize soft ground, allowing reinforcement, restoration, and construction to be carried out in suboptimal ground conditions. Under normal and, even poor conditions, the method is easily applied because the working time is very short. The method is environmentally friendly and results have excellent durability. The purpose of this study was to verify the physical and mechanical properties of high-density polyurethane in the ground. Initial testing of strength, direct shear, and soil environment stability was followed by testing for permeability in order to address environmental concerns. The results of the experiments showed that the internal friction angle was about twice as high and the adhesion was about 2.5 to 3.5 times higher than for dense and hard clay, and that the permeability factor was significantly lower compared with the existing grouting method, within the range of 1.0 × 10<sup>-5</sup>.