Study of the Temperature- and Pressure-Dependent Structural Properties of Alkali Hydrido-closo-borate Compounds.

In this work, we report on the structural properties of alkali hydrido-closo-(car)borates, a promising class of solid-state electrolyte materials, using high-pressure and temperature-dependent X-ray diffraction experiments combined with density functional theory (DFT) calculations. The mechanical properties are determined via pressure-dependent diffraction studies and DFT calculations; the shear moduli appear to be very low for all studied compounds, revealing their high malleability (that can be beneficial for the manufacturing and stable cycling of all-solid-state batteries). The thermodiffraction experiments also reveal a high coefficient of thermal expansion for these materials. We discover a pressure-induced phase transition for K2B12H12 from Fm3̅ to Pnnm symmetry around 2 GPa. A temperature-induced phase transition for Li2B10H10 was also observed for the first time by thermodiffraction, and the crystal structure determined by combining experimental data and DFT calculations. Interestingly, all phases of the studied compounds (including newly discovered high-pressure and high-temperature phases) may be related via a group-subgroup relationship, with the notable exception of the room-temperature phase of Li2B10H10.

Post-Synthesis Amine Borane Functionalization of a Metal-Organic Framework and Its Unusual Chemical Hydrogen Release Phenomenon.

A novel strategy for post-synthesis amine borane functionalization of MOFs under gas-solid phase transformation, utilizing gaseous diborane, is reported. The covalently confined amine borane derivative decorated on the framework backbone is stable when preserved at low temperature, but spontaneously liberates soft chemical hydrogen at room temperature, leading to the development of an unusual borenium type species (-NH=BH2+ ) ion-paired with a hydroborate anion. Furthermore, the unsaturated amino borane (-NH=BH2 ) and the µ-iminodiborane (-µ-NHB2 H5 ) were detected as final products. A combination of DFT based molecular dynamics simulations and solid state NMR spectroscopy, utilizing isotopically enriched materials, were undertaken to unequivocally elucidate the mechanistic pathways for H2 liberation.

Formation of CaB6 in the thermal decomposition of the hydrogen storage material Ca(BH4)2.

Using a combination of high resolution X-ray powder diffraction and X-ray Raman scattering spectroscopy at the B K- and Ca L2,3-edges, we analyzed the reaction products of Ca(BH4)2 after annealing at 350 °C and 400 °C under vacuum conditions. We observed the formation of nanocrystalline/amorphous CaB6 mainly and found only small contributions from amorphous B for annealing times larger than 2 h. For short annealing times of 0.5 h at 400 °C we observed neither CaB12H12 nor CaB6. The results indicate a reaction pathway in which Ca(BH4)2 decomposes to B and CaH2 and finally reacts to form CaB6. These findings confirm the potential of using Ca(BH4)2 as a hydrogen storage medium and imply the desired cycling capabilities for achieving high-density hydrogen storage materials.

In situ characterization of the decomposition behavior of Mg(BH4)2 by X-ray Raman scattering spectroscopy.

We present an in situ study of the thermal decomposition of Mg(BH4)2 in a hydrogen atmosphere of up to 4 bar and up to 500 °C using X-ray Raman scattering spectroscopy at the boron K-edge and the magnesium L2,3-edges. The combination of the fingerprinting analysis of both edges yields detailed quantitative information on the reaction products during decomposition, an issue of crucial importance in determining whether Mg(BH4)2 can be used as a next-generation hydrogen storage material. This work reveals the formation of reaction intermediate(s) at 300 °C, accompanied by a significant hydrogen release without the occurrence of stable boron compounds such as amorphous boron or MgB12H12. At temperatures between 300 °C and 400 °C, further hydrogen release proceeds via the formation of higher boranes and crystalline MgH2. Above 400 °C, decomposition into the constituting elements takes place. Therefore, at moderate temperatures, Mg(BH4)2 is shown to be a promising high-density hydrogen storage material with great potential for reversible energy storage applications.

Elucidating the pressure-induced enhancement of ionic conductivity in sodium closo-hydroborate electrolytes for all-solid-state batteries.

