Pascal Martelli

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.

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.

Mobility and dynamics in the complex hydrides LiAlH4 and LiBH4

The dynamics and bonding of the complex hydrides LiBH4 and LiAlH4 have been investigated by vibrational spectroscopy. The combination of infrared, Raman, and inelastic neutron scattering (INS) spectroscopies on hydrided and deuterided samples reveals a complete picture of the dynamics of the BH4- and AlH4 anions respectively as well as the lattice. The straightforward interpretation of isotope effects facilitates tracer diffusion experiments revealing the diffusion coefficients of hydrogen containing species in LiBH4, and LiAlH4. LiBH4 exchanges atomic hydrogen starting at 200 degrees C. Despite having an iso-electronic structure, the mobility of hydrogen in LiAlH4 is different from that of LiBH4. Upon ball-milling of LiAlH4 and LiAlD4, hydrogen is exchanged with deuterium even at room temperature. However, the exchange reaction competes with the decomposition of the compound. The diffusion coefficients of the alanate and borohydride have been found to be D approximately equal 7 x 10(-14) m2 s(-1) at 473 K and D approximately equal 5 x 10(-16) m2 s(-1) at 348 K, respectively. The BH4 ion is easily exchanged by other ions such as I- or by NH2-. This opens the possibility of tailoring physical properties such as the temperature of the phase transition linked to the Li-ion conductivity in LiBH4 as measured by nuclear magnetic resonance and Raman spectroscopy. Temperature dependent Raman measurements on diffusion gradient samples Li(BH4)1-cIc demonstrate that increasing temperature has a similar impact to increasing the iodide concentration c: the system is driven towards the high-temperature phase of LiBH4. The influence of anion exchange on the hydrogen sorption properties is limited, though. For example, Li4(BH4)(NH2)3 does not exchange hydrogen easily even in the melt.

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.