Atomic and Electronic Structure in MgO-SiO2.

Understanding disordered structure is difficult due to insufficient information in experimental data. Here, we overcome this issue by using a combination of diffraction and simulation to investigate oxygen packing and network topology in glassy (g-) and liquid (l-) MgO-SiO2 based on a comparison with the crystalline topology. We find that packing of oxygen atoms in Mg2SiO4 is larger than that in MgSiO3, and that of the glasses is larger than that of the liquids. Moreover, topological analysis suggests that topological similarity between crystalline (c)- and g-(l-) Mg2SiO4 is the signature of low glass-forming ability (GFA), and high GFA g-(l-) MgSiO3 shows a unique glass topology, which is different from c-MgSiO3. We also find that the lowest unoccupied molecular orbital (LUMO) is a free electron-like state at a void site of magnesium atom arising from decreased oxygen coordination, which is far away from crystalline oxides in which LUMO is occupied by oxygen's 3s orbital state in g- and l-MgO-SiO2, suggesting that electronic structure does not play an important role to determine GFA. We finally concluded the GFA of MgO-SiO2 binary is dominated by the atomic structure in terms of network topology.

Thermodynamic aspect of sulfur, polysulfide anion and lithium polysulfide: plausible reaction path during discharge of lithium-sulfur battery.

The elucidation of elemental redox reactions of sulfur is important for improving the performance of lithium-sulfur batteries. The energies of stable structures of Sn, Sn˙-, Sn2-, [LiSn]- and Li2Sn (n = 1-8) were calculated at the CCSD(T)/cc-pVTZ//MP3/cc-pVDZ level. The heats of reduction reactions of S8 and Li2Sn with Li in the solid phase were estimated from the calculated energies and sublimation energies. The estimated heats of the redox reactions show that there are several redox reactions with nearly identical heats of reaction, suggesting that several reactions can proceed simultaneously at the same discharge voltage, although the discharging process was often explained by stepwise reduction reactions. The reduction reaction for the formation of Li2Sn (n = 2-6 and 8) from S8 normalized as a one electron reaction is more exothermic than that for the formation of Li2S directly from S8, while the reduction reactions for the formation of Li2S from Li2Sn are slightly less exothermic than that for the formation of Li2S directly from S8. If the reduction reactions with large exotherm occur first, these results suggest that the reduction reactions forming Li2Sn (n = 2-6 and 8) from S8 occur first, then Li2S is formed, and therefore, a two-step discharge-curve is observed.

Li-ion transport at the interface between a graphite anode and Li2CO3 solid electrolyte interphase: ab initio molecular dynamics study.

Understanding and the control of Li-ion (Li+) transport across the interface between the anode and solid electrolyte interphase (SEI) film or electrolyte is a key issue in battery electrochemistry and interface science. In this study, we investigated the structural, electronic and free energy properties of Li+ migration between a Li-intercalated graphite anode LiCx and Li2CO3 SEI film, by using ab initio molecular dynamics and free energy calculations. We compared three types of graphite edges: H-, OH- and mixed (H, OH, COOH)-terminations, and three cases of transferred Li-ions: Li+ constructing the SEI, excess Li+ and excess Li0 (excess Li+ + e- in anode). After validation of our calculations with Li2CO3 and LiCx bulk systems, we sampled the interfacial structures under thermodynamic equilibrium and demonstrated that the OH- and mixed-terminations had larger binding energies. The calculated free energy profiles of Li+ intercalation from the Li2CO3 SEI to LiC24 showed barriers larger than 1.2 eV irrespective of the terminations and Li+ cases. We also clarified that the charges of Li ions did not change much upon the intercalation. Based on these results and the calculated Li chemical potential, we constructed the probable free energy profile of Li+ between the anode and cathode under charging and discharging. This profile model suggest a possible electric field approximation for the charging stage, and the resultant free energy profiles with such fields gave a ca. 0.5 eV barrier under charging, which was consistent with the experimental values. The present picture will give a crucial insight into Li-ion transport at the battery interfaces.

Combined Theoretical and Experimental Studies of Sodium Battery Materials.

