New Li Ion Conductors and Solid State Hydrogen Storage Materials: LiM(BH₄)₃Cl, M = La, Gd

Multiple reaction mixtures with different composition ratios of MCl₃–LiBH₄ (M = La, Gd) were studied by mechano-chemical synthesis, yielding two new bimetallic borohydride chlorides, LiM­(BH₄)₃Cl (M = La, Gd). The Gd-containing phase was obtained only after annealing the ball-milled mixture. Additionally, a solvent extracted sample of Gd­(BH₄)₃ was studied to gain insight into the transformation from Gd­(BH₄)₃ to LiGd­(BH₄)₃Cl. The novel compounds were investigated using in situ synchrotron radiation powder X-ray diffraction, thermal analysis combined with mass spectroscopy, Sieverts measurements, Fourier transform infrared spectroscopy, and electrochemical impedance spectroscopy. The two new compounds, LiLa­(BH₄)₃Cl and LiGd­(BH₄)₃Cl, have high lithium ion conductivities of 2.3 × 10^–4 and 3.5 × 10^–4 S·cm^–1 (<i>T</i> = 20 °C) and high hydrogen densities of ρ_m = 5.36 and 4.95 wt % H₂, and both compounds crystallize in the cubic crystal system (space group <i>I</i>-43<i>m</i>) with unit cell parameter <i>a</i> = 11.7955(1) and <i>a</i> = 11.5627(1) Å, respectively. The structures contain isolated tetranuclear anionic clusters [M₄Cl₄(BH₄)₁₂]^4– with distorted cubane M₄Cl₄ cores M = La or Gd. Each lanthanide atom coordinates three chloride ions and three borohydride groups, thus completing the coordination environment to an octahedron. The Li⁺ ions are disordered on 2/3 of the 12<i>d</i> Wyckoff site, which agrees well with the very high lithium ion conductivities. The conductivity is purely ionic, as electronic conductivities were measured to only 1.4 × 10^–8 and 9 × 10^–8 S·cm^–1 at <i>T</i> = 20 °C for LiLa­(BH₄)₃Cl and LiGd­(BH₄)₃Cl, respectively. In situ synchrotron radiation powder X-ray diffraction (SR-PXD) reveals that the decomposition products at 300 °C consist of LaB₆/LaH₂ or GdB₄/GdH₂ and LiCl. The size of the rare-earth metal atom is shown to be crucial for the formation and stability of the borohydride phases in MCl₃–LiBH₄ systems

Nuclear Magnetic Resonance Studies of Reorientational Motion and Li Diffusion in LiBH₄–LiI Solid Solutions

To study the reorientational motion of the BH₄ groups and the translational diffusion of Li⁺ ions in LiBH₄–LiI solid solutions with 2:1, 1:1, and 1:2 molar ratios, we have measured the ¹H, ¹¹B, and ⁷Li NMR spectra and spin–lattice relaxation rates in these compounds over the temperature range 18–520 K. It is found that, at low temperatures, the reorientational motion of the BH₄ groups in LiBH₄–LiI solid solutions is considerably faster than in all other borohydride-based systems studied so far. Our results are consistent with a coexistence of at least two types of reorientational processes with different characteristic rates. For the faster reorientational process, the average activation energies derived from our data are 53 ± 4, 39 ± 4, and 33 ± 4 meV for the LiBH₄–LiI solid solutions with 2:1, 1:1, and 1:2 molar ratios, respectively. In the studied range of iodine concentrations, the Li⁺ jump rates are found to decrease with increasing I^– content. The activation energies for Li diffusion obtained from our data are 0.63 ± 0.01, 0.65 ± 0.01, and 0.68 ± 0.01 eV for the samples with 2:1, 1:1, and 1:2 molar ratios, respectively

Crystal Structure of a Lightweight Borohydride from Submicrometer Crystallites by Precession Electron Diffraction

We demonstrate that precession electron diffraction at low-dose conditions can be successfully applied for structure analysis of extremely electron-beam-sensitive materials. Using LiBH₄ as a test material, complete structural information, including the location of the H atoms, was obtained from submicrometer-sized crystallites. This demonstrates for the first time that, where conventional transmission electron microscopy techniques fail, quantitative precession electron diffraction can provide structural information from submicrometer particles of such extremely electron-beam-sensitive materials as complex lightweight hydrides. We expect the precession electron diffraction technique to be a useful tool for nanoscale investigations of thermally unstable lightweight hydrogen-storage materials