Hydroborates are an emerging class of solid electrolytes for all-solid-state batteries. Here, we investigate the impact of pressure on the crystal structure and ionic conductivity of a close-hydroborate salt consisting of Na2B10H10 and Na2B12H12. Two Na2B10H10:Na2B12H12 ratios were studied, 1:1 and 1:3. The anions of the as-synthesized powder with 1:1 ratio crystallize in a single face-centered cubic phase, while the anions of the powder with 1:3 ratio crystallize in a single monoclinic phase. After applying pressure to densify the powder into a pellet, a partial phase transformation into a body-centered cubic (BCC) phase is observed for both ratios. The BCC content saturates at 50 weight percent (wt%) at 500 MPa for the 1:1 ratio and at 77 wt% at 1000 MPa for the 1:3 sample. The room temperature sodium-ion conductivity follows an analogous trend. For the 1:1 ratio, it increases from 2 × 10-4 Scm-1 at 10 wt% BCC content to about 1.0 × 10-3 Scm-1 at 50 wt% BCC content. For the 1:3 ratio, it increases from 1.3 × 10-5 Scm-1 at 11.9 wt% BCC to 8.1 × 10-4 Scm-1 at 71 wt% BCC content. Our results show that pressure is a prerequisite to achieve high sodium-ion conductivity by formation of the highly conductive BCC phase. Supplementary Information: The online version contains supplementary material available at 10.1007/s10853-022-08121-8.

Pressure and temperature dependence of the decomposition pathway of LiBH4.

The decomposition pathway is crucial for the applicability of LiBH(4) as a hydrogen storage material. We discuss and compare the different decomposition pathways of LiBH(4) according to the thermodynamic parameters and show the experimental ways to realize them. Two pathways, i.e. the direct decomposition into boron and the decomposition via Li(2)B(12)H(12), were realized under appropriate conditions, respectively. By applying a H(2) pressure of 50 bar at 873 K or 10 bar at 700 K, LiBH(4) is forced to decompose into Li(2)B(12)H(12). In a lower pressure range of 0.1 to 10 bar at 873 K and 800 K, the concurrence of both decomposition pathways is observed. Raman spectroscopy and (11)B MAS NMR measurements confirm the formation of an intermediate Li(2)B(12)H(12) phase (mostly Li(2)B(12)H(12) adducts, such as dimers or trimers) and amorphous boron.

CO2 hydrogenation on a metal hydride surface.

The catalytic hydrogenation of CO(2) at the surface of a metal hydride and the corresponding surface segregation were investigated. The surface processes on Mg(2)NiH(4) were analyzed by in situ X-ray photoelectron spectroscopy (XPS) combined with thermal desorption spectroscopy (TDS) and mass spectrometry (MS), and time-of-flight secondary ion mass spectrometry (ToF-SIMS). CO(2) hydrogenation on the hydride surface during hydrogen desorption was analyzed by catalytic activity measurement with a flow reactor, a gas chromatograph (GC) and MS. We conclude that for the CO(2) methanation reaction, the dissociation of H(2) molecules at the surface is not the rate controlling step but the dissociative adsorption of CO(2) molecules on the hydride surface.

Correction: Experimental investigation of Mg(B3H8)2 dimensionality, materials for energy storage applications.

Correction for 'Experimental investigation of Mg(B3H8)2 dimensionality, materials for energy storage applications' by Romain Moury et al., Dalton Trans., 2020, 49, 12168-12173, https://doi.org/10.1039/D0DT02170A.

Transient Elastomers with High Dielectric Permittivity for Actuators, Sensors, and Beyond.

Dielectric elastomers (DEs) are key materials in actuators, sensors, energy harvesters, and stretchable electronics. These devices find applications in important emerging fields such as personalized medicine, renewable energy, and soft robotics. However, even after years of research, it is still a great challenge to achieve DEs with increased dielectric permittivity and fast recovery of initial shape when subjected to mechanical and electrical stress. Additionally, high dielectric permittivity elastomers that show reliable performance but disintegrate under normal environmental conditions are not known. Here, we show that polysiloxanes modified with amide groups give elastomers with a dielectric permittivity of 21, which is 7 times higher than regular silicone rubber, a strain at break that can reach 150%, and a mechanical loss factor tan δ below 0.05 at low frequencies. Actuators constructed from these elastomers respond to a low electric field of 6.2 V µm-1, giving reliable lateral actuation of 4% for more than 30 000 cycles at 5 Hz. One survived 450 000 cycles at 10 Hz and 3.6 V µm-1. The best actuator shows 10% lateral strain at 7.5 V µm-1. Capacitive sensors offer a more than a 6-fold increase in sensitivity compared to standard silicone elastomers. The disintegrated material can be re-cross-linked when heated to elevated temperatures. In the future, our material could be used as dielectric in transient actuators, sensors, security devices, and disposable electronic patches for health monitoring.

A multifaceted approach to hydrogen storage.

