Colossal Reversible Barocaloric Effects in a Plastic Crystal Mediated by Lattice Vibrations and Ion Diffusion.

Solid-state methods for cooling and heating promise a sustainable alternative to current compression cycles of greenhouse gases and inefficient fuel-burning heaters. Barocaloric effects (BCE) driven by hydrostatic pressure (p) are especially encouraging in terms of large adiabatic temperature changes (|ΔT| ≈ 10 K) and isothermal entropy changes (|ΔS| ≈ 100 J K-1 kg-1). However, BCE typically require large pressure shifts due to irreversibility issues, and sizeable |ΔT| and |ΔS| seldom are realized in a same material. Here, the existence of colossal and reversible BCE in LiCB11H12 is demonstrated near its order-disorder phase transition at ≈380 K. Specifically, for Δp ≈ 0.23 (0.10) GPa, |ΔSrev| = 280 (200) J K-1 kg-1 and |ΔTrev| = 32 (10) K are measured, which individually rival with state-of-the-art BCE figures. Furthermore, pressure shifts of the order of 0.1 GPa yield huge reversible barocaloric strengths of ≈2 J K-1 kg-1 MPa-1. Molecular dynamics simulations are performed to quantify the role of lattice vibrations, molecular reorientations, and ion diffusion on the disclosed BCE. Interestingly, lattice vibrations are found to contribute the most to |ΔS| while the diffusion of lithium ions, despite adding up only slightly to the entropy change, is crucial in enabling the molecular order-disorder phase transition.

Mixed alkali-ion transport and storage in atomic-disordered honeycomb layered NaKNi2TeO6.

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 (Na and K) to produce a mixed-alkali honeycomb layered oxide material, namely, NaKNi2TeO6. Via transmission electron microscopy measurements, we reveal the local atomic structural disorders characterised by aperiodic stacking and incoherency in the alternating arrangement of Na and K atoms. We also investigate the possibility of mixed electrochemical transport and storage of Na+ and K+ ions in NaKNi2TeO6. In particular, we report an average discharge cell voltage of about 4 V and a specific capacity of around 80 mAh g-1 at low specific currents (i.e., < 10 mA g-1) when a NaKNi2TeO6-based positive electrode is combined with a room-temperature 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.

Colossal barocaloric effects in the complex hydride Li[Formula: see text]B[Formula: see text]H[Formula: see text].

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 changes in the applied external fields (i.e., magnetic, electric and/or mechanical stress) and represents a promising alternative to current refrigeration methods. However, most of the caloric materials known to date present relatively small adiabatic temperature changes ([Formula: see text] to 10 K) and/or limiting irreversibility issues resulting from significant phase-transition hysteresis. Here, we predict by using molecular dynamics simulations the existence of colossal barocaloric effects induced by pressure (isothermal entropy changes of [Formula: see text] J K[Formula: see text] kg[Formula: see text]) in the energy material Li[Formula: see text]B[Formula: see text]H[Formula: see text]. Specifically, we estimate [Formula: see text] J K[Formula: see text] kg[Formula: see text] and [Formula: see text] K for a small pressure shift of P = 0.1 GPa at [Formula: see text] K. The disclosed colossal barocaloric effects are originated by a fairly reversible order-disorder phase transformation involving coexistence of Li[Formula: see text] diffusion and (BH)[Formula: see text] reorientational motion at high temperatures.

Role of divalent cation (Ba) substitution in the Li+ ion conductor LiTi2(PO4)3: a molecular dynamics study.

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.

Two-Phase Reaction Mechanism for Fluorination and Defluorination in Fluoride-Shuttle Batteries: A First-Principles Study.

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.

Li deposition and desolvation with electron transfer at a silicon/propylene-carbonate interface: transition-state and free-energy profiles by large-scale first-principles molecular dynamics.

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.

Formation of novel transition metal hydride complexes with ninefold hydrogen coordination.

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.

Toward full simulation of the electrochemical oxygen reduction reaction on Pt using first-principles and kinetic calculations.

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.

Fast lithium-ionic conduction in a new complex hydride-sulphide crystalline phase.

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.

Pseudo-binary electrolyte, LiBH4-LiCl, for bulk-type all-solid-state lithium-sulfur battery.

