Molecular dynamics simulation to reveal the transport mechanism of LiPF6 in ethylene carbonate + dimethylcarbonate binary solvent.

This paper presents the molecular dynamics simulation of 1 mol kg-1 LiPF6 in a binary solvent of ethylene carbonate (EC) and dimethylcarbonate, which is a representative electrolyte solution for lithium-ion batteries. The simulation successfully reproduced the diffusion coefficient, ionic conductivity, and shear viscosity as functions of EC content at 300 K, which had been experimentally determined in our previous study. The Yukawa potential was adopted to model intercharge interactions to reduce computational costs, which consequently allowed us to precisely calculate the conductivity and viscosity by directly integrating time-correlation functions without explicitly modeling the molecular polarization. Breaking down microscopic current correlation functions into components revealed that, whereas the cation-anion attractive interaction dominantly impedes the conduction when the EC content is low, it is the cation-cation and anion-anion repulsive interactions that reduce the conductivity at a high EC content. An analysis of the pressure correlations revealed that all components positively contribute to the viscosity in the binary solvent without the electrolyte. On the other hand, negative terms are observed in five out of six cross correlations in the presence of the electrolyte, implying that these correlations negatively contribute to the shear stress and entropy production, both of which are net positive.

Stability of Zirconium-Substituted Face-Centered Cubic Yttrium Hydride.

The stability of a zirconium (Zr)-substituted face-centered cubic (FCC) yttrium (Y) hydride (Y1-xZrx hydride) phase was investigated experimentally and theoretically. Two possible sites for hydrogen atoms exist in the FCC structure, namely, T- and O-sites, where hydrogen is present at the center of the tetrahedron and the octahedron composed of Y and/or Zr metals. The P-C isotherms revealed that the hydrogen content per metal (H/M) with 33% Zr-substituted YH3-δ was 2.2-2.3, which was lower than the expected value calculated from the starting composition of YH3-33% ZrH2 (Y0.67Zr0.33H2.67, H/M = 2.67). Hydrogen at the O-site in Y1-xZrx hydride mainly reacted during hydrogen desorption/absorption. On the basis of theoretical analyses, the hydrogen atoms do not occupy the center of the octahedron, when at least two of the six vertices of the octahedron were composed of Zr. The O-sites, where more than two Zr atoms coordinate, nonlinearly increased with the Zr content, and when the Zr content was >50%, almost no hydrogen atoms occupy the O-sites. The theoretical discussion supported the experimental results, and the Zr substitution was confirmed to reduce the occupancy of H at the O-site in the FCC YH3 significantly.

How does the solvent composition influence the transport properties of electrolyte solutions? LiPF6 and LiFSA in EC and DMC binary solvent.

In this study, we experimentally measured the viscosity, η, and ionic conductivity, σ, of the electrolyte solutions of 1 mol kg-1 of LiPF6 or LiFSA dissolved in the binary mixture solvent of EC and DMC in a temperature range of 288 ≤ T/K ≤ 328 by varying the EC content from 0 to 60 vol%, which translates into the molar fraction of EC of 0 ≤ xEC ≤ 0.7. The diffusion coefficient, D, of each species, Li+, PF6-, FSA-, EC and DMC, was determined by pulse gradient spin-echo NMR. The state of molecules around Li+ was examined using the Raman spectra of the solvents and anions; the quantitative analysis suggests that EC is about twice as much preferred as DMC in the solvation shell at low xEC, while the EC-preference decreases with an increase in xEC. The classical Stokes-Einstein relation still quantitatively holds when evaluating the hydrodynamic radius, rSt, of transporting entities from D and η, in that (i) rSt,EC and rSt,DMC without the solute do not significantly differ from those in the solution; (ii) rSt,Li roughly coincides with the size estimated from the solvation number determined by Raman spectroscopy, which implies that rSt,Li reflects the solvation shell size; and (iii) rSt,anion is close to the static size, suggesting that anions are little solvated. The increase in xEC results in a decrease in rSt for all species, among which anions are most influenced, which is consistent with the view that the highly Li+-solvating EC, with its better dielectric shielding effect than DMC, liberates the anions from Li+, whereby enhancing the anion transfer that positively contributes to the ionic conductivity until the viscosity prevails at high xEC.

Improvement of the Battery Performance of Indigo, an Organic Electrode Material, Using PEDOT/PSS with d-Sorbitol.