Owing to developments in theoretical chemistry and computer power, the combination of calculations and experiments is now standard practice in understanding and developing new materials for battery systems. Here, we briefly review our recent combined studies based on density functional theory and molecular dynamics calculations for electrode and electrolyte materials for sodium-ion batteries. These findings represent case studies of successful combinations of experimental and theoretical methods.

Machine learning prediction of coordination energies for alkali group elements in battery electrolyte solvents.

We combined a data science-driven method with quantum chemistry calculations, and applied it to the battery electrolyte problem. We performed quantum chemistry calculations on the coordination energy (Ecoord) of five alkali metal ions (Li, Na, K, Rb, and Cs) to electrolyte solvent, which is intimately related to ion transfer at the electrolyte/electrode interface. Three regression methods, namely, multiple linear regression (MLR), least absolute shrinkage and selection operator (LASSO), and exhaustive search with linear regression (ES-LiR), were employed to find the relationship between Ecoord and descriptors. Descriptors include both ion and solvent properties, such as the radius of metal ions or the atomic charge of solvent molecules. Our results clearly indicate that the ionic radius and atomic charge of the oxygen atom that is connected to the metal ion are the most important descriptors. Good prediction accuracy for Ecoord of 0.127 eV was obtained using ES-LiR, meaning that we can predict Ecoord for any alkali ion without performing quantum chemistry calculations for ion-solvent pairs. Further improvement in the prediction accuracy was made by applying the exhaustive search with Gaussian process, which yields 0.016 eV for the prediction accuracy of Ecoord.

Prediction and optimization of epoxy adhesive strength from a small dataset through active learning.

Machine learning is emerging as a powerful tool for the discovery of novel high-performance functional materials. However, experimental datasets in the polymer-science field are typically limited and they are expensive to build. Their size (< 100 samples) limits the development of chemical intuition from experimentalists, as it constrains the use of machine-learning algorithms for extracting relevant information. We tackle this issue to predict and optimize adhesive materials by combining laboratory experimental design, an active learning pipeline and Bayesian optimization. We start from an initial dataset of 32 adhesive samples that were prepared from various molecular-weight bisphenol A-based epoxy resins and polyetheramine curing agents, mixing ratios and curing temperatures, and our data-driven method allows us to propose an optimal preparation of an adhesive material with a very high adhesive joint strength measured at 35.8 ± 1.1 MPa after three active learning cycles (five proposed preparations per cycle). A Gradient boosting machine learning model was used for the successive prediction of the adhesive joint strength in the active learning pipeline, and the model achieved a respectable accuracy with a coefficient of determination, root mean square error and mean absolute error of 0.85, 4.0 MPa and 3.0 MPa, respectively. This study demonstrates the important impact of active learning to accelerate the design and development of tailored highly functional materials from very small datasets.

Liquid electrolyte informatics using an exhaustive search with linear regression.

Exploring new liquid electrolyte materials is a fundamental target for developing new high-performance lithium-ion batteries. In contrast to solid materials, disordered liquid solution properties have been less studied by data-driven information techniques. Here, we examined the estimation accuracy and efficiency of three information techniques, multiple linear regression (MLR), least absolute shrinkage and selection operator (LASSO), and exhaustive search with linear regression (ES-LiR), by using coordination energy and melting point as test liquid properties. We then confirmed that ES-LiR gives the most accurate estimation among the techniques. We also found that ES-LiR can provide the relationship between the "prediction accuracy" and "calculation cost" of the properties via a weight diagram of descriptors. This technique makes it possible to choose the balance of the "accuracy" and "cost" when the search of a huge amount of new materials was carried out.

Surface Properties of CH3NH3PbI3 for Perovskite Solar Cells.

