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The ordered–disordered transition temperature is reduced significantly with increasing cationic sizes. A possible principle for creating better ion-conducting materials is to have smaller cations in a larger unit cell.

Honeycomb layered oxides constitute an emerging class of materials that show interesting physicochemical and electrochemical properties. However, the development of these materials is still limited. Here, we report the combined use of alkali atoms ($\rm Na$ and $\rm K$) to produce a mixed alkali honeycomb layered oxide material, namely, $\rm NaKNi_2TeO_6$. Via transmission electron microscopy measurements, we reveal the local atomic structural disorders characterised by aperiodic stacking and incoherency in the alternating arrangement of $\rm Na$ and $\rm K$ atoms. We also investigate the possibility of mixed electrochemical transport and storage of $\rm Na$ and $\rm K$ ions in $\rm NaKNi_2TeO_6$. In particular, we report an average discharge cell voltage of about $4\, \rm V$ and a specific capacity of around $80\, \rm mAh\, g^{-1}$ at low specific currents (i.e., $< 10\, \rm mA\, g^{-1}$) when a $\rm NaKNi_2TeO_6$-based positive electrode is combined with a room-temperature $\rm NaK$ liquid alloy negative electrode using an ionic liquid-based electrolyte solution. These results represent a step towards the use of tailored cathode active materials for dendrite-free electrochemical energy storage systems exploiting room-temperature liquid alkali metal alloy materials., 39 Pages, 7 Figures, 1 Cover Art

Complex hydrides are of great interest for being potential candidates for the solid electrolyte of all-solid-state batteries owing to their exceptionally high ionic conductivity at a high-temperature phase. Hereafter, we study a model system (LiCB11H12) using molecular dynamics (MD) simulations based on a reported force field to investigate the role of cation size in the order-disorder phase-transition temperature (Ttran) and cationic diffusion. The MD results indicate that the lowering of Ttran is due to the high cell volume (by introducing a large-size cation) that eases the anionic rotation. The results are further confirmed by performing a few additional MD simulations by keeping fixed cation size while varying cell volumes. In addition, we obtain comparative insights into cationic density distribution, the bottlenecks of the cationic path, and its hopping mechanism when increasing cationic sizes. These findings are of fundamental importance for guiding a lower transition temperature and higher cationic diffusion for practical applications.

Complex hydrides are potential candidates for the solid electrolyte of all-solid-state batteries owing to their high ionic conductivities in which icosahedral anion reorientational motion plays an ...

Na3PS4 has received significant attention as a solid electrolyte for use in all-solid-state batteries because of its high ionic conductivities. In this work, we performed a series of molecular dyna...

The derivatives of LiTi2(PO4)3 are promising electrolytes for solid-state batteries. An extensive molecular dynamics study is performed employing a refined set of potential parameters to understand the influence of Ba substitution on Li+ ion conductivity in Bax/2Li1-xTi2(PO4)3 (0.0 ≤ x ≤ 0.83). The refined set of potential parameters reveals the structural and dynamical properties of Bax/2Li1-xTi2(PO4)3 which are consistent with experimental results. In the presence of Ba2+, the system endures a persistent competition between the generation of vacant Li-sites and blocking of Li+ ion paths. The diffusivity of Li+ ions enhances with x and increases one order of magnitude higher at x = 0.67, where the creation of vacant Li-sites mainly drives the diffusion. This trend is similar to the experimental report. However, for x > 0.67 compositions, the blocking of the Li+ ion path dominates in the presence of immobile Ba2+ ions, resulting in a reduction of Li+ ion diffusion. The present study also proposes an ordered substitution of Ba2+ ions at crystallographically identified Li1-sites, where an extra Li-site generation is identified at higher compositions. In this case, the vacancy strongly dominates over Li+ ion path blocking, resulting in the possibility to achieve even higher Li+ ion diffusion. The creation of extra Li-sites and mechanism of Li+-ion transport are studied systematically with varying compositions. Further insight into Li+ ion transport is gained by constructing a three-dimensional density map and determining the free energy barrier and clustering of Li+ ion probability density. And the factors affecting the cation diffusion are also systematically investigated.

Honeycomb layered oxides have attracted recent attention because of their rich crystal chemistry. However, the atomistic mechanisms of cationic transport in these structures remain vastly unexplored. Herein, we perform an extensive, systematic molecular dynamics study on Na2LiFeTeO6 using combined force-field and first-principles-based molecular dynamics simulations. We use are fined set of inter-atomic potential parameters of a previously reported potential model that represents various structural and transport properties of this recently reported promising material for all-solid-state battery applications. The present simulation study elucidates the roles of octahedral ordering and entropic contributions in Na+-ion distribution in the ab-plane. Our theoretical simulation also develops a ring-like atomistic diffusion mechanism and relevant atomistic energy barriers that help to understand the origin of fast ion conduction in honeycomb layered oxides.

Although the fascinatingly rich crystal chemistry of honeycomb layered oxides has been accredited as the propelling force behind their remarkable electrochemistry, the atomistic mechanisms surrounding their operations remain unexplored. Thus, herein, we present an extensive molecular dynamics study performed systematically using a reliable set of inter-atomic potential parameters of A2Ni2TeO6 (where A = Li, Na, and K). We demonstrate the effectiveness of the Vashishta-Rahman form of the inter-atomic potential in reproducing various structural and transport properties of this promising class of materials and predict an exponential increase in cationic diffusion with larger inter-layer distances. The simulations demonstrate the correlation between broadened inter-layer (inter-slab) distances associated with the larger ionic radii of K and Na compared to Li and the enhanced cationic conduction exhibited in K2Ni2TeO6 and Na2Ni2TeO6 relative to Li2Ni2TeO6. Whence, our findings connect lower potential energy barriers, favourable cationic paths and wider bottleneck size along the cationic diffusion channel within frameworks (comprised of larger mobile cations) to the improved cationic diffusion experimentally observed in honeycomb layered oxides. Furthermore, we elucidate the role of inter-layer distance and cationic size in cationic diffusion. Our theoretical studies reveal the dominance of inter-layer distance over cationic size, a crucial insight into the further performance enhancement of honeycomb layered oxides.