Nuclear Magnetic Resonance Studies of BH₄ Reorientations and Li Diffusion in LiLa(BH₄)₃Cl

To study the reorientational motion of BH₄ groups and the translational diffusion of Li⁺ ions in the novel bimetallic borohydride chloride LiLa­(BH₄)₃Cl, we have measured the ¹H, ¹¹B, and ⁷Li NMR spectra and spin–lattice relaxation rates in this compound over the temperature range of 23–418 K. At low temperatures (<i>T</i> < 110 K), the proton spin–lattice relaxation rates are governed by fast reorientations of BH₄ groups. This reorientational process can be satisfactorily described in terms of a two-peak distribution of the activation energies with the peak <i>E</i>_a values of 41 and 50 meV. Above 200 K, the NMR data are governed by a combined effect of two types of motion occurring at the same frequency scale: Li ion diffusion and another (slower) reorientational motion of BH₄ groups. These results suggest that the Li ion jumps and the slower reorientational jumps of BH₄ groups in LiLa­(BH₄)₃Cl may be correlated. The estimate of the tracer Li ion diffusion coefficient at room temperature (5.2 × 10^–8 cm²/s) following from our experimental data indicates that LiLa­(BH₄)₃Cl can be considered as a promising solid-state ionic conductor

Pressure-Collapsed Amorphous Mg(BH₄)₂: An Ultradense Complex Hydride Showing a Reversible Transition to the Porous Framework

Hydrogen-storage properties of complex hydrides depend of their form, such as a polymorphic form or an eutectic mixture. This Paper reports on an easy and reproducible way to synthesize a new stable form of magnesium borohydride by pressure-induced collapse of the porous γ-Mg­(BH₄)₂. This amorphous complex hydride was investigated by temperature-programmed synchrotron X-ray diffraction (SXRD), transmission electron microscopy (TEM), thermogravimetric analysis, differential scanning calorimetry analysis, and Raman spectroscopy, and the dynamics of the BH₄^– reorientation was studied by spin–lattice relaxation NMR spectroscopy. No long-range order is observed in the lattice region by Raman spectroscopy, while the internal vibration modes of the BH₄^– groups are the same as in the crystalline state. A hump at 4.9 Å in the SXRD pattern suggests the presence of nearly linear Mg–BH₄–Mg fragments constituting all the known crystalline polymorphs of Mg­(BH₄)₂, which are essentially frameworks built of tetrahedral Mg nodes and linear BH₄ linkers. TEM shows that the pressure-collapsed phase is amorphous down to the nanoscale, but surprisingly, SXRD reveals a transition at ∼90 °C from the dense amorphous state (density of 0.98 g/cm³) back to the porous γ phase having only 0.55 g/cm³ crystal density. The crystallization is slightly exothermic, with the enthalpy of −4.3 kJ/mol. The volumetric hydrogen density of the amorphous form is 145 g/L, one of the highest among hydrides. Remarkably, this form of Mg­(BH₄)₂ has different reactivity compared to the crystalline forms. The parameters of the reorientational motion of BH₄ groups in the amorphous Mg­(BH₄)₂ found from NMR measurements differ significantly from those in the known crystalline forms. The behavior of the nuclear spin–lattice relaxation rates can be described in terms of a Gaussian distribution of the activation energies centered on 234 ± 9 meV with the dispersion of 100 ± 10 meV

The First Halide-Free Bimetallic Aluminum Borohydride: Synthesis, Structure, Stability, and Decomposition Pathway

Interaction of solid KBH₄ with liquid Al­(BH₄)₃ at room temperature yields a solid bimetallic borohydride KAl­(BH₄)₄. According to the synchrotron X-ray powder diffraction, its crystal structure (space group <i>Fddd, a</i> = 9.7405(3), <i>b</i> = 12.4500(4), and <i>c</i> = 14.6975(4) Å) contains a substantially distorted tetrahedral [Al­(BH₄)₄]⁻ anion, where the borohydride groups are coordinated to aluminum atoms via edges. The η²-coordination of BH₄^– is confirmed by the infrared and Raman spectroscopies. The title compound is the first aluminum-based borohydride complex not stabilized by halide anions or by bulky organic cations. It is not isostructural to bimetallic chlorides, where more regular tetrahedral AlCl₄^– anions are present. Instead, it is isomorphic to the LT phase of TbAsO₄ and can be also viewed as consisting of two interpenetrated <i>dia</i>-type nets where BH₄ ligand is bridging Al and K cations. Variable temperature X-ray powder diffraction, TGA, DSC, and TGA-MS data reveal a single step of decomposition at 160 °C, with an evolution of hydrogen and some amount of diborane. Aluminum borohydride is not released in significant amounts; however, some crystalline KBH₄ forms upon decomposition. The higher decomposition temperature than in chloride-substituted Li–Al (70 °C) and Na–Al (90 °C) borohydrides suggests that the larger alkali metal cations (weaker Pearson acids) stabilize the weak Pearson base, [Al­(BH₄)₄]⁻

