Hitoshi Takamura

Lithium superionic conduction in lithium borohydride accompanied by structural transition

The electrical conductivity of lithium borohydride (LiBH4) measured by the ac complex impedance method jumped by three orders of magnitude due to structural transition from orthorhombic to hexagonal at approximately 390K. The hexagonal phase exhibited a high electrical conductivity of the order of 10−3Scm−1. Furthermore, the conductivity calculated from the Nernst-Einstein equation using the correlation time obtained from Li7 nuclear magnetic resonance was in good agreement with the measured electrical conductivity. It was concluded that the electrical conductivity in the hexagonal phase is due to the Li superionic conduction.

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.

Ti-V-Cr b.c.c. alloys with high protium content

Abstract The effects of composition and heat-treatment on the protium absorption–desorption properties of Ti–V–Cr alloys were investigated. It was found that the Ti–35V–40Cr alloy shows a 2.6 mass% protium capacity. The plateau pressure of the Ti–35V–xCr alloys increased with decreasing lattice constants associated with increasing Cr content. The main phase of the as-cast Ti–xV–Cr (Cr/Ti=40/25) alloys containing more than 15%V was a b.c.c. phase. These b.c.c. alloys exhibited a 2.4 mass% protium capacity. Heat-treatment over 1673 K was effective on stabilizing the b.c.c. structure for the Ti–xV–Cr (Cr/Ti=2/3) alloys with low V content. The Ti–5V–57.5Cr alloy heat-treated at 1673 K for 1 h yields a high capacity of 2.8 mass% protium, which is the highest value at 313 K reported so far. The alloy is economically promising since it contains low amounts of expensive V metal.

Complex Hydrides with (BH4)− and (NH2)− 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.

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.

Stabilization of lithium superionic conduction phase and enhancement of conductivity of LiBH4 by LiCl addition

LiBH4 exhibits lithium superionic conduction accompanied by structural transition at around 390 K. Addition of LiCl to LiBH4 drastically affects both the transition and electrical conductivity: Transition from low-temperature (LT) to high-temperature (HT) phases in LiBH4 is observed at 370 K upon heating and the HT phase can be retained at 350–330 K upon cooling. Further, the conductivity in the LT phase is more than one or two orders of magnitude higher than that of pure LiBH4. These properties could be attributed to the dissolution of LiCl into LiBH4, suggested by in situ x-ray diffraction measurement.

New V-based alloys with high protium absorption and desorption capacity

Abstract The hydrogen absorption properties of V–Zr–Ti–M(M=Fe, Mn, Ni) alloys were examined in order to develop the alloys with high hydrogen capacity. It was found that the best composition among the studied alloys is the V–Zr–Ti–Ni system. A suitable amount of the vanadium in the V–Zr–Ti–Ni system was studied in correlation with its hydrogen absorption properties, and turned out to be around 75 at.%V. Substitution with Zr improved the hydrogen absorption properties in forming the grain boundary network phases of C14 Laves phase. Heat treating the alloys (homogenize and quench) drastically improves the plateau region of PCT curves. Zr addition also improves the properties for V–Zr–Ti–Cr alloys.

Experimental and computational studies on structural transitions in the LiBH4–LiI pseudobinary system

Structural transition properties of the LiBH4+xLiI (x=0–1.00) pseudobinary system were examined by powder x-ray diffraction and differential scanning calorimetry combined with periodic density functional theory calculations. We experimentally and computationally confirmed the stabilization of the high-temperature [hexagonal, lithium super(fast-)ionic conduction] phase of LiBH4 with x=0.33 and 1.00, and the results also imply the existence of intermediate phases with x=0.07–0.20. The studies are of importance for further development of LiBH4 and the derived hydrides as solid-state electrolytes.

Protium absorption properties and protide formations of Ti–Cr–V alloys

Abstract Ti–Cr–V alloys are known to absorb about 3.8 mass% (H/M=2) of protium (hydrogen atom), but to desorb about 2.4 mass%. This paper aims to clarify protium absorption properties and protide formations of Ti–Cr–V alloys. It was found that higher protium desorption capacity was achieved by increasing Cr content and controlling measurement temperature in order to control the desorption plateau pressure near atmospheric pressure for the alloys with less than 40 at% V content. However, Cr-rich alloys were found to absorb up to H/M=1 because of the formation of the mono-protides. The region with higher protium desorption capacity was obtained. The lattice parameters of the alloys and the enthalpy changes for di-protide formation were estimated from the compositions of the alloys. Moreover, estimated enthalpy changes for di-protide formation and the lattice parameters of the alloys were found to be generally constant on the limited line between appearance of regions of mono- and di-protides.

Protium absorption properties of Ti–V–Cr–Mn alloys with a b.c.c. structure

The protium absorption–desorption properties of Ti–V–Cr–Mn alloys were studied by varying the contents of Mn, Ti and V in the alloys. It was found that Mn addition of <10 at% was effective in improving the slope of the plateau region without any reduction of the protium absorption–desorption capacity. For Ti–V–Cr–Mn alloys, the α-Ti phase, which does not contribute to the protium storage capacity, appeared along grain boundaries during heat treatment. The formation of the α-Ti phase was found to be suppressed by reducing the Ti content and increasing the V content. By inhibiting the appearance of the α-Ti phase along the grain boundary, the protium absorption–desorption capacity of Ti–xV–Cr–Mn (x=45, 55; Ti/Cr/Mn=24:31:10) heat-treated alloys exhibited 2.7 mass% H, the highest value reported so far.