We demonstrate the feasibility of using a combination of alkali atoms (Na and K) to develop a robust mixed-alkali honeycomb layered oxide NaKNi2TeO6. Through a series of atomic-resolution transmission electron microscopy in multiple zone axes, we reveal for the first time the local atomic structural disorders characterised by aperiodic stackings and incoherency in the alternating arrangement of Na and K atoms. Our findings indicate great structural versatility that renders NaKNi2TeO6 an ideal platform for investigating other fascinating properties such as mixed ionic transport and intriguing electromagnetic and quantum phenomena amongst honeycomb layered oxides. Finally, we unveil the possibility of inducing mixed Na- and K-ion transport electrochemistry of NaKNi2TeO6 at high voltages (~ 4V), thus epitomising it as a competent cathode candidate for the emerging dendrite-free batteries based on NaK liquid metal alloy as anodes. The results not only betoken a new avenue for developing functional materials with fascinating crystal versatility, but also prefigure a new age of ‘dendrite-free’ energy storage system designs that rely on mixed-cation electrochemistry.

Complex hydrides are potential candidates for the solid electrolyte of all-solid-state batteries owing to their high ionic conductivities, in which icosahedral anion reorientational motion plays an essential role in high cation diffusion. Herein, we report molecular dynamics (MD) simulations based on a refined force field and first-principles calculations of the two complex hydride systems Li2B12H12 and LiCB11H12 to investigate their structures, order–disorder phase-transition behavior, anion reorientational motion, and cation conductivities. For both systems, force-field-based MD successfully reproduced the structural and dynamical behavior reported in experiments. Remarkably, it showed an entropy-driven order–disorder phase transition associated with high anion reorientational motion. Furthermore, we obtained comparative insights into the cation around the anion, cation site occupancy in the interstitial space provided by anions, cation diffusion route, role of cation vacancies, anion reorientation, and effect of cation–cation correlation on cation diffusion. We also determined the factors that activate anion reorientational motion leading to a low to high conductivity phase transition. These findings are of fundamental importance in fast ion-conducting solids to diminish the transition temperature for practical applications.

Although the fascinatingly rich crystal chemistry of honeycomb layered oxides has been accredited as the propelling force behind their remarkable electrochemistry, the atomistic mechanisms surrounding their operations remain unexplored. Thus, herein, we present an extensive molecular dynamics study performed systematically using a refined set of inter-atomic potential parameters of A2Ni2TeO6 (where A = Li, Na, and K). We demonstrate the effectiveness of the Vashishta-Rahman form of the interatomic potential in reproducing various structural and transport properties of this promising class of materials and predict an exponential increase in cationic diffusion with larger interlayer distances. The simulations further demonstrate the correlation between broadened inter-layer (inter-slab) distances associated with the larger ionic radii of K and Na compared to Li and the enhanced cationic conduction exhibited in K2Ni2TeO6 and Na2Ni2TeO6 relative to Li2Ni2TeO6. Whence, our findings connect a wider bottleneck along the cationic diffusion channel within frameworks comprised of larger mobile cations to the improved cationic diffusion experimentally observed in honeycomb layered oxides.

The ion coordination and local ion transport of the ternary electrolyte containing sulfolane (SL), lithium bis(trifluoromethanesulfonyl)amide (LiTFSA), and a carbonate solvent were investigated by combining the electrochemical measurement and first-principles molecular dynamics (FPMD) simulation. Li(SL)3TFSA solution with the low-viscosity carbonate solvents, propylene carbonate (PC) and butylene carbonate (BC) was quasi-solidified by nanosized SiO2 and PTFE binder. Electrochemical impedance spectroscopy revealed that the quasi-solidified electrolyte sheets exhibited the Li+ transport number of 0.5, which was higher than that of Li(G4)TFSA containing PC and the conventional organic liquid electrolyte. The higher Li+ transport number contributed to an enhanced discharge capacity of the LIB at a high rate, and the LIB with Li(SL)3TFSA-PC and Li(SL)3TFSA-BC based electrolyte exhibited higher capacity retention in cycle test compared with Li(G4)TFSA-PC. The FPMD analysis revealed that a bridging coordination by SL and TFSA, where a single SL or TFSA coordinates two Li+, was observed even under coexistence with the carbonate solvent. It was also confirmed that some Li+ tend to transfer independently to the other molecules/anions via the bridging coordination, leading to the high Li+ transport number. In conclusion, the composition design of the electrolyte to keep the bridging coordination is desirable for a high Li+ transport number and superior LIB performances.

