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

Roman Zettl; Katharina Hogrefe; Bernhard Gadermaier; Ilie Hanzu; Peter Ngene; Petra E de Jongh; H Martin R Wilkening

doi:10.1021/acs.jpcc.1c03789

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

J Phys Chem C Nanomater Interfaces. 2021 Jul 15;125(27):15052-15060. doi: 10.1021/acs.jpcc.1c03789. Epub 2021 Jul 6.

Authors

Roman Zettl^{1

2}, Katharina Hogrefe¹, Bernhard Gadermaier¹, Ilie Hanzu¹, Peter Ngene², Petra E de Jongh², H Martin R Wilkening¹

Affiliations

¹ Institute for Chemistry and Technology of Materials, Christian-Doppler-Laboratory for Lithium Batteries, Graz University of Technology (NAWI Graz), Stremayrgasse 9, 8010 Graz, Austria.
² Materials Chemistry and Catalysis, Debye Institute for Nanomaterials Science, Utrecht University, 3584 Utrecht, Netherlands.

Abstract

Synthesizing Li-ion-conducting solid electrolytes with application-relevant properties for new energy storage devices is a challenging task that relies on a few design principles to tune ionic conductivity. When starting with originally poor ionic compounds, in many cases, a combination of several strategies, such as doping or substitution, is needed to achieve sufficiently high ionic conductivities. For nanostructured materials, the introduction of conductor-insulator interfacial regions represents another important design strategy. Unfortunately, for most of the two-phase nanostructured ceramics studied so far, the lower limiting conductivity values needed for applications could not be reached. Here, we show that in nanoconfined LiBH₄/Al₂O₃ prepared by melt infiltration, a percolating network of fast conductor-insulator Li⁺ diffusion pathways could be realized. These heterocontacts provide regions with extremely rapid ⁷Li NMR spin fluctuations giving direct evidence for very fast Li⁺ jump processes in both nanoconfined LiBH₄/Al₂O₃ and LiBH₄-LiI/Al₂O₃. Compared to the nanocrystalline, Al₂O₃-free reference system LiBH₄-LiI, nanoconfinement leads to a strongly enhanced recovery of the ⁷Li NMR longitudinal magnetization. The fact that almost no difference is seen between LiBH₄-LiI/Al₂O₃ and LiBH₄/Al₂O₃ unequivocally reveals that the overall ⁷Li NMR spin-lattice relaxation rates are solely controlled by the spin fluctuations near or in the conductor-insulator interfacial regions. Thus, the conductor-insulator nanoeffect, which in the ideal case relies on a percolation network of space charge regions, is independent of the choice of the bulk crystal structure of LiBH₄, either being orthorhombic (LiBH₄/Al₂O₃) or hexagonal (LiBH₄-LiI/Al₂O₃). ⁷Li (and ¹H) NMR shows that rapid local interfacial Li-ion dynamics is corroborated by rather small activation energies on the order of only 0.1 eV. In addition, the LiI-stabilized layer-structured form of LiBH₄ guarantees fast two-dimensional (2D) bulk ion dynamics and contributes to facilitating fast long-range ion transport.