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.

Isotopic Exchange in Porous and Dense Magnesium Borohydride.

Magnesium borohydride (Mg(BH4)2) is one of the most promising complex hydrides presently studied for energy-related applications. Many of its properties depend on the stability of the BH4(-) anion. The BH4(-) stability was investigated with respect to H→D exchange. In situ Raman measurements on high-surface-area porous Mg(BH4 )2 in 0.3 MPa D2 have shown that the isotopic exchange at appreciable rates occurs already at 373 K. This is the lowest exchange temperature observed in stable borohydrides. Gas-solid isotopic exchange follows the BH4(-) +D˙ →BH3D(-) +H˙ mechanism at least at the initial reaction steps. Ex situ deuteration of porous Mg(BH4)2 and its dense-phase polymorph indicates that the intrinsic porosity of the hydride is the key behind the high isotopic exchange rates. It implies that the solid-state H(D) diffusion is considerably slower than the gas-solid H→D exchange reaction at the surface and it is a rate-limiting steps for hydrogen desorption and absorption in Mg(BH4)2.

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.

Selective reversible hydrogenation of Mg(B3H8)2/MgH2 to Mg(BH4)2: pathway to reversible borane-based hydrogen storage?

Mg(B3H8)2·2THF (THF = tetrahydrofuran) was prepared by the addition of BH3·THF to Mg/Hg amalgam. Heating a 1:2 molar mixture of Mg(B3H8)2·2THF and MgH2 to 200 °C under 5 MPa H2 for 2 h leads to nearly quantitative conversion to Mg(BH4)2. The differential scanning calorimetry profile of the reaction measured under 5 MPa H2 shows an initial endothermic feature at ∼65 °C for a phase change of the compound followed by a broad exothermic feature that reaches a maximum at 130 °C corresponding to the hydrogenation of Mg(B3H8)2 to Mg(BH4)2. Heating Mg(B3H8)2·2THF to 200 °C under 5 MPa H2 pressure in the absence of MgH2 gives predominantly MgB12H12 as well as significant amounts of MgB10H10 and Mg(BH4)2. Hydrogenation of a mixture of Mg(B3H8)2·2THF and LiH in a 1:4 molar ratio at 130 °C under 5 MPa H2 yields [B12H12](2-) in addition to [BH4](-), while a 1:4 molar ratio of Mg(B3H8)2·2THF and NaH yields [BH4](-) and a new borane, likely [B2H7](-). Hydrogenation of the NaH-containing mixture at 130 °C gives primarily the alternative borane, indicating it is an intermediate in the two-step conversion of the triborane to [BH4](-). The solvent-free triborane Mg(B3H8)2, derived from the low-temperature dehydrogenation of Mg(BH4)2, also produces Mg(BH4)2, but higher temperature and pressure is required to effect the complete transformation of the Mg(B3H8)2. These results show that the reversible transformation of the triborane depends on the stability of the metal hydride. The more stable the metal hydride, that is, LiH > NaH > MgH2, the lower is the "regeneration" efficiency.

Complex transition metal hydrides incorporating ionic hydrogen: thermal decomposition pathway of Na2Mg2FeH8 and Na2Mg2RuH8.

Complex transition metal hydrides have potential technological application as hydrogen storage materials, smart windows and sensors. Recent exploration of these materials has revealed that the incorporation of anionic hydrogen into these systems expands the potential number of viable complexes, while varying the countercation allows for optimisation of their thermodynamic stability. In this study, the optimised synthesis of Na2Mg2TH8 (T = Fe, Ru) has been achieved and their thermal decomposition properties studied by ex situ Powder X-ray Diffraction, Gas Chromatography and Pressure-Composition Isotherm measurements. The temperature and pathway of decomposition of these isostructural compounds differs considerably, with Na2Mg2FeH8 proceeding via NaMgH3 in a three-step process, while Na2Mg2RuH8 decomposes via Mg2RuH4 in a two-step process. The first desorption maxima of Na2Mg2FeH8 occurs at ca. 400 °C, while Na2Mg2RuH8 has its first maxima at 420 °C. The enthalpy and entropy of desorption for Na2Mg2TH8 (T = Fe, Ru) has been established by PCI measurements, with the ΔHdes for Na2Mg2FeH8 being 94.5 kJ mol(-1) H2 and 125 kJ mol(-1) H2 for Na2Mg2RuH8.