Traditional refrigeration technologies based on compression cycles of greenhouse gases pose serious threats to the environment and cannot be downscaled to electronic device dimensions. Solid-state cooling exploits the thermal response of caloric materials to external fields and represents a promising alternative to current refrigeration methods. However, most of the caloric materials known to date present relatively small adiabatic temperature changes ($|\Delta T| \sim 1$ K) and/or limiting irreversibility issues resulting from significant phase-transition hysteresis. Here, we predict the existence of colossal barocaloric effects (isothermal entropy changes of $|\Delta S| \sim 100$ JK$^{-1}$kg$^{-1}$) in the energy material Li$_{2}$B$_{12}$H$_{12}$ by means of molecular dynamics simulations. Specifically, we estimate $|\Delta S| = 387$ JK$^{-1}$kg$^{-1}$ and $|\Delta T| = 26$ K for an applied pressure of $P = 0.4$ GPa at $T = 475$ K. The disclosed colossal barocaloric effects are originated by an order-disorder phase transformation that exhibits a fair degree of reversibility and involves coexisting Li$^{+}$ diffusion and (BH)$_{12}^{-2}$ reorientational motion at high temperatures., Comment: 8 pages, 3 figures, 1 table

The derivatives of LiTi2(PO4)3 are promising electrolytes for solid-state batteries. An extensive molecular dynamics study is performed employing a refined set of potential parameters to understand the influence of Ba substitution on Li+ ion conductivity in Bax/2Li1-xTi2(PO4)3 (0.0 ≤ x ≤ 0.83). The refined set of potential parameters reveals the structural and dynamical properties of Bax/2Li1-xTi2(PO4)3 which are consistent with experimental results. In the presence of Ba2+, the system endures a persistent competition between the generation of vacant Li-sites and blocking of Li+ ion paths. The diffusivity of Li+ ions enhances with x and increases one order of magnitude higher at x = 0.67, where the creation of vacant Li-sites mainly drives the diffusion. This trend is similar to the experimental report. However, for x0.67 compositions, the blocking of the Li+ ion path dominates in the presence of immobile Ba2+ ions, resulting in a reduction of Li+ ion diffusion. The present study also proposes an ordered substitution of Ba2+ ions at crystallographically identified Li1-sites, where an extra Li-site generation is identified at higher compositions. In this case, the vacancy strongly dominates over Li+ ion path blocking, resulting in the possibility to achieve even higher Li+ ion diffusion. The creation of extra Li-sites and mechanism of Li+-ion transport are studied systematically with varying compositions. Further insight into Li+ ion transport is gained by constructing a three-dimensional density map and determining the free energy barrier and clustering of Li+ ion probability density. And the factors affecting the cation diffusion are also systematically investigated.

Fluoride-shuttle batteries (FSBs), which are based on fluoride-ion transfer, have attracted attention because of their high theoretical energy densities. The fluorination and defluorination reactions at the electrodes are the possible rate-determining steps in FSBs, and understanding the mechanism is important to achieve smooth charge/discharge. In this study, we discuss the thermodynamically favored pathways for the fluorination and defluorination reactions and compare the reactions through the solid-solution and two-phase-coexistent states by density functional theory (DFT) calculations. The free energies of the solid-solution and two-phase states approximate the energies calculated by DFT, and their accuracy was validated by comparison with experimental formation enthalpies and free energies. The relative formation enthalpies of typical, transition, and relativistic metal (Tl, Pb, and Bi) fluorides are well reproduced by DFT calculations within 0.1, 0.2, and 0.4 eV, respectively. We also show that the reaction pathway can be determined by comparing the formation enthalpies of the metal fluoride H, a fluorine vacancy HV, and an interstitial fluorine defect HI from the simple selection rule. The enthalpy relation of HI > H > -HV observed in all the calculations strongly suggests that fluorination and defluorination in FSB electrodes occur by a two-phase reaction. This fluorination and defluorination mechanism will be useful to clarify the rate-determining step in FSBs.

Degradation reactions of perfluorosulfonic acid ionomers via H or OH radicals were investigated employing B3LYP/6-311++G (2d,2p) and ωB97XD/6-311++G (2d,2p). All important degradation reaction mech...

The charge transfer reaction at the electrode/solution interface is regarded as a major component that limits the current densities of Li-ion batteries. A combination of density functional theory for the electronic structure calculations of an electrode and a reacting component with implicit solvation theory for an electrolyte solution is used for the lithium insertion/desorption reaction (R). At the interfaces between tilted-graphite electrodes and a 1 M LiPF6 ethylene carbonate solution, the energy landscapes of reaction R are revealed under constant electron chemical potential conditions. Across the transition state where the Li forms a half solvation shell, the reacting Li inside the electrode changes to a full solvation structure in the solution accompanied by electron transfer. On graphite with two tilt angles, the activation energies at the equilibrium potentials of reaction R are approximately 0.6 eV, which is consistent with the electrochemical impedance spectroscopy measurements. However, opposi...

High-throughput prediction of chemical and physical properties of materials is a promising approach to the discovery and development of novel materials. We perform high-throughput first-principles calculations of more than 65 000 Li/vacancy configurations at different concentrations in the surface region of Li2MnO3 to exhaustively investigate its structural evolution during the delithiation process. Through this computation, we establish a complete picture of all stable and unstable configurations of the material that is difficult to obtain with nonexhaustive calculations. Given the picture, we identify the free energy and the ground-state energy together with their electrochemical potential by considering the energy levels of all configurations. Examination into the vacancy population and vacancy site map shows that the Li atoms are extracted from the surface region first, and then from the bulk during the process. The Li extraction is found to destabilize the O atoms near the surface and help in generat...

closo-type complex hydrides contain large cage-type complex polyanions in their crystal structures and thus can exhibit superior ion-conducting properties (e.g., Li and Na). However, the unique structures of complex polyanions have made it challenging to modify crystal structures, making systematic control of ion conductivity difficult. Here, we report an atom deficiency approach to enhance lithium-ion conductivity of complex hydrides. We find that lithium and hydrogen could be simultaneously extracted from Li2B12H12 by applying a small external energy, enabling the formation of atom deficiencies. These atom deficiencies lead to an increase in carrier concentration, improving lithium-ion conductivity by 3 orders of magnitude compared to that of a pristine material. An all-solid-state TiS2/Li battery employing atom-deficient Li2B12H12 as a solid electrolyte exhibits superior battery performance during repeated discharge–charge cycles. The current study suggests that the atom deficiency can be a useful stra...

