13 research outputs found
Electronic Transport through QD in the whole temperature range including both the high- and the low-T limits with the equation-of-motion technique
We have studied theoretically the Kondo effect in the quantum dot(QD) within
the whole range of temperature by using the equation-of-motion(EOM) technique
based on the non-equilibrium Green function formalism. We have taken the
finiteness of Coulomb correlation and the non-equilibrium effect into account
by calculating the correlation terms emerged from the decoupling approximation
using EOM method for the lesser Green function. We showed that the result is in
good qualitative agreement with the results of NCA, NRG and NRPT, etc., even
using EOM method which is being recognized as a 'conventional' method.
The results are the generalization into the pseudo-equilibrium state of the
Refs. 32,33 and can be used to describe a non-equilibrium state under the bias
voltage which is not so large
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 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 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 to 54 mAh g 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
formation, and reveal new prospects for developing graphite based cathode
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
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)C compound has the negatively lowest intercalation energy at
21, the solvated Na(digl) 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
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
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), denoted as AM-nC-GICs (AM=Li, Na, K, =1, 2,
3, 4). The orthorhombic unit cells have been used to build the models for
crystalline stage-I AM-nC-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.Comment: 22 pages, 9 figure
Ionic Diffusion and Electronic Transport in Eldfellite NaFe(SO)
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
NaFe(SO), 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
The Number of Irreducible Polynomials over Finite Fields of Characteristic 2 with Given Trace and Subtrace
In this paper we obtained the formula for the number of irreducible
polynomials with degree over finite fields of characteristic two with given
trace and subtrace. This formula is a generalization of the result of Cattell
et al.(2003) [2].Comment: 16 page
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
Ab initio thermodynamic study of SnO(110) surface in an O and NO environment: a fundamental understanding of gas sensing mechanism for NO and NO
For the purpose of elucidating the gas sensing mechanism of SnO for NO
and NO gases, we calculate the phase diagram of SnO(110) surface in
contact with an O 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(110) surfaces using (11) and (21) 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(110) surfaces in
(, ) 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 adsorption on the
surface, we provide a meaningful understanding about the gas sensing mechanism.Comment: 10 figure