arXiv subjects

Gum-Chol Ri

Publications and source records attributed to Gum-Chol Ri.

9 recordsLinked to original sources

arXiv2018

Role of Water Molecule in Enhancing the Proton Conductivity on Graphene Oxide at Humidity Condition

Recent experimental reports on in-plane proton conduction in reduced graphene oxide (rGO) films open a new way for the design of proton exchange membrane essential in fuel cells and chemical filters. At high humidity condition, water molecules attached on the rGO sheet are expected to play a critical role, but theoretical works for such phenomena have been scarcely found in the literature. In this study, we investigate the proton migration on water-adsorbed monolayer and bilayer rGO sheets using first-principles calculations in order to reveal the mechanism. We devise a series of models for the water-adsorbed rGO films as systematically varying the reduction degree and water content, and optimize their atomic structures in reasonable agreement with the experiment, using a density functional that accounts for van der Waals correction. Upon suggesting two different transport mechanisms, epoxy-mediated and water-mediated hoppings, we determine the kinetic activation barriers for these in-plane proton transports on the rGO sheets. Our calculations indicate that the water-mediated transport is more likely to occur due to its much lower activation energy than the epoxy-mediated one and reveal new prospects for developing efficient solid proton conductors.

cond-mat.mtrl-sci↗

arXiv2018

Revealing the formation and electrochemical properties of bis(trifluoromethanesulfonyl) imide intercalated graphite with first-principles calculations

Graphite has been reported to have anion as well as cation intercalation capacities as both cathode and anode host materials for the dual ion battery. In this work, we study the intercalation of bis(trifluoromethanesulfonyl) imide (TFSI) anion from ionic liquid electrolyte into graphite with first-principles calculations. We build models for TFSI-C$_n$ compounds with systematically increasing unit cell sizes of graphene sheet and investigate their stabilities by calculating the formation energy, resulting in the linear decrease and arriving at the limit of stability. With identified unit cell sizes for stable compound formation, we reveal that the interlayer distance and relative volume expansion ratio of TFSI-C$_n$ increase as increasing the concentration of TFSI intercalate during the charge process. The electrode voltage is determined to be ranged from 3.8 V to 3.0 V at the specific capacity ranging from 30 mAh g$^{-1}$ to 54 mAh g$^{-1}$ in agreement with experiment. Moreover, a very low activation barrier of under 50 meV for TFSI migration and good electronic conductivity give a proof of using these compounds as a promising cathode. Through the analysis of charge transfer, we clarify the mechanism of TFSI-C$_n$ formation, and reveal new prospects for developing graphite based cathode.

cond-mat.mtrl-sci↗

arXiv2017

Influence of water intercalation and hydration on chemical decomposition and ion transport in methylammonium lead halide perovskites

The use of methylammonium (MA) lead halide perovskites \ce{CH3NH3PbX3} (X=I, Br, Cl) in perovskite solar cells (PSCs) has made great progress in performance efficiency during recent years. However, the rapid decomposition of \ce{MAPbI3} in humid environments hinders outdoor application of PSCs, and thus, a comprehensive understanding of the degradation mechanism is required. To do this, we investigate the effect of water intercalation and hydration of the decomposition and ion migration of \ce{CH3NH3PbX3} using first-principles calculations. We find that water interacts with \ce{PbX6} and MA through hydrogen bonding, and the former interaction enhances gradually, while the latter hardly changes when going from X=I to Br and to Cl. Thermodynamic calculations indicate that water exothermically intercalates into the perovskite, while the water intercalated and monohydrated compounds are stable with respect to decomposition. More importantly, the water intercalation greatly reduces the activation energies for vacancy-mediated ion migration, which become higher going from X=I to Br and to Cl. Our work indicates that hydration of halide perovskites must be avoided to prevent the degradation of PSCs upon moisture exposure.

cond-mat.mtrl-sci↗

arXiv2017

Revealing the stability and efficiency enhancement in mixed halide perovskites MAPb(I$_{1-x}$Cl$_x$)$_3$ with ab initio calculations

A little addition of Cl to \ce{MAPbI3} has been reported to improve the material stability as well as light harvesting and carrier conducting properties of organometal trihalide perovskites, the key component of perovskite solar cell (PSC). However, the mechanism of performance enhancement of PSC by Cl addition is still unclear. Here, we apply the efficient virtual crystal approximation method to revealing the effects of Cl addition on the structural, electronic, optical properties and material stability of \ce{MAPb(I_{1-x}Cl_x)3}. Our {\it ab initio} calculations present that as the increase of Cl content cubic lattice constants and static dielectric constants decrease linearly, while band gaps and exciton binding energies increase quadratically. Moreover, we find the minimum of exciton binding energy at the Cl content of 7\%, at which the chemical decomposition reaction changes coincidentally to be from exothermic to endothermic. Interactions among constituents of compound and electronic charge transferring during formation are carefully discussed. This reveals new prospects for understanding and designing of stable, high efficiency PSCs.

cond-mat.mtrl-sci↗

arXiv2017

Ab initio thermodynamic study of SnO$_2$(110) surface in an O$_2$ and NO environment: a fundamental understanding of gas sensing mechanism for NO and NO$_2$