Aluminum Borohydride Complex with Ethylenediamine: Crystal Structure and Dehydrogenation Mechanism Studies

We report the structure of an aluminum borohydride ethylenediamine complex, Al­(EDA)₃·3BH₄·EDA. This structure was successfully determined using X-ray powder diffraction and was supported by first-principles calculations. The complex can be described as a mononuclear complex exhibiting three-dimensional supramolecular structure, built from units of Al­[C₂N₂H₈]₃, BH₄, and ethylenediamine (EDA) molecules. Examination of the chemical bonding indicates that this arrangement is stabilized via dihydrogen bonding between the NH₂ ligand in EDA and the surrounding BH₄. The partial ionic bonding between the Al and N atoms in EDA forms a five-member ring (5MR), an Al­[NCCN] unit. The calculated H₂ removal energies confirm that it is energetically favorable to remove the loosely bonded EDA and H atoms with N–H···H–B dihydrogen bonds upon heating. Our results suggest that the NH₂ terminal ligand in the EDA molecule combines with a H atom in the BH₄ group to release H₂ at elevated temperature, and our results confirm that the experimental result Al­(EDA)₃·3BH₄·EDA can release 8.4 wt % hydrogen above 149 °C with detectable EDA molecules. This work provides insights into the dehydrogenation behavior of Al­(EDA)₃·3BH₄·EDA and has implications for future development of promising high-performance metal borohydride ethylenediamine complexes

Lithium Hydrazinidoborane: A Polymorphic Material with Potential for Chemical Hydrogen Storage

Herein, we describe the synthesis and characterization (chemical, structural, and thermal) of a new crystal phase of lithium hydrazinidoborane (LiN₂H₄­BH₃, LiHB), which is a new material for solid-state chemical hydrogen storage. We put in evidence that lithium hydrazinidoborane is a polymorphic material, with a stable low-temperature phase and a metastable high-temperature phase. The former is called β-LiHB and the latter α-LiHB. Results from DSC and XRD showed that the transition phase occurs at around 90 °C. On this basis, the crystal structure of the novel β-LiHB phase was solved. The potential of this material for solid-state chemical hydrogen storage was verified by TGA, DSC, and isothermal dehydrogenations. Upon the formation of the α-LiHB phase, the borane dehydrogenates. At 150 °C, it is able to generate 10 wt % of pure H₂ while a solid residue consisting of polymers with linear and cyclic units forms. Reaction mechanisms and formation of bis­(lithium hydrazide) of diborane [(LiN₂H₃)₂­BH₂]⁺­[BH₄]⁻ as a reaction intermediate are tentatively proposed to highlight the decomposition of β-LiHB in our conditions

Potassium Zinc Borohydrides Containing Triangular [Zn(BH₄)₃]⁻ and Tetrahedral [Zn(BH₄)_<i>x</i>Cl_4–<i>x</i>]^2– Anions