The ionic conduction and electrochemical and thermal stabilities of the LiBH4-LiCl solid-state electrolyte were investigated for use in bulk-type all-solid-state lithium-sulfur batteries. The LiBH4-LiCl solid-state electrolyte exhibiting a lithium ionic conductivity of [Formula: see text] at 373 K, forms a reversible interface with a lithium metal electrode and has a wide electrochemical potential window up to 5 V. By means of the high-energy mechanical ball-milling technique, we prepared a composite powder consisting of elemental sulfur and mixed conductive additive, i.e., Ketjen black and Maxsorb. In that composite powder, homogeneous dispersion of the materials is achieved on a nanometer scale, and thereby a high concentration of the interface among them is induced. Such nanometer-scale dispersals of both elemental sulfur and carbon materials play an important role in enhancing the electrochemical reaction of elemental sulfur. The highly deformable LiBH4-LiCl electrolyte assists in the formation of a high concentration of tight interfaces with the sulfur-carbon composite powder. The LiBH4-LiCl electrolyte also allows the formation of the interface between the positive electrode and the electrolyte layers, and thus the Li-ion transport paths are established at that interface. As a result, our battery exhibits high discharge capacities of 1377, 856, and 636 mAh g(-1) for the 1st, 2nd, and 5th discharges, respectively, at 373 K. These results imply that complex hydride-based solid-state electrolytes that contain Cl-ions in the crystal would be integrated into rechargeable batteries.

True boundary for the formation of homoleptic transition-metal hydride complexes.

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.

Ab initio studies on the proton dissociation and infrared spectra of sulfonated poly(ether ether ketone) (SPEEK) membranes.

SPEEK is known to possess high proton conductivity at high water content, being comparable with other popular membranes used in fuel cells, such as Nafion and sulfonated polyethersulfone (SPES). However, much less is known about its fundamental properties, including the status of proton dissociation and spectral features. In this work, the properties of two model molecules of SPEEK, M1 (20 atoms), M2 (50 atoms) and their hydrated systems, M1 + nH2O and M2 + nH2O (n = 1-9), have been investigated using static electronic structure calculations and the ab initio molecular dynamics (MD) method. Optimized structures for all of the systems and the trajectories of M1 + nH2O (n = 3-6) at finite temperatures have been computed using density functional theory at the B3LYP level of theory. Proton dissociation has been discussed in detail, especially for n = 3 and n = 4. In addition, the infrared spectra of SPEEK and its hydrated systems have been studied using a combination of theory and experiment. The characteristic bands of SPEEK and the surrounding water clusters have been assigned with emphasis on their relationship with the degree of proton dissociation. We have found that the hydronium ion stretching modes, which appear in the 2000-3000 cm(-1) region in static electronic structure calculations, are not observed experimentally. This discrepancy is explained by the stationary structure and the temperature effect.

Large-scale first-principles molecular dynamics for electrochemical systems with O(N) methods.

A method for large-scale first-principles molecular dynamics (MD) simulations on electrochemical systems has been developed by combining the effective screening medium (ESM) method with O(N) density functional theory (DFT). This implementation has been significantly simplified by the introduction of neutral atom potentials, which minimizes the modifications to existing DFT code. In order to demonstrate ability of this implementation, it has been applied to an electrochemical system consisting of a H-Si(111) electrode, which is a candidate anode for high-capacity Li-ion secondary batteries, and a propylene carbonate (PC) solvent to simulate how PC molecules in the vicinity of the electrode surface respond to an imposed electric field. The large-scale MD simulation clearly demonstrates that the combination of the ESM and O(N) DFT methods provides a useful tool for first-principles investigation of complicated electrochemical systems such as high-capacity batteries.

Hydrogen quantum effects in hydride LaNi(5)H(7).

Energy eigenvalues and wave functions of hydrogen atoms in hydride LaNi(5)H(7) are calculated. First-principles electronic structure calculations are employed to obtain the three-dimensional potential energy structure of each hydrogen site. These quantum effects are not negligibly small in evaluation of enthalpy of formation, an important property of hydrogen storage. Including the temperature effect from hydrogen gas, experimental values are well reproduced. The excitation probability of inelastic neutron scattering is also calculated using the wave functions obtained.

Reversible redox reaction and water configuration on a positively charged platinum surface: first principles molecular dynamics simulation.

