Abstract
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.
Highlights
The increase of anthropogenic CO2 in the atmosphere is responsible for the global warming observed; this warming is generally known as the greenhouse effect
We show that nuclear spin relaxation, pulsed field gradient (PFG) nuclear magnetic resonance (NMR), and electrical relaxation provide consistent diffusion coefficients characterizing long-range ion dynamics
Solid-state diffusion coefficients estimated from conductivity values and variable-temperature resistivity data recorded at fixed frequency agree very well with those obtained from macroscopic 7Li PFG NMR
Summary
The increase of anthropogenic CO2 in the atmosphere is responsible for the global warming observed; this warming is generally known as the greenhouse effect. Nanoconfinement is, in general, an elegant approach to increase the room-temperature conductivity of LiBH4.28−31 Bulk LiBH4 undergoes a reversible structural change at approximately Tpt = 110 °C and changes from the highly conductive hexagonal phase to the poorly conductive orthorhombic phase.[32] The scaffolds of the nanoconfined complex hydrides are ionic insulators In such conductor/ insulator composites,[33−37] enhanced ion transport along the heterointerfaces can occur due to both structural disorder[38−40] and/or space charge effects.[35,41−43]. The present paper is aimed at giving insights into the mechanisms of Li+ diffusion in LiBH4-LiI/Al2O3 For this purpose, we used solid-state NMR techniques including variable-temperature 1H, 7Li, and 11B spin−lattice relaxation measurements and 7Li pulsed-field gradient (PFG) experiments to study ion dynamics on different length scales. Via the combination of these techniques, we were able to study ion transport in the nanoconfined ion conductor over a dynamic range of 6−8 orders of magnitude
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