The widespread adoption of hydrogen as an energy carrier could bring significant benefits, but only if a number of currently intractable problems can be overcome. Not the least of these is the problem of storage, particularly when aimed at use onboard light-vehicles. The aim of this overview is to look in depth at a number of areas linked by the recently concluded HYDROGEN research network, representing an intentionally multi-faceted selection with the goal of advancing the field on a number of fronts simultaneously. For the general reader we provide a concise outline of the main approaches to storing hydrogen before moving on to detailed reviews of recent research in the solid chemical storage of hydrogen, and so provide an entry point for the interested reader on these diverse topics. The subjects covered include: the mechanisms of Ti catalysis in alanates; the kinetics of the borohydrides and the resulting limitations; novel transition metal catalysts for use with complex hydrides; less common borohydrides; protic-hydridic stores; metal ammines and novel approaches to nano-confined metal hydrides.

Rotational motion in LiBH4/LiI solid solutions.

We investigated the localized rotational diffusion of the (BH(4))(-) anions in LiBH(4)/LiI solid solutions by means of quasielastic and inelastic neutron scattering. The (BH(4))(-) motions are thermally activated and characterized by activation energies in the order of 40 meV. Typical dwell times between jumps are in the picosecond range at temperatures of about 200 K. The motion is dominated by 90° reorientations around the 4-fold symmetry axis of the tetrahedraly shaped (BH(4))(-) ions. As compared to the pure system, the presence of iodide markedly reduces activation energies and increases the rotational frequencies by more than a factor of 100. The addition of iodide lowers the transition temperature, stabilizing the disordered high temperature phase well below room temperature.

Thermal and Electrochemical Interface Compatibility of a Hydroborate Solid Electrolyte with 3 V-Class Cathodes for All-Solid-State Sodium Batteries.

Thermal stability of solid electrolytes and their compatibility with battery electrodes are key factors to ensure stable cycling and high operational safety of all-solid-state batteries. Here, we study the compatibility of a hydroborate solid electrolyte Na4(B12H12)(B10H10) with 3 V-class cathode active materials: NaCrO2, NaMnO2, and NaFeO2. Among these layered sodium transition metal oxide cathodes, NaCrO2 shows the highest thermal compatibility in contact with the hydroborate solid electrolyte up to 525 °C in the discharged state. Furthermore, the electrolyte remains intact upon the internal thermal decomposition of the charged, that is, desodiated, cathode (Na0.5CrO2) above 250 °C, demonstrating the potential for highly safe hydroborate-based all-solid-state batteries with a wide operating temperature range. The experimentally determined onset temperatures of thermal decomposition of Na4(B12H12)(B10H10) in contact with 3 V-class cathodes surpass those of sulfide and selenide solid electrolytes, exceeding previous thermodynamic calculations. Our results also highlight the need to identify relevant decomposition pathways of hydroborates to enable more valid theoretical predictions.

Effect of the surface oxidation of LiBH(4) on the hydrogen desorption mechanism.

The surface oxidation behavior of LiBH(4) and NaBH(4) was investigated in view of the formation and structure of the surface oxidation and its effect on the hydrogen desorption kinetics. The sample surfaces were intentionally modified by exposure to oxygen in the pressure range from 10(-10) mbar up to 200 mbar. The induced surface changes were systematically studied by means of X-ray photoelectron spectroscopy. NaBH(4) shows a low reactivity with oxygen, while LiBH(4) oxidizes rapidly, accompanied by surface segregation of Li. The hydrogen desorption kinetics of LiBH(4) were studied by thermal desorption spectroscopy with particular emphasis on the analysis of the desorbed gases, i.e. diborane and hydrogen. The surface oxidation induces the formation of a Li(2)O layer on LiBH(4), significantly reduces the desorption of diborane, and enhances the rate of hydrogen desorption.

BH4− self-diffusion in liquid LiBH4.

The hydrogen dynamics in solid and in liquid LiBH4 was studied by means of incoherent quasielastic neutron scattering. Rotational jump diffusion of the BH4- subunits on the picosecond scale was observed in solid LiBH4. The characteristic time constant is significantly shortened when the system transforms from the low-temperature phase to the high-temperature phase at 383 K. In the molten phase of LiBH4 above 553 K, translational diffusion of the BH4- units is found. The measured diffusion coefficients are in the 10(-5)cm2/s range at temperatures around 700 K, which is in the same order of magnitude as the self-diffusion of liquid lithium or the diffusion of ions in molten alkali halides. The temperature dependence of the diffusion coefficient shows an Arrhenius behavior, with an activation energy of Ea = 88 meV and a prefactor of D0 = 3.1 × 10(-4)cm2/s.

