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 LiBH 4 /Al 2 O 3 prepared by melt infiltration, a percolating network of fast conductor–insulator Li + diffusion pathways could be realized. These heterocontacts provide regions with extremely rapid 7 Li NMR spin fluctuations giving direct evidence for very fast Li + jump processes in both nanoconfined LiBH 4 /Al 2 O 3 and LiBH 4 -LiI/Al 2 O 3 . Compared to the nanocrystalline, Al 2 O 3 -free reference system LiBH 4 -LiI, nanoconfinement leads to a strongly enhanced recovery of the 7 Li NMR longitudinal magnetization. The fact that almost no difference is seen between LiBH 4 -LiI/Al 2 O 3 and LiBH 4 /Al 2 O 3 unequivocally reveals that the overall 7 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 4 , either being orthorhombic (LiBH 4 /Al 2 O 3 ) or hexagonal (LiBH 4 -LiI/Al 2 O 3 ). 7 Li (and 1 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 4 guarantees fast two-dimensional (2D) bulk ion dynamics and contributes to facilitating fast long-range ion transport.

Extremely Fast Interfacial Li Ion Dynamics in Crystalline LiTFSI Combined with EMIM-TFSI

“…Hence, we assign the fast diffusing spins to those 7 Li ions that benefit from the LiTFSI|EMIM-TFSI interfacial regions in the composite with the overall composition Li 0.9 EMIN 0.1 TFSI. This assignment is in agreement with earlier studies focusing on similar (nanocrystalline or nanoconfined) two-phase systems. ,,,, …”

“…Such an assignment would be in agreement with nanocrystalline and nanoconfined systems studied earlier by NMR; see above. 13,16,19,21 These regions, as has been shown for nanocrystalline ceramics, are extended over a region of 2−4 nm. 42 In the present case, Li + ion dynamics of this fast spin ensemble is characterized by a very low activation energy as low as 73 meV.…”

“…This assignment is in agreement with earlier studies focusing on similar (nanocrystalline or nanoconfined) two-phase systems. 13,16,21,22,42 To quantify the dynamic properties the two spin ensembles, we recorded 7 Li NMR spin−lattice relaxation transients M z (t d ) and studied their evolution as a function of temperature T, see Figure 3. We assume that magnetization recovery is most likely induced by both magnetic dipolar and to a larger extent by electric quadrupolar interactions.…”

“…12−15 In so-called dispersed ion conductors, a Li-ion conducting phase is mixed on the nanometer length scale with a second compound that acts as insulating phase, such as Al 2 O 3 , TiO 2 , or B 2 O 3 . Examples of such nanostructured composites 16 include LiI:Al 2 O 3 , 17 Li 2 O:X 2 O 3 (X = Al, B), 5,14,18−20 nanoconfined LiBH 4 :Al 2 O 3 , 21 LiF:Al 2 O 3 , 22 and LiF:TiO 2 . 23 As has been shown earlier, 24 poorly conducting LiTFSI (lithium bis(trifluoromethylsulfonyl)imide, LiX, X = TFSI (N(CF 3 SO 2 ) 2 − )) can be turned into a highly conducting composite material by adding a small amount of an ionic liquid (IL) with the same anion, i.e., by adding EMIM-TFSI (EMIM means the 1-ethyl-3-methylimidazolium cation, C 6 H 11 N 2 + ).…”

“…In so-called dispersed ion conductors, a Li-ion conducting phase is mixed on the nanometer length scale with a second compound that acts as insulating phase, such as Al 2 O 3 , TiO 2 , or B 2 O 3 . Examples of such nanostructured composites include LiI:Al 2 O 3 , Li 2 O:X 2 O 3 (X = Al, B), ,,− nanoconfined LiBH 4 :Al 2 O 3 , LiF:Al 2 O 3 , and LiF:TiO 2 …”