Hydrogen evolution reaction (HER) on Cu and Ag electrodes in neutral and alkaline media (but not in acidic media) is significantly accelerated by the addition of 4,4′-bipyridine (BiPy). The mechanism of the HER acceleration is discussed at a molecular level by using surface-enhanced infrared absorption spectroscopy (SEIRAS) coupled with electrochemical techniques and density functional theory (DFT) calculations. Simultaneous SEIRAS and cyclic voltammetry measurements reveal that the onset potential of the accelerated HER coincides with the 1e − + 1H + reduction potential of adsorbed BiPy to yield monohydro BiPy radical (BiPy-H ). Tafel plot analysis and DFT calculations suggest that BiPy-H is further reduced to N , C(3) -dihydro BiPy via a subsequent 1e − + 1H + step and finally H 2 is evolved by the combination of two H atoms on N and C(3) atoms (BiPy-H 2 → BiPy + H 2 ). Since adsorbed BiPy is readily reduced again to BiPy-H in the potential range of HER, the reaction process is catalytically cycled. The enhanced HER is not observed on Au electrodes due to the desoption of BiPy from the surface. Also the enhanced HER is not observed on Pt electrodes despite strong adsorption on Pt because the redox potential of adsorbed BiPy, the first step of the HER, is more negative than the onset of HER on bare Pt surface.

Electrochemical hydrogen evolution reaction (HER) on silver is significantly accelerated by adding 4,4 ′ -bipyridine (BiPy) into neutral or alkaline aqueous solution (T. Uchida et al. J. Am. Chem. Soc. , 130 (2008) 10826). The mechanism of the accelerated HER by BiPy is studied by the first-principles molecular dynamics simulation with the constant Fermi energy scheme of the effective screening medium (ESM) method. The simulation shows that H 2 is evolved through the following three steps: First, BiPy adsorbed on the electrode via one N-end directing another N-end toward the solution is reduced to N -hydro BiPy radical (BiPy-H⋅), in which H is bonded to N atom of the solution side. BiPy-H⋅ is further reduced to N , C (3)-dihydro BiPy with an activation free energy of 0.5 eV by the second proton-coupled electron transfer. Finally, H 2 is released from the two H atoms on N and C(3) of N , C (3)-dihydro BiPy by the assist of H 2 O or OH − . BiPy is reproduced by the H 2 evolution and the processes are catalytic-cycled.

${\mathrm{Li}}_{2}{\mathrm{B}}_{12}{\mathrm{H}}_{12}$ and its derivatives are promising solid electrolytes for solid-state batteries. In this work, a potential model is proposed, and an extensive classical molecular dynamics study is performed to understand the origin of the fast ion conduction in ${\mathrm{Li}}_{2}{\mathrm{B}}_{12}{\mathrm{H}}_{12}$. The proposed potential model reveals structural and dynamical properties of ${\mathrm{Li}}_{2}{\mathrm{B}}_{12}{\mathrm{H}}_{12}$ that are consistent with first-principles molecular dynamics simulation and experimental results. The mechanism of ${\mathrm{Li}}^{+}$-ion transport is studied systematically. The low-temperature $\ensuremath{\alpha}$ phase exhibits negligible diffusivity within a timescale of a few nanoseconds, whereas the high-temperature $\ensuremath{\beta}$ phase with a similar crystal structure and larger lattice parameter exhibits entropy-driven high ${\mathrm{Li}}^{+}$-ion diffusion assisted by anionic reorientation. We explicitly demonstrate the role of closo-borane anionic reorientational motion in the ${\mathrm{Li}}^{+}$-ion diffusion and explain how the cell parameter facilitates the anionic reorientational motion. In addition, enhancing the degree of H freedom (by changing the B-B-H angular force parameters) results in significantly high cationic diffusivity at low temperature. Further insight into ${\mathrm{Li}}^{+}$-ion transport is obtained by constructing a three-dimensional density map and determining the free-energy barrier, and the factors affecting cationic diffusion are thoroughly investigated with high precision using long simulations (5 ns).

