Hydrogen Dynamics in Nanoconfined Lithiumborohydride

Abstract

Lithiumborohydride (LiBH4) contains 18.5 wt % hydrogen and exhibits a structural phase transition (orthorhombic→ hexagonal) at 381 K, which is associated with a large increase in hydrogen and lithium mobility in the solid. Confining metal hydrides in a nanoporous matrix may change the hydrogen desorption kinetics and reversibility, and influence phase equilibria. The hydrogen mobility in nanoconfined LiBH4 was studied using inelastic and quasielastic neutron scattering. Confinement in nanoporous carbon leads to a greater anion mobility and a reduced activation energy of 8 kJ/mol at room temperature as compared to 17.3 kJ/mol in bulk LiBH4. In the nanoconfined phase, the mobility resembles that of the high-temperature bulk phase, and no distinct phase transition was observed. However, a substantial fraction of the hydrogen is immobile, leading to effectively reduced anion dynamics as compared to the bulk high-temperature phase. We tentatively attribute these effects to lattice distortions due to the finite pore size, and to thermally induced stress leading to a loss in long-range order and an increase in dynamical disorder, as supported by first principle calculations.