Perovskite solar cells (PSCs) have attracted considerable interest because of their high potential for solar energy conversion. Power conversion efficiencies of the PSCs have rapidly increased from 3.8 to over 20% only in the past few years. PSCs have several similarities to dye-sensitized solar cells in their device compositions; mesoporous TiO2 (mp-TiO2) is sensitized by light-absorbing components and placed into a medium containing hole transporting materials (HTMs). On the other hand, the perovskite materials for the light-harvesting, for example, CH3NH3PbI3 (MAPbI3), have a greater advantage for the photovoltaic applications; extremely long photocarrier diffusion lengths (over 1 µm) enable carrier transports without singnificant loss. In this respect, the surface states, that can be possible recombination centers, are also of great importance. Availability of solution processes is another important aspect in terms of low cost fabrication of PSCs. Two-step methods, where PbI2 is first introduced from solution onto a mp-TiO2 film and subsequently transformed into the MAPbI3 by the exposition of a solution containing MAI, suggest that use of such a high PbI2 concentration is crucial to obtain higher performance. The experiments also indicate that the PbI2-rich growth condition modifies TiO2/ or HTM/MAPbI3 interfaces in such a way that the photocarrier transport is improved. Thus, the characteristics of surfaces and interfaces play key roles in the high efficiencies of the PSCs. In this Account, we focus on the structural stability and electronic states of the representative (110), (001), (100), and (101) surfaces of tetragonal MAPbI3, which can be regarded as reasonable model HTM/MAPbI3 interfaces, by use of first-principles calculations. By examining various types of PbIx polyhedron terminations, we found that there are two major phases on all of the four surface facets. They can be classified as vacant- and flat-type terminations, and the former is more stable than the latter under thermodynamically equilibrium conditions. More interestingly, both terminations can coexist especially on the more probable (110) and (001) surfaces. Electronic states, that is, projected density of states, of the stable-vacant and PbI2-rich-flat terminations on the two surfaces are almost the same as that in bulk MAPbI3. These surfaces can contribute to the long carrier lifetime actually observed for the PSCs because they have no midgap surface states. Furthermore, the shallow surface states on the (110) and (001) flat terminations can be efficient intermediates for hole transport to HTMs. Consequently, the formation of the flat terminations under the PbI2-rich condition will be beneficial for the improvement of PSC performance.

Decomposition of the fluoroethylene carbonate additive and the glue effect of lithium fluoride products for the solid electrolyte interphase: an ab initio study.

Additives in the electrolyte solution of lithium-ion batteries (LIBs) have a large impact on the performance of the solid electrolyte interphase (SEI) that forms on the anode and is a key to the stability and durability of LIBs. We theoretically investigated effects of fluoroethylene carbonate (FEC), a representative additive, that has recently attracted considerable attention for the enhancement of cycling stability of silicon electrodes and the improvement of reversibility of sodium-ion batteries. First, we intensively examined the reductive decompositions by ring-opening, hydrogen fluoride (HF) elimination to form a vinylene carbonate (VC) additive and intermolecular chemical reactions of FEC in the ethylene carbonate (EC) electrolyte, by using density functional theory (DFT) based molecular dynamics and the blue-moon ensemble technique for the free energy profile. The results show that the most plausible product of the FEC reductive decomposition is lithium fluoride (LiF), and that the reactivity of FEC to anion radicals is found to be inert compared to the VC additive. We also investigated the effects of the generated LiF on the SEI by using two model systems; (1) LiF molecules distributed in a model aggregate of organic SEI film components (SFCs) and (2) a LiF aggregate interfaced with the SFC aggregate. DFT calculations of the former system show that F atoms form strong bindings with the Li atoms of multiple organic SFC molecules and play as a joint connecting them. In the latter interface system, the LiF aggregate adsorbs the organic SFCs through the F-Li bindings. These results suggest that LiF moieties play the role of glue in the organic SFC within the SEI film. We also examined the interface structure between a LiF aggregate and a lithiated silicon anode, and found that they are strongly bound. This strong binding is likely to be related to the effectiveness of the FEC additive in the electrolyte for the silicon anode.

First-Principles Study of Ion Diffusion in Perovskite Solar Cell Sensitizers.

Hysteresis in current-voltage curves has been an important issue for conversion efficiency evaluation and development of perovskite solar cells (PSCs). In this study, we explored the ion diffusion effects in tetragonal CH3NH3PbI3 (MAPbI3) and trigonal (NH2)2CHPbI3 (FAPbI3) by first-principles calculations. The calculated activation energies of the anionic and cationic vacancy migrations clearly show that I(-) anions in both MAPbI3 and FAPbI3 can easily diffuse with low barriers of ca. 0.45 eV, comparable to that observed in ion-conducting materials. More interestingly, typical MA(+) cations and larger FA(+) cations both have rather low barriers as well, indicating that the cation molecules can migrate in the perovskite sensitizers when a bias voltage is applied. These results can explain the ion displacement scenario recently proposed by experiments. With the dilute diffusion theory, we discuss that smaller vacancy concentrations (higher crystallinity) and replacement of MA(+) with larger cation molecules will be essential for suppressing hysteresis as well as preventing aging behavior of PSC photosensitizers.