Introduction The rechargeable Li-ion battery (LIB) is a successful energy storage device because of its high energy density and long cycle life. In order to improve its performance, quantitative understanding of elementary reactions in the LIB such as the reaction of Li intercalation from electrolyte solution into graphite and crystal structure change in graphite during the reaction must be a great help. Structure change of Li-intercalated graphite during the charge/discharge processes was recently observed by operando X-ray diffraction measurements using a synchrotron radiation at SPring-8.1 The operando XRD data at LiC18 charge/discharge state exhibited two peaks corresponding two different C-C grid distances in the same plane of graphite, while no change in interlayer distance was detected during the appearance and disappearance of these peaks; it implies only in-plane structure transition such as Li/6C layer to Li/9C layer changes.1 An AA to AB stack transition in graphite was characterized by (101) peak disappearance in the XRD data at the composition between LiC63–LiC72.2 To interpret and discuss these experimental results, we conduct first-principles calculations for the phase stability of graphite. Methods and Models The formation enthalpy ∆H f of Li-intercalated graphite is defined as ∆H f (Li n C m ) = ( E DFT(Li n C m ) − nE DFT(Li) − mE DFT(C) ) / (m+n) (1), where E DFT are the total energies of Li-intercalated graphite Li n C m , simple metal Li, and AB stacked graphite C from density functional theory (DFT) calculations. Formation enthalpy as a function of C-ratio, x C = m / (m + n) (2), was used to investigate the phase stability of Li n C m . Quantum ESPRESSO with a plane-wave basis3 and van der Waals functional4 was used to obtain DFT energies. Our previous study using this functional well reproduced the experimental lattice constants and electromotive forces (EMFs) of Li-intercalated graphite.5 It is known that Li-intercalated graphite exhibits stages from 1 to 8. These stages may take different combination of in-plane configurations and interlayer configurations. In-plane configurations considered are Li/6C, Li/9C, Li/18C, and Li/24C, which means the ratios of Li/C atoms are located on a single layer as shown in Fig. 1 (a). Three interlayer structures are considered here; (1) graphite layers stacked with AA overlap, (2) with AB, and (3) mixed (Li including layers are AA stacked, the other layers are AB stacked). Conventional LiC6 intercalated phase corresponds to the stage 1 of Li/6C AA stack structure (denoted as s1-Li/6C-AA, here s1, s2, s3, ... represents stage 1, stage 2, stage 3, ..., respectively). Results The calculated formation enthalpies ∆H f are plotted as a function of x C in Fig. 1 (b). At the composition of LiC6 (x C = 0.857), LiC12 (x C = 0.923), and C (x C = 1), respective most stable phases are s1-Li/6C-AA, s2-Li/6C-AA, and AB stack graphite ones as we could expect. At the composition of LiC18 (x C = 0.947), s2-Li/9C-AA structure is more stable than the s3-Li/6C-AA structure, which is consistent with the experimental observation.1 AB stack structures are more stable than AA at x C > 0.97 from the theoretical calculation, while the AA-AB transition has been observed at 0.984 < x C < 0.986 by experiments. At higher stages (x C > 0.94), mixed stack structure is predicted to be more stable than simple AA and AB structures. The formation enthalpies of s2-Li/9C-mixed and s3-Li/6C-mixed are almost the same (within 0.01 meV/atom) and these configurations are predicted to be the most stable at LiC18. However, these mixed stackings have not been detected by experiments. In the presentation, we will also present the theoretical EMF values of the Li-intercalated graphite electrode and compare them with experimental results. Acknowledgments This work was supported in part by the Research and Development Initiative for Scientific Innovation of New Generation Batteries 2 Project (RISING2) administrated by the New Energy and Industrial Technology Development Organization (NEDO). References S. Takagi, K. Shimoda, H. Fujimoto, H. Kiuchi, T. Naka, T. Murata, K.-I. Okazaki, T. Fukunaga, E. Matsubara, Z. Ogumi, and T. Abe, “Operando Structure Analysis of Graphite Electrode by Synchrotron Radiation Diffraction” The 60th Battery Symposium, 1B20 (2019). H. Fujimoto , S. Takagi , H. Kiuchi , K. Shimoda , K.-I. Okazaki, T. Murata , T. Abe, Z. Ogumi, and E. Matsubara, “Operando analysis of charge/discharge reaction mechanism of graphite anode of Li ion battery using synchrotron radiation diffraction, Second report”, The 59th Battery Symposium, 2E16 (2018). P. Giannozzi et al., J. Phys.: Condens. Matter 21, 395502 (2009); ibid. 29, 465901 (2017). I. Hamada, Phys. Rev. B 89, 121103(R) (2014). J. Haruyama, T. Ikeshoji, and M. Otani, J. Chem. Phys. C 122, 9804 (2018). Figure 1

The structural and dynamic properties of a brush-type hydrocarbon copolymer are investigated using first-principles molecular dynamics simulations. Two model compounds, one with mainly hydrophilic domains and one with mainly hydrophobic domains, were selected and used in the simulations. A series of radial distribution functions of different groups, such as water–water, sulfonic group–hydrogen, and ether–hydrogen, is obtained to investigate the structure of the whole systems. The radial distribution functions of sulfonic groups, gS–S(r), and the structure of water clusters indicate the formation of a well-developed water channel in the studied copolymer. Analysis of proton dissociation reveals that the protons in both systems are not completely dissociated when the number of water molecules per sulfonic group is equal to 4. The low dissociation nature of this copolymer compared with that of Nafion is explained by its intrinsic acid strength and the presence of ineffective hydrogen bonds in the system, whe...

