Lithium borohydride (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.

The Journal of Physical Chemistry C 2013