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 Li + transport in Li 4 ( BH 4 ) ( NH 2 ) 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 [ BH 4 ] - units within the crystalline lattice, characterized by a low rotational activation barrier of ∼0.16 eV. This barrier is substantially lower than the Li + transport activation energy (∼0.26 eV), demonstrating that thermally activated [ BH 4 ] - reorientation provides a dynamic pathway that promotes Li + hopping through the lattice. In contrast, [ NH 2 ] - anions exhibit limited quasielastic broadening, indicating more restricted dynamics. Upon melting, the emergence of Q-dependent quasielastic broadening reveals translational diffusion of [ BH 4 ] - species, yielding a diffusion coefficient of ∼. This value closely agrees with the Li + diffusion coefficient obtained from ionic conductivity measurements, evidencing a transition from rotation-assisted Li + migration in the solid state to coupled cation-anion diffusion in the molten phase. Complementary NSE measurements directly capture long-range Li + motion on nanosecond timescales, providing a unified molecular-level picture of how anion dynamics regulate ion transport in mixed-anion complex hydrides.