INTRODUCTION Rechargeable batteries with energy densities higher than those of the present Li-ion batteries (LIBs) are required for electric vehicles. We focus on fluoride-ion (F−) transfer batteries whose electrodes are composed of metal/metal fluoride (M|MF n ) combinations. Electrochemical charge-transfer reactions occur during the discharge/charge processes at the two electrodes. Defluorination of the metal fluoride MF n and fluorination of another metal M’ occurs at the positive and negative electrodes, respectively, during the discharging process as M1/n F + e− → M1/n + F− (1), M’1/m + F− → M’1/m F + e− (2). This type of battery is called a fluoride-shuttle battery (FSB), which is expected to produce high voltage and have large capacity.1 In 2017, Okazaki et al. fabricated a Bi|BiF3 electrode with a liquid electrolyte prepared by dissolving an organic fluoride in an ionic liquids (ILs) at room temperature. 2 In 2018, Davis et al. reported another type of liquid electrolyte composed of a simple organic solvents (OSs) with F−conductivity greater than 1 mS/cm.3 From the development of these liquid electrolytes, the F− conductivity has been dramatically increased in recent years. After improvement of the electrolyte conductivity, the fluorination and defluorination reactions at the electrodes are considered to be the rate-determining steps in FSBs. However, the mechanism of the fluorination and defluorination reactions in FSBs has not been clarified in contrast to the well-investigated Li-intercalation reaction in LIBs. METHODS In this study, we classify the fluorination and defluorination reactions into two possible reaction pathways from a thermodynamic point of view: reaction through the solid-solution state and reaction through the two-phase state. It is possible to find which reaction pathway is realized from the free energy difference. Selection of these reaction pathways can be determined from the enthalpies of all of the intermediate states, since the entropy term in the free energy is fairly cancelled when we consider the difference of free energies. Selection rule for the fluorination and defluorination reactions in FSBs is schematically illustrated in Figure 1. The solid-solution states of M1/n F x for x ≈ 0 and for x ≈ 1 are assumed to be formed by introducing an interstitial and a vacancy defect, respectively. In this case, the free energy relation for the selection can be described as G ss(x) < G tp(x) → H I(M1/n ) < H(M1/n F) or H(M1/n F) < −H V(M1/n F) (3), where G ss(x), G tp(x), H (M1/n F), H I(M1/n F) , H V(M1/n F) are the free energy of the solid-solution state of M1/n F x , that of two-phase state, the formation enthalpy of M1/n F, that of interstitial defect in M1/n , and that of vacancy defect in M1/n F, respectively. We use the energies from first-principles calculations for the enthalpy differences, whose accuracy is validated by comparing them with experimentally obtained thermodynamic enthalpy data.4 The first-principles calculations were performed with the configurations of the bulk crystal systems using Quantum Espresso package.5 The crystal data were taken from the AtomWork database.6 RESULTS The relative formation enthalpies of typical, transition, and relativistic metal fluorides are well reproduced by first-principles calculations within 0.1, 0.2, and 0.4 eV, respectively. By comparing the formation enthalpies of H, H V, and H I, the relation H I > H > −H V is always satisfied in all of the calculated metal and metal-fluoride combinations (M = Li, Mg, Ca, Al, Co, Ni, Cu, Pb, Bi). Thus, the selection rule of eq. (3) strongly suggests that fluorination and defluorination in FSB electrodes occur by two-phase reaction.7 This fluorination and defluorination mechanism will be useful to clarify the rate-determining step in FSBs. Acknowledgments This work was supported by the Research and Development Initiative for Scientific Innovation of New Generation Batteries 2 Project (RISING2) administrated by the New Energy and Industrial Technology Development Organization (NEDO). REFERENCES 1) F. Gschwind, G. Rodriguez-Garcia, D. Sandbeck, A. Gross, M. Weil, M. Fichtner, N. Hörmann, J. Fluorine Chem. 182, 76 (2016). 2) K.-I. Okazaki, Y. Uchimoto, T. Abe, Z. Ogumi, ACS Energy Lett. 2, 1460 (2017). 3) V. K. Davis et al., Science 362, 1144 (2018). 4) Handbook of Chemistry: Pure Chemistry, 5th ed. 5) P. Giannozzi et al., J. Phys.: Condens. Matter. 29, 465901 (2017). 6) National Institute for Materials Science (NIMS) AtomWork 7) J. Haruyama, K.-I. Okazaki, Y. Morita, H. Nakamoto, E. Matsubara, T. Ikeshoji, M. Otani, “Two-Phase Reaction Mechanism for Fluorination and Defluorination in Fluoride-Shuttle Batteries: A First-Principles Study” in revision. Figure 1. Two reaction pathways considered in this study. In the fluorination and defluorination reactions, the solid-solution (M1/n F x , 0 < x< 1) and two-phase (M1/n and M1/n F) states are considered to be possible intermediate states. Figure 1

Solid-state materials containing rotatable polyanions, such as B12 H122−, constitute a peculiar class of ionic conductors due to their unique transport behavior, where rotating polyanions promote phase transitions to disordered phases with several orders of magnitude enhancement in cation conductivities. A major drawback is the high temperature required to activate rotation and thereby low conductivities at room temperature. Here, we elucidate a mechanism to drastically reduce the temperature based on the use of pseudorotation in high-H coordination hydride complexes. We demonstrate this mechanism for an existing complex transition metal hydride Li5MoH11 containing MoH93−, and we present a strong potential of this material to unprecedentedly exhibit a high lithium ion conductivity of 7.9 × 10–2 S cm−1 at room temperature.