The water dissociation reaction and water molecule configuration on a positively charged platinum (111) surface were investigated by means of first principles molecular dynamics under periodic boundary conditions. Water molecules on the Pt surface were mostly in the O-down orientation but some H-down structures were also found. OH(-) ion, generated by removing H from H(2)O in the bulk region, moved to the Pt surface, on which a positive charge is induced, by a Grotthuss-like proton-relay mechanism and adsorbed on it as OH(Pt). Hydrogen atom exchange between OH(Pt) and a near-by water molecule frequently occurred on the Pt surface and had a low activation energy of the same order as room temperature energy. When a positive charge (7 µC cm(-2)) was added to the Pt surface, H(3)O(+) and OH(Pt) were generated from 2H(2)O on the Pt. This may be coupled with an electron transfer to the Pt electrode [2H(2)O → H(3)O(+) + OH(Pt) + e(-)]. The opposite reaction was also observed on the same charged surface during a simulation of duration about 10 ps; it is a reversible redox reaction. When further positive charge (14 µC cm(-2)) was added, the reaction shifted to the right hand side completely. Thus, this one-electron transfer reaction, which is a part of the oxygen electrode reaction in fuel cells and water electrolysis, was confirmed to be a low activation energy process.

Novel numerical method for calculating the pressure tensor in spherical coordinates for molecular systems.

We propose a novel method for computing the pressure tensor along the radial axis of a molecular system with spherical symmetry. The proposed method uses the slice averaged pressure to improve the numerical stability and precision significantly. Simplified expressions of the local pressure are derived for a conventional molecular force field including non-bond, bond stretching, angle bending, and torsion interactions; these expressions are advantageous in terms of the computational cost. We also discuss an algorithm to avoid numerical singularity. Finally, the method is successfully applied to three different molecular systems, i.e., a water droplet in oil, a spherical micelle, and a liposome.

An ab initio modeling study on a modeled hydrated polymer electrolyte membrane, sulfonated polyethersulfone (SPES).

The nature of proton dynamics as well as a pendant side chain's ability for proton dissociation and capture in low-hydration sulfonated polyethersulfone (SPES) (lambda = 2, 4) have been studied theoretically by means of quantum chemical calculations and first-principles molecular dynamics simulations. A detailed comparison of results on SPES with those on Nafion has been made. It is found that the sulfonic groups of Nafion tend to dissociate protons more easily than do those of SPES. Hydration by four water molecules allows the dissociation of a proton from the sulfonic groups in both SPES and Nafion. The results of the first-principles MD simulations on SPES show that the nature of proton transfer kinetics for both hydration levels is very similar. Compared with low-hydration Nafion, however, hydration around the sulfonic groups in SPES is not sufficient to fully dissociate protons from the sulfonic groups, which results from the fact that some of the water molecules participate in hydrating SO(2) groups in SPES rather than SO(3)(-). Such a feature affects the performance of SPES under low-hydration conditions.

First-principles study of hydrogen vacancies in sodium alanate with Ti substitution.

In order to clarify the effect of hydrogen vacancies on the stability and structure of sodium alanate, NaAlH(4), with and without Ti substitution for Al, first-principles electronic structure calculations were carried out. The relative thermodynamic stability of the Ti dopant and the H vacancy in a supercell was obtained. For the Ti-doped Na(16)Al(16)H(64) supercell calculations, it was preferable to perform the initial substitution with a cluster of TiAlH(n). We showed that substitution of a Ti atom for an Al atom in Na(16)Al(15)TiH(63) with H vacancies increases the stability of the structure. A density of states analysis revealed weakening of the bond strength corresponding to increase in the bond length.

Nature of proton dynamics in a polymer electrolyte membrane, nafion: a first-principles molecular dynamics study.

First-principles molecular dynamics simulations have been carried out to investigate the nature of proton dynamics in Nafion, a representative polymer electrolyte membrane (PEM) widely used in PEM fuel cells. From the trajectories of the simulations, diffusion coefficients for the protonic defects were calculated to be 0.3 x 10(-5) cm(2) s(-1) and 7.1 x 10(-5) cm(2) s(-1) for lambda = 4.25 and 12.75, respectively, where lambda denotes hydration levels inside Nafion defined as a number of water molecules per sulfonic group. Our simulations show that proton hopping probability does not depend much on the water content inside Nafion. This finding indicates that the classical vehicular (or en masse) diffusion model, which has been employed to account for the slow diffusion process of protons in low water-content Nafion, is an oversimplification and does not correctly describe proton dynamics. Furthermore, it is found that difference in the value of the proton diffusion coefficient with respect to water content inside Nafion is related to the different character of proton hopping occurring in the water hydrogen bond network. When the water content is low, the proton hopping occurs in a manner that does not contribute constructively to proton mobility, while when the water content is high, it occurs in a manner which is beneficial to overall proton mobility. Such a different nature of proton hoppings arises mainly from the difference in the connectivity of water hydrogen bond network. Our results broadly support earlier simulation studies and provide the molecular level origin of properties arising from the proton dynamics in Nafion.