Three novel potassium–zinc borohydrides/chlorides are described. KZn(BH₄)₃ and K₂Zn(BH₄)_<i>x</i>Cl_4–<i>x</i> form in ball-milled KBH₄:ZnCl₂ mixtures with molar ratios ranging from 1.5:1 up to 3:1. On the other hand, K₃Zn(BH₄)_<i>x</i>Cl_5–<i>x</i> forms only in the 2:1 mixture after longer milling times. The new compounds have been studied by a combination of in situ synchrotron powder diffraction, thermal analysis, Raman spectroscopy, and the solid state DFT calculations. Rhombohedral KZn(BH₄)₃ contains an anionic complex [Zn(BH₄)₃]⁻ with <i>D</i>₃ (32) symmetry, located inside a rhombohedron K₈. KZn(BH₄)₃ contains 8.1 wt % of hydrogen and decomposes at ∼385 K with a release of hydrogen and diborane similar to other Zn-based bimetallic borohydrides like MZn₂(BH₄)₅ (M = Li, Na) and NaZn(BH₄)₃. The decomposition temperature is much lower than for KBH₄. Monoclinic K₂Zn(BH₄)_<i>x</i>Cl_4–<i>x</i> contains a tetrahedral complex anion [Zn(BH₄)_<i>x</i>Cl_4–<i>x</i>]²^– located inside an Edshammar polyhedron (pentacapped trigonal prism) K₁₁. The compound is a monoclinically distorted variant of the paraelectric orthorhombic <i>ht</i>-phase of K₂ZnCl₄ (structure type K₂SO₄). K₂Zn(BH₄)_<i>x</i>Cl_4–<i>x</i> releases BH₄ starting from 395 K, forming Zn and KBH₄. As the reaction proceeds and <i>x</i> decreases, the monoclinic distortion of K₂Zn(BH₄)_<i>x</i>Cl_4–<i>x</i> diminishes and the structure transforms at 445 K into the orthorhombic <i>ht</i>-phase of K₂ZnCl₄. Tetragonal K₃Zn(BH₄)_<i>x</i>Cl_5–<i>x</i> is a substitutional and deformation variant of the tetragonal (<i>I</i>4/<i>mcm</i>) Cs₃CoCl₅ structure type possibly with the space group <i>P</i>4₂/<i>ncm</i>. K₃Zn(BH₄)_<i>x</i>Cl_5–<i>x</i> decomposes nearly at the same temperature as KZn(BH₄)₃, i.e., at ∼400 K, with the formation of K₂Zn(BH₄)_<i>x</i>Cl_4–<i>x</i> and KBH₄, indicating that the compound is an adduct of the two latter compounds

Potassium Zinc Borohydrides Containing Triangular [Zn(BH₄)₃]⁻ and Tetrahedral [Zn(BH₄)_<i>x</i>Cl_4–<i>x</i>]^2– Anions

Three novel potassium–zinc borohydrides/chlorides are described. KZn(BH₄)₃ and K₂Zn(BH₄)_<i>x</i>Cl_4–<i>x</i> form in ball-milled KBH₄:ZnCl₂ mixtures with molar ratios ranging from 1.5:1 up to 3:1. On the other hand, K₃Zn(BH₄)_<i>x</i>Cl_5–<i>x</i> forms only in the 2:1 mixture after longer milling times. The new compounds have been studied by a combination of in situ synchrotron powder diffraction, thermal analysis, Raman spectroscopy, and the solid state DFT calculations. Rhombohedral KZn(BH₄)₃ contains an anionic complex [Zn(BH₄)₃]⁻ with <i>D</i>₃ (32) symmetry, located inside a rhombohedron K₈. KZn(BH₄)₃ contains 8.1 wt % of hydrogen and decomposes at ∼385 K with a release of hydrogen and diborane similar to other Zn-based bimetallic borohydrides like MZn₂(BH₄)₅ (M = Li, Na) and NaZn(BH₄)₃. The decomposition temperature is much lower than for KBH₄. Monoclinic K₂Zn(BH₄)_<i>x</i>Cl_4–<i>x</i> contains a tetrahedral complex anion [Zn(BH₄)_<i>x</i>Cl_4–<i>x</i>]²^– located inside an Edshammar polyhedron (pentacapped trigonal prism) K₁₁. The compound is a monoclinically distorted variant of the paraelectric orthorhombic <i>ht</i>-phase of K₂ZnCl₄ (structure type K₂SO₄). K₂Zn(BH₄)_<i>x</i>Cl_4–<i>x</i> releases BH₄ starting from 395 K, forming Zn and KBH₄. As the reaction proceeds and <i>x</i> decreases, the monoclinic distortion of K₂Zn(BH₄)_<i>x</i>Cl_4–<i>x</i> diminishes and the structure transforms at 445 K into the orthorhombic <i>ht</i>-phase of K₂ZnCl₄. Tetragonal K₃Zn(BH₄)_<i>x</i>Cl_5–<i>x</i> is a substitutional and deformation variant of the tetragonal (<i>I</i>4/<i>mcm</i>) Cs₃CoCl₅ structure type possibly with the space group <i>P</i>4₂/<i>ncm</i>. K₃Zn(BH₄)_<i>x</i>Cl_5–<i>x</i> decomposes nearly at the same temperature as KZn(BH₄)₃, i.e., at ∼400 K, with the formation of K₂Zn(BH₄)_<i>x</i>Cl_4–<i>x</i> and KBH₄, indicating that the compound is an adduct of the two latter compounds