Experimental investigation of Mg(B3H8)2 dimensionality, materials for energy storage applications.

Mg(B3H8)2 is a crucial reaction intermediate in the thermal decomposition of the hydrogen storage material Mg(BH4)2 and is discussed as a potential solid-state Mg-ion conductor. We successfully synthesized unsolvated Mg(B3H8)2 and highlight that Mg(B3H8)2 exists mainly as a low-dimensional solid. In addition, the Mg2+ conductivity was evaluated to be 1.4.10-4 S cm-1 at 80 °C.

Complex hydrides with (BH(4))(-) and (NH(2))(-) anions as new lithium fast-ion conductors.

Some of the authors have reported that a complex hydride, Li(BH(4)), with the (BH(4))(-) anion exhibits lithium fast-ion conduction (more than 1 x 10(-3) S/cm) accompanied by the structural transition at approximately 390 K for the first time in 30 years since the conduction in Li(2)(NH) was reported in 1979. Here we report another conceptual study and remarkable results of Li(2)(BH(4))(NH(2)) and Li(4)(BH(4))(NH(2))(3) combined with the (BH(4))(-) and (NH(2))(-) anions showing ion conductivities 4 orders of magnitude higher than that for Li(BH(4)) at RT, due to being provided with new occupation sites for Li(+) ions. Both Li(2)(BH(4))(NH(2)) and Li(4)(BH(4))(NH(2))(3) exhibit a lithium fast-ion conductivity of 2 x 10(-4) S/cm at RT, and the activation energy for conduction in Li(4)(BH(4))(NH(2))(3) is evaluated to be 0.26 eV, less than half those in Li(2)(BH(4))(NH(2)) and Li(BH(4)). This study not only demonstrates an important direction in which to search for higher ion conductivity in complex hydrides but also greatly increases the material variations of solid electrolytes.

Low-temperature synthesis of LiBH4 by gas-solid reaction.

The solvent-free synthesis of LiBH(4) from LiH in a borane atmosphere at 120 degrees C and ambient pressures is demonstrated. The source of borane is a milled LiBH(4)/ZnCl(2) mixture, in which Zn(BH(4))(2) is generated by a metathesis reaction. The yield of the reaction of about 74 % LiBH(4) shows that a bulk reaction is taking place upon borane absorption by LiH. This indicates that the formation of B-H bonds is the limiting step for the formation of LiBH(4) from the elements. Therefore, the use of diborane as a starting reactant allows one to circumvent the reaction barrier for the B-B bond dissociation and explains the rather moderate synthesis conditions.

Pressure-induced phase transitions in Na2B12H12, structural investigation on a candidate for solid-state electrolyte.

closo-Borates, such as Na2B12H12, are an emerging class of ionic conductors that show promising chemical, electrochemical and mechanical properties as electrolytes in all-solid-state batteries. Motivated by theoretical predictions, high-pressure in situ powder X-ray diffraction on Na2B12H12 was performed and two high-pressure phases are discovered. The first phase transition occurs at 0.5 GPa and it is persistent to ambient pressure, whereas the second transition takes place between 5.7 and 8.1 GPa and it is fully reversible. The mechanisms of the transitions by means of group theoretical analysis are unveiled. The primary-order parameters are identified and the stability at ambient pressure of the first polymorph is explained by density functional theory calculations. Finally, the parameters relevant to engineer and build an all-solid-state battery, namely, the bulk modulus and the coefficient of the thermal expansion are reported. The relatively low value of the bulk modulus for the first polymorph (14 GPa) indicates a soft material which allows accommodation of the volume change of the cathode during cycling.

Direct Solution-Based Synthesis of Na4 (B12 H12 )(B10 H10 ) Solid Electrolyte.

All-solid-state batteries (ASSBs) promise higher power and energy density than batteries based on liquid electrolytes. Recently, a stable 3 V ASSB based on the super ionic conductor (1 mS cm-1 near room temperature) Na4 (B12 H12 )(B10 H10 ) has demonstrated excellent cycling stability. This study concerns the development of a five-step, scalable, and solution-based synthesis of Na4 (B12 H12 )(B10 H10 ). The use of a wet chemistry approach allows solution processing with high throughput and addresses the main drawbacks for this technology, specifically, the limited electrode-electrolyte contact and high cost. Moreover, a cost-efficient synthesis of the expensive precursors Na2 B10 H10 and Na2 B12 H12 is also achieved through the same process. The mechanism of the reactions is investigated and two key parameters to tune the kinetics and selectivity are highlighted: the choice of counter cation (tetraethylammonium) and solvent.