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

Solid electrolytes based on LiBH 4 receive much attention because of their high ionic conductivity, electrochemical robustness, and low interfacial resistance against Li metal. The highly conductive hexagonal modification of LiBH 4 can be stabilized via the incorporation of LiI. If the resulting LiBH 4 -LiI is confined to the nanopores of an oxide, such as Al 2 O 3 , interface-engineered LiBH 4 -LiI/Al 2 O 3 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 1 H, 6 Li, 7 Li, 11 B, and 27 Al nuclear magnetic resonance (NMR) to study structural and dynamic features of nanoconfined LiBH 4 -LiI/Al 2 O 3 . In particular, diffusion-induced 1 H, 7 Li, and 11 B NMR spin-lattice relaxation measurements and 7 Li-pulsed field gradient (PFG) NMR experiments were used to extract activation energies and diffusion coefficients. 27 Al magic angle spinning NMR revealed surface interactions of LiBH 4 -LiI with pentacoordinated Al sites, and two-component 1 H 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 LiBH 4 -LiI regions turned out to be fully homogenous. This view is supported by 7 Li 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 LiBH 4 -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 LiBH 4 -LiI/Al 2 O 3 is characterized by an activation energy of 0.43 eV.