Unusual stability of acetonitrile-based superconcentrated electrolytes for fast-charging lithium-ion batteries.

The development of a stable, functional electrolyte is urgently required for fast-charging and high-voltage lithium-ion batteries as well as next-generation advanced batteries (e.g., Li-O2 systems). Acetonitrile (AN) solutions are one of the most promising electrolytes with remarkably high chemical and oxidative stability as well as high ionic conductivity, but its low stability against reduction is a critical problem that hinders its extensive applications. Herein, we report enhanced reductive stability of a superconcentrated AN solution (>4 mol dm(-3)). Applying it to a battery electrolyte, we demonstrate, for the first time, reversible lithium intercalation into a graphite electrode in a reduction-vulnerable AN solvent. Moreover, the reaction kinetics is much faster than in a currently used commercial electrolyte. First-principle calculations combined with spectroscopic analyses reveal that the peculiar reductive stability arises from modified frontier orbital characters unique to such superconcentrated solutions, in which all solvents and anions coordinate to Li(+) cations to form a fluid polymeric network of anions and Li(+) cations.

Arylpyrrole oligomers as tunable anion receptors.

A novel type of arylpyrrole oligomer possessing an appropriate electropositive cavity has been designed, prepared and analysed for use as readily accessible receptors for negatively charged guests. Affinities of the receptors for various anions were determined by UV/Vis titration experiments and in depth insights into the host-guest interactions were gained by performing (1)H NMR titration experiments and X-ray crystallographic structure analyses. Experimentally determined association constants were correlated with the calculated maximum electrostatic potentials of the electropositive cavities of the receptors, allowing estimation of the strengths of host-guest associations in similar compounds. The joint contribution of aryl C-H and pyrrole N-H hydrogens was shown to be key to a strong guest association, resulting in the arylpyrrole oligomers being efficient anion receptors.

A double-QM/MM method for investigating donor-acceptor electron-transfer reactions in solution.

We developed a double-quantum mechanical/molecular mechanical (d-QM/MM) method for investigation of full outer-sphere electron transfer (ET) processes between a donor and an acceptor (DA) in condensed matter. In the d-QM/MM method, which employs the novel concept of multiple QM regions, one can easily specify the number of electrons, spin states and appropriate exchange-correlation treatment in each QM region, which is especially important in the cases of ET involving transition metal sites. We investigated Fe(2+/3+) self-exchange and Fe(3+) + Ru(2+) → Fe(2+) + Ru(3+) in aqueous solution as model reactions, and demonstrated that the d-QM/MM method gives reasonable accuracy for the redox potential, reorganization free energy and electronic coupling. In particular, the DA distance dependencies of those quantities are clearly shown at the density functional theory hybrid functional level. The present d-QM/MM method allows us to explore the intermediate DA distance region, important for long-range ET phenomena observed in electrochemistry (on the solid-liquid interfaces) and biochemistry, which cannot be dealt by the half-reaction scheme with the conventional QM/MM.

Optimization of the Jastrow factor using the random-phase approximation and a similarity-transformed Hamiltonian: application to band-structure calculation for some semiconductors and insulators.

Based on the random-phase approximation and the transcorrelated (TC) method, we optimize the Jastrow factor together with one-electron orbitals in the Slater determinant in the correlated wave function with a new scheme for periodic systems. The TC method is one of the promising wave function theories for first-principles electronic structure calculation, where the many-body wave function is approximated as a product of a Slater determinant and a Jastrow factor, and the Hamiltonian is similarity-transformed by the Jastrow factor. Using this similarity-transformed Hamiltonian, we can optimize the one-electron orbitals without evaluating 3N-dimensional integrations for the N-electron system. In contrast, optimization of the Jastrow factor within the framework of the TC method is computationally much more expensive and has not been performed for solid-state calculations before. In this study, we also benefit from the similarity-transformation in optimizing the Jastrow factor. Our optimization scheme is tested in applications to some solids from narrow-gap semiconductors to wide-gap insulators, and it is verified that the band gap of a wide-gap insulator and the lattice constants of some solids are improved by this optimization with reasonable computational cost.