Unparalleled Lithium and Sodium Superionic Conduction in Solid Electrolytes with Large Monovalent Cage-like Anions.

Solid electrolytes with sufficiently high conductivities and stabilities are the elusive answer to the inherent shortcomings of organic liquid electrolytes prevalent in today's rechargeable batteries. We recently revealed a novel fast-ion-conducting sodium salt, Na2B12H12, which contains large, icosahedral, divalent B12H122- anions that enable impressive superionic conductivity, albeit only above its 529 K phase transition. Its lithium congener, Li2B12H12, possesses an even more technologically prohibitive transition temperature above 600 K. Here we show that the chemically related LiCB11H12 and NaCB11H12 salts, which contain icosahedral, monovalent CB11H12- anions, both exhibit much lower transition temperatures near 400 K and 380 K, respectively, and truly stellar ionic conductivities (> 0.1 S cm-1) unmatched by any other known polycrystalline materials at these temperatures. With proper modifications, we are confident that room-temperature-stabilized superionic salts incorporating such large polyhedral anion building blocks are attainable, thus enhancing their future prospects as practical electrolyte materials in next-generation, all-solid-state batteries.

Exceptional superionic conductivity in disordered sodium decahydro-closo-decaborate.

Na2 B10 H10 exhibits exceptional superionic conductivity above ca. 360 K (e.g., ca. 0.01 S cm(-1) at 383 K) concomitant with its transition from an ordered monoclinic structure to a face-centered-cubic arrangement of orientationally disordered B10 H10 (2-) anions harboring a vacancy-rich Na(+) cation sublattice. This discovery represents a major advancement for solid-state Na(+) fast-ion conduction at technologically relevant device temperatures.

Site occupancy of interstitial deuterium atoms in face-centred cubic iron.

Hydrogen composition and occupation state provide basic information for understanding various properties of the metal-hydrogen system, ranging from microscopic properties such as hydrogen diffusion to macroscopic properties such as phase stability. Here the deuterization process of face-centred cubic Fe to form solid-solution face-centred cubic FeDx is investigated using in situ neutron diffraction at high temperature and pressure. In a completely deuterized specimen at 988 K and 6.3 GPa, deuterium atoms occupy octahedral and tetrahedral interstitial sites with an occupancy of 0.532(9) and 0.056(5), respectively, giving a deuterium composition x of 0.64(1). During deuterization, the metal lattice expands approximately linearly with deuterium composition at a rate of 2.21 Å(3) per deuterium atom. The minor occupation of the tetrahedral site is thermally driven by the intersite movement of deuterium atoms along the direction in the face-centred cubic metal lattice.

Sodium superionic conduction in Na2B12H12.

Impedance measurements indicate that Na2B12H12 exhibits dramatic Na(+) conductivity (on the order of 0.1 S cm(-1)) above its order-disorder phase-transition at ≈529 K, rivaling that of current, solid-state, ceramic-based, Na-battery electrolytes. Superionicity may be aided by the large size, quasispherical shape, and high rotational mobility of the B12H12(2-) anions.

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.

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.

Halide-stabilized LiBH4, a room-temperature lithium fast-ion conductor.

Solid state lithium conductors are attracting much attention for their potential applications to solid-state batteries and supercapacitors of high energy density to overcome safety issues and irreversible capacity loss of the currently commercialized ones. Recently, we discovered a new class of lithium super ionic conductors based on lithium borohydride (LiBH(4)). LiBH(4) was found to have conductivity as high as 10(-2) Scm(-1) accompanied by orthorhombic to hexagonal phase transition above 115 degrees C. Polarization to the lithium metal electrode was shown to be extremely low, providing a versatile anode interface for the battery application. However, the high transition temperature of the superionic phase has limited its applications. Here we show that a chemical modification of LiBH(4) can stabilize the superionic phase even below room temperature. By doping of lithium halides, high conductivity can be obtained at room temperature. Both XRD and NMR confirmed room-temperature stabilization of superionic phase for LiI-doped LiBH(4). The electrochemical measurements showed a great advantage of this material as an extremely lightweight lithium electrolyte for batteries of high energy density. This material will open alternative opportunities for the development of solid ionic conductors other than previously known lithium conductors.