We present a method that makes it possible to determine an electrode potential in an electrode/electrolyte solution system. We consider the electrode potential of the standard hydrogen electrode (SHE) reaction $\mathrm{R}:\phantom{\rule{0.28em}{0ex}}1/2\phantom{\rule{0.16em}{0ex}}{\mathrm{H}}_{2}(\mathrm{gas})+{\mathrm{H}}_{2}\mathrm{O}\phantom{\rule{0.16em}{0ex}}(1\phantom{\rule{0.16em}{0ex}}\mathrm{M}\phantom{\rule{0.16em}{0ex}}\mathrm{HCl}\phantom{\rule{0.16em}{0ex}}\mathrm{aq}.)\ensuremath{\leftrightarrow}{\mathrm{H}}_{3}{\mathrm{O}}^{+}(1\phantom{\rule{0.16em}{0ex}}\mathrm{M}\phantom{\rule{0.16em}{0ex}}\mathrm{HCl}\phantom{\rule{0.16em}{0ex}}\mathrm{aq}.)+{e}^{\ensuremath{-}}$ (electrode M), and conduct density functional theory (DFT) calculations combined with the effective screening medium (ESM) method and the reference interaction site model (RISM). The electrostatic field from a charged slab described by DFT with ESM is screened by that from the charge distribution in an electrolyte solution. This screening enables us to define the inner potential ${\mathrm{\ensuremath{\Phi}}}_{\mathrm{S}}$ at the bulk solution region, which is the reference potential for the electrode potential, that is, the chemical potential of electrons (${\ensuremath{\mu}}_{\mathrm{e}}$). Grand potentials of the left and right sides in reaction R at the equilibrium point derive the corresponding SHE potential of ${\ensuremath{\mu}}_{\mathrm{e}}^{\mathrm{SHE}}\phantom{\rule{0.16em}{0ex}}=\phantom{\rule{0.16em}{0ex}}\ensuremath{-}5.27\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$ vs ${\mathrm{\ensuremath{\Phi}}}_{\mathrm{S}}$ for a Pt(111) electrode. Another pathway that uses the free energy difference gives the same SHE potential; the equivalence of the electrode potentials from chemical potential and from free energy difference is validated within ESM-RISM. Even using a different electrode of Al yields the value of ${\ensuremath{\mu}}_{\mathrm{e}}^{\mathrm{SHE}}=\ensuremath{-}5.22\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$ vs ${\mathrm{\ensuremath{\Phi}}}_{\mathrm{S}}$, which indicates that the electrode potential is independent of the electrodes. Finally, the potential energy profile in a vacuum/metal/solution/vacuum region shows that a difference between the inner and outer potentials is necessary to compare an absolute SHE potential and the SHE potential vs ${\mathrm{\ensuremath{\Phi}}}_{\mathrm{S}}$.

We report the result of a large-scale first-principles molecular dynamics simulation under different electric biases performed to understand the charge transfer process coupling with lithium deposition and desolvation processes. We applied the effective screening medium (ESM) method to control the bias across the electrode/solution interface, and simulated a series of Li de-solvation and Li-deposition reactions occurring under the bias. Solvated Li+ in the bulk propylene carbonate migrates to the Si electrode surface and gradually de-solvates through the transition state. Introducing the blue-moon ensemble method, we determined the possible structures and activation energies for the transition states.

Details on the degradation mechanism of hydrocarbon ionomers for polymer electrolyte membrane fuel cells (PEFCs) have been investigated using density functional theory (DFT). Two model compounds of a hydrocarbon ionomer were selected to investigate the nature of the degradation, where the compounds selected represented the general features of available hydrocarbon polymer electrolyte membranes (PEMs). The results show that two degradation reactions are energetically favorable, indicating two possible weak sites in the hydrocarbon PEMs susceptible to OH or H radical attack. One site has an aryl sulfonated bond and a sulfonate group, and the other site has an aryl ether bond. The alkane chains in the PEMs were calculated to be relatively stable against radical attack. In addition, we found that in contrast to the degradation of perfluorinated PEMs, hydrocarbon PEMs are relatively robust against H radical attack.

In this study, we assembled a bulk-type all-solid-state battery comprised of a TiS2 positive electrode, LiBH4 electrolyte, and Li negative electrode. Our battery retained high capacity over 300 discharge–charge cycles when operated at 393 K and 0.2 C. The second discharge capacity was as high as 205 mAh g–1, corresponding to a TiS2 utilization ratio of 85%. The 300th discharge capacity remained as high as 180 mAh g–1 with nearly 100% Coulombic efficiency from the second cycle. Negligible impact of the exposure of LiBH4 to atmospheric-pressure oxygen on battery cycle life was also confirmed. To investigate the origin of the cycle durability for this bulk-type all-solid-state TiS2/Li battery, electrochemical measurements, thermogravimetry coupled with gas composition analysis, powder X-ray diffraction measurements, and first-principles molecular dynamics simulations were carried out. Chemical and/or electrochemical oxidation of LiBH4 occurred at the TiS2 surface at the battery operating temperature of 393 K...

The dehydriding process and hydrogen–deuterium exchange (H–D exchange) of xLiBH4 + (1 − x)Mg2FeD6 (x = 0.25, 0.75) composites has been studied in detail. For the composition with x = 0.25, only one overlapping mass peak of all hydrogen and deuterium related species was observed in mass spectrometry. This implied the simultaneous dehydriding of LiBH4 and Mg2FeD6, despite an almost 190 °C difference in the dehydriding temperatures of the respective discrete complex hydrides. In situ infrared spectroscopy measurements indicated that H–D exchange between [BH4]− and [FeD6]4− had occurred during ball-milling and was promoted upon heating. The extent of H–D exchange was estimated from the areas of the relevant mass signals: immediately prior to the dehydriding, more than two H atoms in [BH4]− was replaced by D atoms. For x = 0.75, H–D exchange also occurred and about one to two H atoms in [BH4]− was replaced by D atoms immediately before the dehydriding. In contrast to the situation for x = 0.25, firstly LiBH4 and Mg2FeD6 dehydrided simultaneously with a special molar ratio = 1:1 at x = 0.75, and then the remaining LiBH4 reacted with the Mg and Fe derived from the dehydriding of Mg2FeD6.

