Li-Ion Diffusion in Nanoconfined LiBH₄-LiI/Al₂O₃: From 2D Bulk Transport to 3D Long-Range Interfacial Dynamics

Solid electrolytes based on LiBH₄ receive much attention because of their high ionic conductivity, electrochemical robustness, and low interfacial resistance against Li metal. The highly conductive hexagonal modification of LiBH₄ can be stabilized via the incorporation of LiI. If the resulting LiBH₄-LiI is confined to the nanopores of an oxide, such as Al₂O₃, interface-engineered LiBH₄-LiI/Al₂O₃ is obtained that revealed promising properties as a solid electrolyte. The underlying principles of Li⁺ conduction in such a nanocomposite are, however, far from being understood completely. Here, we used broadband conductivity spectroscopy and ¹H, ⁶Li, ⁷Li, ¹¹B, and ²⁷Al nuclear magnetic resonance (NMR) to study structural and dynamic features of nanoconfined LiBH₄-LiI/Al₂O₃. In particular, diffusion-induced ¹H, ⁷Li, and ¹¹B NMR spin-lattice relaxation measurements and ⁷Li-pulsed field gradient (PFG) NMR experiments were used to extract activation energies and diffusion coefficients. ²⁷Al magic angle spinning NMR revealed surface interactions of LiBH₄-LiI with pentacoordinated Al sites, and two-component ¹H NMR line shapes clearly revealed heterogeneous dynamic processes. These results show that interfacial regions have a determining influence on overall ionic transport (0.1 mS cm^-1 at 293 K). Importantly, electrical relaxation in the LiBH₄-LiI regions turned out to be fully homogenous. This view is supported by ⁷Li NMR results, which can be interpreted with an overall (averaged) spin ensemble subjected to uniform dipolar magnetic and quadrupolar electric interactions. Finally, broadband conductivity spectroscopy gives strong evidence for 2D ionic transport in the LiBH₄-LiI bulk regions which we observed over a dynamic range of 8 orders of magnitude. Macroscopic diffusion coefficients from PFG NMR agree with those estimated from measurements of ionic conductivity and nuclear spin relaxation. The resulting 3D ionic transport in nanoconfined LiBH₄-LiI/Al₂O₃ is characterized by an activation energy of 0.43 eV.