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

Abstract

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.

Conductor–Insulator Interfaces in Solid Electrolytes: A Design Strategy to Enhance Li-Ion Dynamics in Nanoconfined LiBH4/Al2O3

“… 8 The LiBH 4 /Al 2 O 3 zones are, however, expected to be rich in defects, thus facilitating ion transport. 33 A similar effect has been described very recently by first-principles calculations for the interface in LiBH 4 /MoS 2 composites. 44 Importantly, in LiBH 4 /SiO 2 composites, the role of surface groups should not be underestimated.…”

“…As discussed earlier, 31 , 33 the overall coupling constant C q (≈ 20 kHz) turned out to be clearly reduced compared to that of LiBH 4 -LiI. Importantly, for LiBH 4 /Al 2 O 3 , almost the same line shapes are detected as for the LiBH 4 -LiI/Al 2 O 3 .…”

“…Therefore, the combination of nanoconfinement and anion substitution enables facile, overall Li + long-range ion transport as it is necessary for, e.g., battery applications. 31 , 33 …”

“…As ionic conductivity depends on both the mobility μ and the charge carrier density N , the combination of anion substitution and nanoconfinement is a perfect tool to tune overall Li + transport. 31 , 33 …”

The effect of nanoscaffold porosity and surface chemistry on the Li-ion conductivity of LiBH4–LiNH2/metal oxide nanocomposites

“…In fact, via the synthesis of nanoconned anion-substituted complex hydrides, such as nanoconned LiBH 4 -LiI and LiBH 4 -LiNH 2 , conductivity could be substantially improved. [53][54][55] Although the increased conductivity in the anion-substituted/oxide nanocomposites was attributed to the synergetic effect of anion substitution and connement in the oxide nanopores, the exact role of the connement remained unclear. For nanocomposites with pure complex hydrides, e.g.…”