For the purpose of elucidating the gas sensing mechanism of SnO$_2$ for NO and NO$_2$ gases, we calculate the phase diagram of SnO$_2$(110) surface in contact with an O$_2$ and NO gas environment by means of {\it ab initio} thermodynamic method. Firstly we build a range of surface slab models of oxygen pre-adsorbed SnO$_2$(110) surfaces using (1$\times$1) and (2$\times$1) surface unit cells and calculate their Gibbs free energies considering only oxygen chemical potential. The fully reduced surface containing the bridging and in-plane oxygen vacancies in the oxygen-poor condition, while the fully oxidized surface containing the bridging oxygen and oxygen dimer in the oxygen-rich condition, and the stoichiometric surface in between, were proved to be most stable. Using the selected plausible NO-adsorbed surfaces, we then determine the surface phase diagram of SnO$_2$(110) surfaces in ($Δμ_\text{O}$, $Δμ_\text{NO}$) space. In the NO-rich condition, the most stable surfaces were those formed by NO adsorption on the most stable surfaces in contact with only oxygen gas. Through the analysis of electronic charge transferring and density of states during NO$_x$ adsorption on the surface, we provide a meaningful understanding about the gas sensing mechanism.

cond-mat.mtrl-sci↗

arXiv2017

First-principles study of ternary graphite compounds cointercalated with alkali atoms (Li, Na, and K) and alkylamines towards alkali ion battery applications

Using density functional theory calculations, we have investigated the structural, energetic, and electronic properties of ternary graphite intercalation compounds (GICs) containing alkali atoms (AM) and normal alkylamine molecules (nC$x$), denoted as AM-nC$x$-GICs (AM=Li, Na, K, $x$=1, 2, 3, 4). The orthorhombic unit cells have been used to build the models for crystalline stage-I AM-nC$x$-GICs. By performing the variable cell relaxations and the analysis of results, we have found that with the increase in the atomic number of alkali atoms the layer separations decreases in contrast to AM-GICs, while the bond lengths of alkali atoms with graphene layer and nitrogen atom of alkylamine decreases. The formation and interlayer binding energies of AM-nC3-GICs have been calculated, indicating the increase in stability from Li to K. The calculated energy barriers for migration of alkali atoms suggest that alkali cation with larger ionic radius diffuses in graphite more smoothly, being similar to AM-GICs. The analysis of density of states, electronic density differences, and atomic populations illustrates a mechanism how the insertion of especially Na among alkali atoms into graphite with first stage can be made easy by cointercalation with alkylamine, more extent of electronic charge transfer is occurred from more electropositive alkali atom to carbon ring of graphene layer, while alkylamine molecules interact strongly with graphene layer through the hybridization of valence electron orbitals.

cond-mat.mtrl-sci↗

arXiv2017

Ionic Diffusion and Electronic Transport in Eldfellite Na$_x$Fe(SO$_4$)$_2$

Discovering new electrodes for sodium-ion battery requires clear understanding of the material process during battery operation. Using first-principles calculations, we identify mechanisms of ionic diffusion and electronic transfer in newly developed cathode material, eldfellite Na$_x$Fe(SO$_4$)$_2$, reproducing the electrochemical properties in good agreement with experiment. The inserted sodium atom is suggested to diffuse along the two-dimensional pathway with preceding movement of the host sodium atom, and the activation energy is calculated to be reasonable for fast insertion. We calculate the electronic properties, showing the band insulating at low composition of inserted sodium, for which the electron polaron formation and hoping are also suggested. Our results may contribute to opening a new way of developing innovative cathode materials based on iron and sulfate ion.

cond-mat.mtrl-sci↗

arXiv2017

Mixed eldfellite compounds \ce{Na(Fe_{1/2}M_{1/2})(SO4)2} (M = Mn, Co, Ni): A new family of high electrode potential cathodes for the sodium-ion battery

Natural abundance of sodium and its similar behavior to lithium triggered recent extensive studies of cost-effective sodium-ion batteries (SIBs) for large-scale energy storage systems. A challenge is to develop electrode materials with a high electrode potential, specific capacity and a good rate capability. In this work we propose mixed eldfellite compounds \ce{Na_x(Fe_{1/2}M_{1/2})(SO4)2} (M = Mn, Co, Ni) as a new family of high electrode potential cathodes of SIBs and present their material properties predicted by first-principles calculations. The structural optimizations show that these materials have significantly small volume expansion rates below 5\% upon Na insertion/desertion with negative Na binding energies. Through the electronic structure calculations, we find band insulating properties and hole (and/or electron) polaron hoping as a possible mechanism for the charge transfer. Especially we confirm the high electrode voltages over 4 V with reasonably high specific capacities. We also investigate the sodium ion mobility by estimating plausible diffusion pathways and calculating the corresponding activation barriers, demonstrating the reasonably fast migrations of sodium ions during the operation. Our calculation results indicate that these mixed eldfellite compounds can be suitable materials for high performance SIB cathodes.

cond-mat.mtrl-sci↗

arXiv2017

Ab initio study of sodium cointercalation with diglyme molecule into graphite

The cointercalation of sodium with the solvent organic molecule into graphite can resolve difficulty of forming the stage-I Na-graphite intercalation compound, which is a predominant anode of Na-ion battery. To clarify the mechanism of such cointercalation, we investigate the atomistic structure, energetics, electrochemical properties, ion and electron conductance, and charge transferring upon de/intercalation of the solvated Na-diglyme ion into graphite with {\it ab initio} calculations. It is found that the Na(digl)$_2$C$_n$ compound has the negatively lowest intercalation energy at $n\approx$21, the solvated Na(digl)$_2$ ion diffuses fast in the interlayer space, and their electronic conductance can be enhanced compared to graphite. The calculations reveal that the diglyme molecules as well as Na atom donates electrons to the graphene layer, resulting in the formation of ionic bonding between the graphene layer and the moiety of diglyme molecule. This work will contribute to the development of innovative anode materials for alkali-ion battery applications.

cond-mat.mtrl-sci↗