Rare-metal-free and high-performance secondary batteries are necessary for improving the efficiency of renewable energy systems. Organic compounds are attractive candidates for the active material of such batteries. Many studies have reported organic active materials that show high energy density per active material weight. However, organic active materials, most of which exhibit low conductivity and low specific density, typically require a large amount of a conductive additive (>50 wt %) to obtain a high utilization rate. Therefore, organic active materials rarely display high energy density per electrode weight. High energy densities per electrode weight can be obtained using high weight fractions of active materials and low weight fractions of conductive additives. Herein, we report that a low-conductivity organic active material, indigo, showed improved net discharge capacity density when even a small amount of a conductive polymer composite, poly(3,4-ethylenedioxythiophene)/polystyrene sulfonic acid (PEDOT/PSS) with d-sorbitol, was used as both a binder and conductive additive. The cycle life was also improved by coating one side of the separator with the composite, which probably hindered the dissolution of the active material. A discharge capacity of 96% of the theoretical capacity of indigo and an improved cycle life were achieved with an electrode containing 80 wt % indigo and with a PEDOT/PSS-coated separator. The optimal fraction of the conductive binder was examined, and the mechanism of conductivity enhancement was discussed. The present scheme allows us to replace the dispersion solvent of the slurry, N-methylpyrrolidone, with water, which can reduce the environmental load during battery manufacturing processes.

Analytical Measurements to Elucidate Structural Behavior of 2,5-Dimethoxy-1,4-benzoquinone During Charge and Discharge.

Organic compounds as electrode materials can contribute to sustainability because they are nontoxic and environmentally abundant. The working mechanism during charge-discharge for reported organic compounds as electrode materials is yet to be completely understood. In this study, the structural behavior of 2,5-dimethoxy-1,4-benzoquinone (DMBQ) during charge-discharge is investigated by using NMR spectroscopy, energy-dispersive X-ray spectroscopy, magnetic measurements, operando Raman spectroscopy, and operando X-ray diffraction. For both lithium and sodium systems, DMBQ works as a cathode accompanied with the insertion and deinsertion of Li and Na ions during charge-discharge processes. The DMBQ sample is found to be in two-phase coexistence state at the higher voltage plateau, and the radical monoanion and dianion phases have no long-distance ordering. These structures reversibly change into the original neutral phase with long-distance ordering. These techniques can show the charge-discharge mechanism and the factors that determine the deterioration of organic batteries, thus guiding the design of future high-performance organic batteries.

Viologen Derivatives Extended with Aromatic Rings Acting as Negative Electrode Materials for Use in Rechargeable Molecular Ion Batteries.

Many types of batteries have been proposed as next-generation energy-storage systems. One candidate is a rocking-chair-type "molecular ion battery" in which a molecular ion, instead of Li+ , works as a charge carrier. Previously, we reported a viologen-type derivative as a negative electrode material that releases and receives PF6 - anions during the charge-discharge process; however, its redox potential was not satisfactorily low. Further, the two potential plateaus of this material (difference=0.5 V) should be reduced. In this study, PF6 - salts of viologen (bipyridinium) derivatives extended by aromatic rings were synthesized to obtain a negative electrode material with a lower redox potential and small potential change during the charge and discharge processes. Some of the synthesized viologen derivatives were fluorescent even in solid-state electrodes. In the half-cell configuration, the prepared negative electrode materials showed average voltages of approximately 2 V (vs. Li+ /Li), which is lower than that of conventional viologen derivatives.

How does the Li-distribution in the 16d sites determine the stability of A3(Li,Ti5)O12 (A = Li and Na)?

Li3(Li,Ti5)O12 (LTO) is a stable and safe negative electrode material for Li-ion batteries, and its Na substitute Na3(Li,Ti5)O12 (NTO) is a counterpart for the Na-ion battery. In LTO and NTO, a sixth of the Ti-sites (16d) in the spinel framework are replaced by Li: Li mixing in the 16d sites. For conducting theoretical studies on these materials, e.g., density functional theory (DFT) calculations, one has to confront the astronomical number of combinations of Li distribution in 16d sites to construct model structures, of which the size is sufficiently large to represent the bulk material properties. Only a limited number of models, whose structures are a priori specified by "researcher intuition," have been examined thus far, and how Li-mixing determines the material stability has yet to be clarified. Herein, we statistically analyzed the DFT total energy of more than 2 × 104 model structures of LTO and NTO that were extracted from the 4 × 108 possible combinations of Li-mixing with computer-aided symmetry analysis and an automated model building system. The local energy analysis further revealed the local stability/instability of each structure. We found that LTO and NTO stability can be well explained by the apparent coulombic repulsion between Li+ in the 16d sites as if they were placed in a matrix of dielectric constants of 1.92 and 2.04 for LTO and NTO, respectively. That is, the sum of the inverse of the Li-Li distance (S) serves as a good descriptor in predicting the stability of these materials. The extent to which the O2- anions are displaced from the Wyckoff position (32e) is considered to differentiate NTO from LTO. However, the electronic structure of NTO does not significantly differ from that of LTO unless S exceeds a certain limit. These results suggest that the spinel framework tolerates the structural instability and variety to some extent, which is important in constructing a spinel structure with the mixing of other cations, thereby replacing the rare element Li.

