Hydrogen dynamics in nanoconfined lithiumborohydride

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.