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Kim, Duho,Cho, Maenghyo,Cho, Kyeongjae The Royal Society of Chemistry 2018 Journal of Materials Chemistry A Vol.6 No.37
<P>Anionic redox reactions (O<SUP>2−</SUP>/O<SUP>−</SUP>), an alternative to conventional cationic redox reactions (M<SUP>n+</SUP>/M<SUP>(n+1)+</SUP>; M: transition metal), have recently been identified as essential to achieve high energy density cathodes for sodium-ion batteries (SIBs). To overcome the drawbacks of anionic redox reactions leading to phase change and separation in the newly discovered Na(Li1/3Mn2/3)O2 material (NLMO, ∼4.2 V <I>vs.</I> Na/Na<SUP>+</SUP> with a high charge capacity of 190 mAh g<SUP>−1</SUP>), we have rationally designed high energy density Na(Li1/3Mn1/2Cr1/6)O2 (NLMCO) in which the Cr 3d-electron is coupled with the labile O 2p-electron coordinated with Mn<SUP>4+</SUP> for charge compensation during desodiation processes. NLMCO exhibits reduced phase change and separation, and chemomechanical strain and stress compared to NLMO and is thus expected to show high electrochemical performance, where the formation of short O-O bonds is not observed. By correlating the thermodynamic energy behavior with the redox mechanism in NLMO, it is concluded that our systematically designed cation-anion-coupled NLMCO is an excellent cathode material, introducing advanced materials of formula Na(Li1/3M2/3(1−y)Mcy)O2 (M and Mc: transition metals with stabilized M<SUP>4+</SUP> species and cationic redox active Mc<SUP>4+</SUP> species) for next-generation SIBs.</P>
제일 원리법을 이용한 리튬 이온베터리 음극 물질의 물성치 예측
문장혁(Janghyuk Moon),조경재(Kyeongjae Cho),조맹효(Maenghyo Cho) 대한기계학회 2010 대한기계학회 춘추학술대회 Vol.2010 No.11
An investigation of Li-M (M:Si, Sn) alloys using density functional theory is presented. DFT calculation methods performed total energy calculations, structural optimizations, bulk modulus and elastic constant with Li-Sn, Li-Si. To simulate the lithiation of amorphous Si at room temperature, we simply make up amorphous Si cell with additional Li atom at the center of the largest void. The cells optimize was used in conjunction with DFT methods. These cell volume changes agree with experiment data and Li-Si crystalline.
제일원리 계산법을 이용한 비정질 실리콘 내에서의 리튬 이온 확산 예측
문장혁(Janghyuk Moon),조경재(Kyeongjae Cho),조맹효(Maenghyo Cho) 대한기계학회 2012 대한기계학회 춘추학술대회 Vol.2012 No.11
We have studied the lithium absorption in crystalline silicon with the strain effects on unit cell using density functional theory calculation. We have concluded that the dependences of the lithium diffusion were on the local volume and environmental. In the various strained cells, the effect of the lattice deformation about migration barriers for the motion of the lithium atom has been fit on the linear regression equation based on the volume of silicon surrounding lithium impurity and the migration distance of lithium atom. This result has applied to the calculation of diffusion coefficient in amorphous silicon which was generated by annealing from crystalline structure at 3000K. The migration barriers and attempt frequency, by Arrhenius formula, of lithium in the amorphous silicon structures has been determined by local environment using the linear regression equation. Then, the statistical method, kinetic Monte Carlo method, has been demonstrated for the diffusion coefficient of lithium. Finally, we have parameterized in terms of the amorphous effects into Arrhenius diffusion formula. This study have supported that the diffusion of lithium in amorphous silicon is faster than that in crystalline silicon.
제일원리 계산법을 이용한 리튬 이차 전지의 음극 물질의 비교 및 예측
문장혁(Janghyuk Moon),조경재(Kyeongjae Cho),조맹효(Maenghyo Cho) 대한기계학회 2011 대한기계학회 춘추학술대회 Vol.2011 No.10
An investigation of Li-M (M:Si, Ge, Sn) alloys using ab initio calculation is presented. Ab initio calculation methods performed for total energy calculations, structural optimizations, electric and mechanical properties with Li-Sn, Li-Ge, Li-Si. To achieve anode materials design, specific characteristics, such as large volume change and elastic softening, are compared in Group 14 chemistry.
제일원리 계산법을 이용한 실리콘 음극 소재의 그래핀 코팅 효과 분석
문장혁(Janghyuk Moon),조경재(Kyeongjae Cho),조맹효(Maenghyo Cho) 대한기계학회 2013 대한기계학회 춘추학술대회 Vol.2013 No.12
Computational study on the effect of graphene coating to Si anode material is performed by using density functional theory calculations. We construct the atomic model to examine interactions between amorphous silicon and graphene during lithiation. The lithiation of Si anode increases the mechanical contact force between outer graphene layer and amorphous silicon and the shear resistance is also increased. To explain the interaction between graphene and silicon we examine the charge distribution of silicon and graphene considering lithiation insertion. The number of density, electro field distribution and electric potential are also calculated. Charge-non polar interaction between Li-ion and graphene increase the contact energy between graphene-silicon. To calculate the contact force, we simplified interaction atomic force model. In this theoretical study, potential improvement of cyclability and improved mechanical properties of graphene coating for Si anode have been investigated.
Chenzhe Li,Maenghyo Cho,Kyeongjae Cho 대한기계학회 2014 대한기계학회 춘추학술대회 Vol.2014 No.11
We present theoretical evidence for the phase evolution and stability of Fe substituted RMn₂O<SUB>5-δ</SUB> (R=Bi, Sm or Y) mullite structures. Our density-functional simulations show evidence for the R site dependence of the mullite phase evolution with the Fe content in the solid solution. For Fe in R=Y or Sm compounds, the orthorhombic mullite phases are predicted to possess substitution limits. Compounds with x more than 0.5 or 0.75 in R[Fe<SUB>x</SUB>Mn<SUB>1-x</SUB>]₂O<SUB>5-δ</SUB> stoichiometry formula are found to be unstable under the high temperature calcination condition. For Fe in R=Bi compounds, a sub-phase transition behavior is found to preserve the orthorhombic structure with oxygen content variation in the tested compounds.
Bhatt, Mahesh Datt,Cho, Maenghyo,Cho, Kyeongjae Canadian Science Publishing 2011 Canadian journal of chemistry Vol.89 No.12
<P> The interaction of lithium (Li<SUP>+</SUP>) cation and hexafluorophosphate (PF6<SUP>-</SUP>) anion with nonaqueous electrolytes is studied by using density functional theory at the B3LYP/6-311++G(d,p) level in the gas phase in terms of the coordination of Li<SUP>+</SUP> and PF6<SUP>-</SUP> with these solvents. Ethylene carbonate (EC) coordinates with Li<SUP>+</SUP> and PF6<SUP>-</SUP> most strongly and reaches the anode and cathode most easily because of its highest dielectric constant among all the solvent molecules, resulting in its preferential reduction on the anode and oxidation on the cathode. For cyclic carbonates EC and propylene carbonate (PC), the structure Li<SUP>+</SUP>(S)4 is found to be the most stable. However, for linear carbonates dimethyl carbonate (DMC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC), the formation of PF6<SUP>-</SUP>(S)n=1-3 is not favorable. Such analysis may be useful in applications for lithium ion batteries. </P>