Ionic conductivity of molten alkali-metal carbonates A2CO3 (A = Li, Na, K, Rb, and Cs) and binary mixtures (Li1-xCsx)2CO3 and (Li1-xKx)2CO3: A molecular dynamics simulation.

Based on experimental data, we optimized the potential parameters for the classical molecular dynamics simulation to reproduce the volume and ionic conductivity of the molten alkali-metal carbonates A2CO3 where A = Li, Na, K, Rb, and Cs at T/K = 1223 and ambient pressure. The force field was then applied to the binary mixtures (Li1-xCsx)2CO3 and (Li1-xKx)2CO3. In (Li1-xCsx)2CO3, the diffusion coefficient DCs exceeds DLi at x > 0.6, testifying to the Chemla effect. The net ionic conductivity was broken down into the contributions from the velocity auto- and cross-correlations of each ionic species. The significant negative deviation of the real conductivity of (Li1-xCsx)2CO3 from the one estimated by the Nernst-Einstein (NE) relation is clearly explained by the contribution from the cross correlations; specifically, the cross term between Li+and CO3 2-, which is negative at x = 0, significantly shifts to the positive side when x increases, which is dominantly responsible for dampening the conductivity from the NE conductivity. A similar behavior was observed in (Li1-xKx)2CO3 with a less pronounced manner than in (Li1-xCsx)2CO3. These observations corroborate the precedent studies pointing to the trapping of Li+ by the anion when a lithium salt is mixed with another salt of which the cation size is greater than that of Li+.

Monte-Carlo simulation combined with density functional theory to investigate the equilibrium thermodynamics of electrode materials: lithium titanates as model compounds.

By using lithium titanate (LTO) as a model electrode material, the present study proposes a method to describe its equilibrium thermodynamics based on the Monte-Carlo simulation (MC), for which the energetic parameters are determined by the density functional theory (DFT). The electrochemical potential profile is simulated by a simple topological model which consists only of three parameters representing the Li site energies; namely, the potential energy of the 8a site (ε8a), the difference in the site energy between the 8a and 16c sites (Δε) and the repulsion between two Li atoms situated at the adjacent 8a and 16c sites (J). Parameter physics by the MC revealed that the term Δε plays a decisive role, with a collateral effect from J, for characterizing the shape of the potential profile whereas the term ε8a determines its position along the electrochemical potential. For instance, if Δε exceeds the thermal energy at the temperature under consideration, i.e., if Δε > 3kT, the first-order phase transition takes place during which two phases coexist, resulting in a plateau region in the potential profile. On the other hand, if Δε < 3kT, the lithiation of LTO is viewed as a phenomenon above the critical point, above which the material is in a homogeneous uniphasic state. A multiple regression analysis of a set of the total energy calculated by DFT allows us to determine these energetic terms. The MC simulation with the determined parameters well reproduces the shape and position of the experimental potential profile of LTO. Since the determined value, Δε/eV ∼ 0.4, far exceeds the thermal energy at ambient temperature, the potential plateau of LTO is explained by the first-order phase transition as long as the equilibrium state is concerned.

Anthraquinone-Based Oligomer as a Long Cycle-Life Organic Electrode Material for Use in Rechargeable Batteries.

An anthraquinone (AQ)-based dimer and trimer linked by a triple bond (-C≡C-) were newly synthesized as active materials for the positive electrode of rechargeable lithium batteries. These synthesized oligomers exhibited an initial discharge capacity of about 200 mAh g-1 with an average voltage of 2.2-2.3 V versus Li(C.E.) . These capacity values are similar to that of the AQ-monomer, reflecting the two-electron transfer redox per AQ unit. Regarding their cycling stability, the capacity of the monomer electrode quickly decreased; however, the electrodes of the prepared oligomers showed an improved cycling performance. In particular, the discharge capacities of the trimer remained almost constant for 100 cycles. A theoretical calculation revealed that the intermolecular binding energy can be increased to the level of a weak covalent bonding by oligomerization, which would be beneficial to suppress the dissolution of the organic active materials into the electrolyte solutions. These results show that the cycle-life of organic active materials can be extended without lowering the discharge capacity by the oligomerization of the redox active molecule unit.

A Monte-Carlo simulation of ionic conductivity and viscosity of highly concentrated electrolytes based on a pseudo-lattice model.

