Disentangling Multiscale Cation-Anion Dynamics in Li₄(BH₄)(NH₂)₃ Using Quasielastic Neutron Scattering

Elucidating the interplay between anion dynamics and cation transport is essential for the rational design of solid-state ionic conductors. Here, we investigate the relationship between local anion motion and long-range ${\mathrm{Li}}^{+}$ transport in ${\mathrm{Li}}_4\left({\mathrm{BH}}_4\right){\left({\mathrm{NH}}_2\right)}_3$ by combining quasielastic neutron scattering (QENS) and neutron spin-echo (NSE) spectroscopy with complementary thermal, structural, and electrochemical investigations. Time-of-flight QENS analysis reveals ultrafast reorientational motion of ${\left[{\mathrm{BH}}_4\right]}^{-}$ units within the crystalline lattice, characterized by a low rotational activation barrier of ∼0.16 eV. This barrier is substantially lower than the ${\mathrm{Li}}^{+}$ transport activation energy (∼0.26 eV), demonstrating that thermally activated ${\left[{\mathrm{BH}}_4\right]}^{-}$ reorientation provides a dynamic pathway that promotes ${\mathrm{Li}}^{+}$ hopping through the lattice. In contrast, ${\left[{\mathrm{NH}}_2\right]}^{-}$ anions exhibit limited quasielastic broadening, indicating more restricted dynamics. Upon melting, the emergence of Q-dependent quasielastic broadening reveals translational diffusion of ${\left[{\mathrm{BH}}_4\right]}^{-}$ species, yielding a diffusion coefficient of ∼. This value closely agrees with the ${\mathrm{Li}}^{+}$ diffusion coefficient obtained from ionic conductivity measurements, evidencing a transition from rotation-assisted ${\mathrm{Li}}^{+}$ migration in the solid state to coupled cation-anion diffusion in the molten phase. Complementary NSE measurements directly capture long-range ${\mathrm{Li}}^{+}$ motion on nanosecond timescales, providing a unified molecular-level picture of how anion dynamics regulate ion transport in mixed-anion complex hydrides.