Hideki Maekawa

The structural groups of alkali silicate glasses determined from 29Si MAS-NMR

Abstract Lithium, sodium and potassium silicate glasses containing 20–56 mol% alkali oxide were investigated by 29 Si nuclear magnetic resonance (MAS-NMR) spectroscopy. In the spectrum of each sample, at least two to four distinct peaks were identified. The distributions of SiO 4 structural units, Q n , where n is the number of bridging oxygen atoms bound to other Si atoms, were determined as a function of composition. The equilibrium constants of the reactions, 2Q n ⇌ Q n −1 + Q n +1 ( n = 3, 2, 1), were determined. The reaction proceeds to the right direction as cationic power of alkali ion ( Z / r ) increases (Li + >Na + >K + ) at the same alkali oxide concentration. The apparent equilibrium constants of the above reactions are discussed along with a proposed thermodynamic model. The 29 Si chemical shifts assigned to each structural unit increase linearly with alkali oxide contents. The slope of these lines decreases as the numbers of attached bridging oxygen (BO) atoms decrease. The average chemical shifts also increase linearly with an increase of alkali content. A close relationship between the average chemical shifts and the theoretical optical basicity was observed.

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.

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.

Protonic conduction and defect structures in Sr-doped LaPO4

Abstract Protonic conduction and defect structures in Sr-doped LaPO 4 (Sr=0–7 mol%) were investigated by using conductivity, MAS-NMR spectroscopy and FT-Raman spectroscopy measurements. The results of MAS-NMR and FT-Raman spectroscopy measurements revealed that hydrogen phosphate groups, possibly hydrogen phosphate ions HPO 4 2− , and pyrophosphate ions P 2 O 7 4− existed in Sr-doped LaPO 4 under wet and dry conditions, respectively, in addition to orthophosphate ions PO 4 3− . Based on these results, we proposed a defect structure model for Sr-doped LaPO 4 . According to this model, La substitution with Sr leads to formation of P 2 O 7 4− as intrinsic positive defects, and in wet atmosphere, protons are introduced into the phosphate forming hydrogen phosphate groups through the equilibrium between P 2 O 7 4− and water vapor. The conductivity behavior of Sr-doped LaPO 4 versus water vapor and oxygen partial pressures supported this defect structure model.

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.

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.

Lithium-ion conduction in complex hydrides LiAlH4 and Li3AlH6

Lithium-ion conduction in complex hydrides LiAlH4 and Li3AlH6 was investigated using ac complex impedance measurements. The conductivities at room temperature were 8.7×10−9 S/cm in the case of LiAlH4 and 1.4×10−7 S/cm in the case of Li3AlH6. To enhance the conductivity of Li3AlH6 having good thermal stability in heating/cooling cycles, mechanical milling, and addition of lithium halides (LiCl, LiI) were implemented. The maximum value of 2.5×10−4 S/cm at 393 K was observed when 0.33 M ratio of LiI was added to Li3AlH6. This study demonstrated two research directions to enhance the lithium-ion conductivity in a variety of complex hydrides.

High temperature 1H NMR study of proton conducting oxide SrCe0.95Y0.05H0.004O3-δ

Abstract The ionic motion of protons in yttrium doped SrCeO 3 perovskite has been investigated by means of high temperature 1 H NMR and ac electric conductivity measurements. The temperature dependences of line shapes and spin–lattice relaxation times ( T 1 ) were obtained. The inverse temperature versus logarithm of T 1 plot shows asymmetric temperature dependence with respect to its minimum value, and was well interpreted using the classical Bloembergen, Purcell and Pound (BPP) formula with introducing distributions of the activation energies. The averaged correlation times of the proton hopping were obtained from the T 1 data. The conductivity calculated from the Einstein equation using NMR correlation times assuming the jump distance of proton hopping as 1 A yields quantitative agreement with the measured conductivity in a wide temperature range.

Analysis of (π K+) and (K-, K+) hypernuclear production spectra in distorted-wave impulse approximation

Abstract.We study the hyperon-nucleus potential with distorted-wave impulse wave approximation (DWIA) using the Greens function method. In order to include the nucleon and hyperon potential effects in Fermi averaging, we introduce the local optimal momentum approximation of target nucleons. We can describe the quasi-free Λ , Σ and Ξ production spectra in a better way than in the standard Fermi-averaged t -matrix treatments.