Despite many exploratory studies over the past several decades, the presently known transition metals that form homoleptic transition-metal hydride complexes are limited to the Groups 7-12. Here we present evidence for the formation of Mg3 CrH8 , containing the first Group 6 hydride complex [CrH7 ](5-) . Our theoretical calculations reveal that pentagonal-bipyramidal H coordination allows the formation of σ-bonds between H and Cr. The results are strongly supported by neutron diffraction and IR spectroscopic measurements. Given that the Group 3-5 elements favor ionic/metallic bonding with H, along with the current results, the true boundary for the formation of homoleptic transition-metal hydride complexes should be between Group 5 and 6. As the H coordination number generally tends to increase with decreasing atomic number of transition metals, the revised boundary suggests high potential for further discovery of hydrogen-rich materials that are of both technological and fundamental interest.

The dehydriding process and hydrogen–deuterium exchange (H–D exchange) of x LiBH 4 + (1 − x )Mg 2 FeD 6 ( x = 0.25, 0.75) composites has been studied in detail. For the composition with x = 0.25, only one overlapping mass peak of all hydrogen and deuterium related species was observed in mass spectrometry. This implied the simultaneous dehydriding of LiBH 4 and Mg 2 FeD 6 , despite an almost 190 °C difference in the dehydriding temperatures of the respective discrete complex hydrides. In situ infrared spectroscopy measurements indicated that H–D exchange between [BH 4 ] − and [FeD 6 ] 4− had occurred during ball-milling and was promoted upon heating. The extent of H–D exchange was estimated from the areas of the relevant mass signals: immediately prior to the dehydriding, more than two H atoms in [BH 4 ] − was replaced by D atoms. For x = 0.75, H–D exchange also occurred and about one to two H atoms in [BH 4 ] − was replaced by D atoms immediately before the dehydriding. In contrast to the situation for x = 0.25, firstly LiBH 4 and Mg 2 FeD 6 dehydrided simultaneously with a special molar ratio = 1:1 at x = 0.75, and then the remaining LiBH 4 reacted with the Mg and Fe derived from the dehydriding of Mg 2 FeD 6 .

A new crystalline phase derived from a 90LiBH4:10P2S5 mixture displays high lithium-ionic conductivity of log(σ/S cm(-1)) = -3.0 at 300 K. It is stable up to 473 K and has both a wide potential window of 0-5 V and favorable mechanical properties for battery assembly. Its incorporation into a bulk-type all-solid-state TiS2/InLi battery enabled repeated battery operation at 300 K.

Ninefold coordination of hydrogen is very rare, and has been observed in two different hydride complexes comprising rhenium and technetium. Herein, based on a theoretical/experimental approach, we present evidence for the formation of ninefold H- coordination hydride complexes of molybdenum ([MoH9]3−), tungsten ([WH9]3−), niobium ([NbH9]4−) and tantalum ([TaH9]4−) in novel complex transition-metal hydrides, Li5MoH11, Li5WH11, Li6NbH11 and Li6TaH11, respectively. All of the synthesized materials are insulated with band gaps of approximately 4 eV, but contain a sufficient amount of hydrogen to cause the H 1s-derived states to reach the Fermi level. Such hydrogen-rich materials might be of interest for high-critical-temperature superconductivity if the gaps close under compression. Furthermore, the hydride complexes exhibit significant rotational motions associated with anharmonic librations at room temperature, which are often discussed in relation to the translational diffusion of cations in alkali-metal dodecahydro-closo-dodecaborates and strongly point to the emergence of a fast lithium conduction even at room temperature.

Present fuel cells must increase the activity of the oxygen reduction reaction (ORR) on platinum (or Pt alloy) electrodes. Detailed simulation analyses can direct future investigations by providing a better understanding of the ORR. We adopted a density functional theory (DFT)-based, first-principles molecular dynamics simulation for the elementary steps of the electrochemical ORR on Pt(111). The two-step process involves successive protonation of O and OH, which are adsorbed on Pt. The relevant redox potentials were estimated by changing the coverage of OH(ad) and O(ad). The reaction energy profiles were determined along the reaction coordinate using the Blue-Moon ensemble method and a constant-bias scheme in the DFT calculations. These profiles at different biases were then used to generate activation energies and symmetry factors. Cyclic voltammetry (CV) and linear sweep voltammetry profiles were then calculated from the Butler-Volmer rate, Nernst equilibrium, and mass diffusion equations using these obtained parameters, literature values and appropriate prefactors in the rate equations. The experimentally observed reversible and irreversible peaks in CV were obtained. The irreversibility of the protonation of O(ad), , attributed to its higher activation energy, affects the ORR potential and thus fuel cell performance. It is therefore necessary not only to tune the adsorption energy of the O(ad) and OH(ad) intermediates, which are the origin of the "volcano plot", but also to tune 's activation energy to elevate the performance above that of the volcano-top.

We report ab initio molecular dynamics studies with electric field that reveal chemical stability of room temperature ionic liquid for charge transfer from lithium and nickel electrodes. Bis(trifluoromethanesulfonyl)imide (TFSI) is oxidized on the nickel electrode under a high positive bias condition as expected. However, TFSI is reduced on the lithium electrode under both positive and negative bias conditions, because the lithium electrode acts as a strong reductant. The decomposition of TFSI anion might induce the formation of LiF as a solid electrolyte interphase, which could restrain the TFSI reduction. The stability of an cation under reductant conditions is presented.

We use large-scale first-principles molecular dynamics simulations to find possible ways to make magnesium dication conductors based on Mg(BH4)2. The Mg atoms are confined in a tetrahedral cage of BH4 groups and coordinated by eight hydrogen atoms from these groups. These Mg atoms do not move outside the cage either in high- or low-temperature phases. After introducing larger-sized AlH−4 anions by substitution of 20% BH−4, the Mg atoms are able to move outside the cage with coordination from hydrogen atoms of BH−4 located outside the cage. Substitution by Cl− and I− anions did not allow large Mg movements.