Additive effect on reductive decomposition and binding of carbonate-based solvent toward solid electrolyte interphase formation in lithium-ion battery.

The solid-electrolyte interphase (SEI) formed through the reductive decomposition of solvent molecules plays a crucial role in the stability and capability of a lithium-ion battery (LIB). Here we investigated the effects of adding vinylene carbonate (VC) to ethylene carbonate (EC) solvent, a typical electrolyte in LIBs, on the reductive decomposition. We focused on both thermodynamics and kinetics of the possible processes and used density functional theory-based molecular dynamics with explicit solvent and Blue-moon ensemble technique for the free energy change. We considered Li(+) in only EC solvent (EC system) and in EC solvent with a VC additive (EC/VC system) to elucidate the additive effects. In addition to clarifying the equilibrium properties, we evaluated the free energy changes along several EC or VC decomposition pathways under one-electron (1e) reduction condition. Two-electron (2e) reduction and attacks of anion radicals to intact molecules were also examined. The present results completely reproduce the gaseous products observed in the experiments. We also found a new mechanism involving the VC additive: the VC additive preferentially reacts with the EC anion radical to suppress the 2e reduction of EC and enhance the initial SEI formation, contrary to the conventional scenario in which VC additive is sacrificially reduced and its radical oligomerization becomes the source of SEI. Because our mechanism needs only 1e reduction, the irreversible capacity at the SEI formation will decrease, which is also consistent with the experimental observations. These results reveal the primary role of VC additive in the EC solvent.

Trimer effects in fragment molecular orbital-linear combination of molecular orbitals calculation of one-electron orbitals for biomolecules.

The fragment molecular orbital (FMO)-linear combination of molecular orbitals (LCMO) method incorporates as an efficient post-process calculation of one-electron orbitals of the whole system after the FMO total energy calculation. A straightforward way to increase the accuracy is inclusion of the trimer effect. Here, we derive a comprehensive formulation called the FMO3-LCMO method. To keep the computational costs of the trimer term low enough, we use a matrix-size reduction technique. We evaluated the accuracy and efficiency of the FMO3-LCMO scheme in model biological systems (alanine oligomer and chignolin). The results show that delocalized electronic orbitals with covalent and hydrogen bonds are better described at the trimer level, and the FMO3-LCMO method is applicable to quantitative evaluations of a wide range of frontier orbitals in large biosystems.

Efficient algorithm of the transcorrelated method for periodic systems.

The transcorrelated (TC) method is one of the promising wave-function-based approaches for the first-principles electronic structure calculations. In this method, the many-body wave function is approximated as the Jastrow-Slater type and one-electron orbitals in the Slater determinant are optimized with a one-body self-consistent-field equation such as that in the Hartree-Fock (HF) method. Although the TC method has yielded good results for both molecules and solids, its computational cost in solid-state calculations, being of order O(N(k)(3)N(b)(3)) with N(k) and N(b) the respective numbers of k-points and bands, has for some years hindered its wide application in condensed matter physics. Although an efficient algorithm was proposed for a Gaussian basis set, that algorithm is not applicable to a plane-wave basis that is suited to and widely used in solid-state calculations. In this paper, we present a new efficient algorithm of the TC method for the plane-wave basis or an arbitrary basis function set expanded in terms of plane waves, with which the computational cost of the TC method scales as O(N(k)(2)N(b) (2)). This is the same as that of the HF method. We applied the TC method with the new algorithm to obtain converged band structure and cell parameters of some semiconductors.

Determining Factor on the Polarization Behavior of Magnesium Deposition for Magnesium Battery Anode.