Simulating three transport phenomena-ionic conductivity, viscosity, and self-diffusion coefficient-in a common Monte-Carlo framework, we discuss their relationship to the intermolecular interactions of electrolyte solutions at high concentrations (C/mol l-1 ∼ 1). The simulation is predicated on a pseudolattice model of the solution. The ions and solvents (collectively termed "molecules") are considered dimensionless points occupying the lattice sites. The molecular transport is realized by a repetition of swapping two adjacent molecules by the stochastic Gibbs sampling process based on simple intermolecular interactions. The framework has been validated by the fact that the simulated ionic conductivity and dynamic viscosity of 1:1- and 2:1-salts qualitatively well represent the experimental data. The magnitude of the Coulombic interaction itself is not reflected in the ionic conductivity, but the extent to which the Coulombic interaction is shielded by the dielectric constant has a significant influence. On the other hand, the dielectric constant barely influences the viscosity, while the magnitude of the Coulombic interaction is directly reflected in the viscosity.

Molecular ion battery: a rechargeable system without using any elemental ions as a charge carrier.

Is it possible to exceed the lithium redox potential in electrochemical systems? It seems impossible to exceed the lithium potential because the redox potential of the elemental lithium is the lowest among all the elements, which contributes to the high voltage characteristics of the widely used lithium ion battery. However, it should be possible when we use a molecule-based ion which is not reduced even at the lithium potential in principle. Here we propose a new model system using a molecular electrolyte salt with polymer-based active materials in order to verify whether a molecular ion species serves as a charge carrier. Although the potential of the negative-electrode is not yet lower than that of lithium at present, this study reveals that a molecular ion can work as a charge carrier in a battery and the system is certainly a molecular ion-based "rocking chair" type battery.

Indigo carmine: an organic crystal as a positive-electrode material for rechargeable sodium batteries.

Using sodium, instead of lithium, in rechargeable batteries is a way to circumvent the lithium's resource problem. The challenge is to find an electrode material that can reversibly undergo redox reactions in a sodium-electrolyte at the desired electrochemical potential. We proved that indigo carmine (IC, 5,5'-indigodisulfonic acid sodium salt) can work as a positive-electrode material in not only a lithium-, but also a sodium-electrolyte. The discharge capacity of the IC-electrode was ~100 mAh g(-1) with a good cycle stability in either the Na or Li electrolyte, in which the average voltage was 1.8 V vs. Na(+)/Na and 2.2 V vs. Li(+)/Li, respectively. Two Na ions per IC are stored in the electrode during the discharge, testifying to the two-electron redox reaction. An X-ray diffraction analysis revealed a layer structure for the IC powder and the DFT calculation suggested the formation of a band-like structure in the crystal.

Highly-thermostable metal-organic frameworks (MOFs) of zinc and cadmium 4,4'-(hexafluoroisopropylidene)diphthalates with a unique fluorite topology.

Two novel zinc and cadmium 4,4'-(hexafluoroisopropylidene)diphthalate metal-organic frameworks have been synthesized and characterized using single crystal X-ray diffraction analysis, and exhibit a unique fluorite topology and high thermal stabilities.

The aromaticity of pyracylene: an experimental and computational study of the energetics of the hydrogenation of acenaphthylene and pyracylene.

In this work, the aromaticity of pyracylene (2) was investigated from an energetic point of view. The standard enthalpy of hydrogenation of acenaphthylene (1) to acenaphthene (3) at 298.15 K was determined to be minus sign(114.5 +/- 4.2) kJ x mol(-1) in toluene solution and minus sign(107.9 +/- 4.2) kJ x mol(-1) in the gas phase, by combining results of combustion and reaction-solution calorimetry. A direct calorimetric measurement of the standard enthalpy of hydrogenation of pyracylene (2) to pyracene (4) in toluene at 298.15 K gave -(249.9 plus minus 4.6) kJ x mol(-1). The corresponding enthalpy of hydrogenation in the gas phase, computed from the Delta(f)H(o)m(cr) and DeltaH(o)m(sub) values obtained in this work for 2 and 4, was -(236.0 +/- 7.0) kJ x mol(-1). Molecular mechanics calculations (MM3) led to Delta(hyd)H(o)m(1,g) = -110.9 kJ x mol(-1) and Delta(hyd)H(o)m(2,g) = -249.3 kJ x mol(-1) at 298.15 K. Density functional theory calculations [B3LYP/6-311+G(3d,2p)//B3LYP/6-31G(d)] provided Delta(hyd)H(o)m(2,g) = -(244.6 +/- 8.9) kJ x mol(-1) at 298.15 K. The results are put in perspective with discussions concerning the "aromaticity" of pyracylene. It is concluded that, on energetic grounds, pyracylene is a borderline case in terms of aromaticity/antiaromaticity character.