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

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.