Conductor-Insulator Interfaces in Solid Electrolytes: A Design Strategy to Enhance Li-Ion Dynamics in Nanoconfined LiBH/AlO.

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 LiBH/AlO prepared by melt infiltration, a percolating network of fast conductor-insulator Li diffusion pathways could be realized. These heterocontacts provide regions with extremely rapid Li NMR spin fluctuations giving direct evidence for very fast Li jump processes in both nanoconfined LiBH/AlO and LiBH-LiI/AlO. Compared to the nanocrystalline, AlO-free reference system LiBH-LiI, nanoconfinement leads to a strongly enhanced recovery of the Li NMR longitudinal magnetization. The fact that almost no difference is seen between LiBH-LiI/AlO and LiBH/AlO unequivocally reveals that the overall Li 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 LiBH, either being orthorhombic (LiBH/AlO) or hexagonal (LiBH-LiI/AlO). Li (and H) 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 LiBH guarantees fast two-dimensional (2D) bulk ion dynamics and contributes to facilitating fast long-range ion transport.