To clarify the origin of the polarization of magnesium deposition/dissolution reactions, we combined electrochemical measurement, operando soft X-ray absorption spectroscopy (operando SXAS), Raman, and density functional theory (DFT) techniques to three different electrolytes: magnesium bis(trifluoromethanesulfonyl)amide (Mg(TFSA)2)/triglyme, magnesium borohydride (Mg(BH4)2)/tetrahydrofuran (THF), and Mg(TFSA)2/2-methyltetrahydrofuran (2-MeTHF). Cyclic voltammetry revealed that magnesium deposition/dissolution reactions occur in Mg(TFSA)2/triglyme and Mg(BH4)2/THF, while the reactions do not occur in Mg(TFSA)2/2-MeTHF. Raman spectroscopy shows that the [TFSA]- in the Mg(TFSA)2/triglyme electrolyte largely does not coordinate to the magnesium ions, while all of the [TFSA]- in Mg(TFSA)2/2-MeTHF and [BH4]- in Mg(BH4)2/THF coordinate to the magnesium ions. In operando SXAS measurements, the intermediate, such as the Mg+ ion, was not observed at potentials above the magnesium deposition potential, and the local structure distortion around the magnesium ions increases in all of the electrolytes at the magnesium electrode|electrolyte interface during the cathodic polarization. Our DFT calculation and X-ray photoelectron spectroscopy results indicate that the [TFSA]-, strongly bound to the magnesium ion in the Mg(TFSA)2/2-MeTHF electrolyte, undergoes reduction decomposition easily, instead of deposition of magnesium metal, which makes the electrolyte inactive electrochemically. In the Mg(BH4)2/THF electrolyte, because the [BH4]- coordinated to the magnesium ions is stable even under the potential of the magnesium deposition, the magnesium deposition is not inhibited by the decomposition of [BH4]-. Conversely, because [TFSA]- is weakly bound to the magnesium ion in Mg(TFSA)2/triglyme, the reduction decomposition occurs relatively slowly, which allows the magnesium deposition in the electrolyte.

Two-electron reduction of a Rh-Mo-Rh dithiolato complex to form a triplet ground state associated with a change in CO coordination mode.

We synthesized a dithiolato-bridged heterometal trinuclear complex [{(eta(5)-C(5)Me(5))Rh(S(2)C(6)H(4))}(2)Mo(CO)(2)] (1) in which two rhodadithiolene complex units are bridged by a Mo(CO)(2) moiety. Complex 1 with a Rh(III)-Mo(0)-Rh(III) bond exhibits reversible one-step two-electron reduction with potential inversion. This redox process between 1 and 1(2-) accompanies a reversible structural change, which is an alternation in the CO coordination mode between semibridging and bridging. The ground state of dianion 1(2-) with a Rh(II)-Mo(0)-Rh(II) bond is assigned to spin triplet. These alternations of CO coordination mode and spin state are fully consistent with the density functional theory calculation results. This is the first example of multinuclear metalladithiolene complex which was successful in elucidating a reversible multielectron redox process associated with structural change and spin state change.

Cation Mixing Properties toward Co Diffusion at the LiCoO2 Cathode/Sulfide Electrolyte Interface in a Solid-State Battery.

All-solid-state Li-ion batteries (ASS-LIBs) are expected to be the next-generation battery, however, their large interfacial resistance hinders their widespread application. To understand and resolve the possible causes of this resistance, we examined mutual diffusion properties of the cation elements at LiCoO2 (LCO) cathode/ß-Li3PS4 (LPS) solid electrolyte interface as a representative system as well as the effect of a LiNbO3 buffer layer by first-principles calculations. Evaluating energies of exchanging ions between the cathode and the electrolyte, we found that the mixing of Co and P is energetically preferable to the unmixed states at the LCO/LPS interface. We also demonstrated that the interposition of the buffer layer suppresses such mixing because the exchange of Co and Nb is energetically unfavorable. Detailed analyses of the defect levels and the exchange energies by using the individual bulk crystals as well as the interfaces suggest that the lower interfacial states in the energy gap can make a major contribution to the stabilization of the Co ↔ P exchange, although the anion bonding preference of Co and P as well as the electrostatic interactions may have effects as well. Finally, the Co ↔ P exchanges induce interfacial Li sites with low chemical potentials, which enhance the growth of the Li depletion layer. These atomistic understandings can be meaningful for the development of ASS-LIBs with smaller interfacial resistances.