Journal
JOURNAL OF PHYSICAL CHEMISTRY C
Volume 125, Issue 27, Pages 15052-15060Publisher
AMER CHEMICAL SOC
DOI: 10.1021/acs.jpcc.1c03789
Keywords
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Funding
- German Science Foundation (Deutsche Forschungsgemeinschaft (DFG)) [FOR1277]
- Dutch Research Council (Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NOW)), ECHO grant [712.015.005]
- NWO materials for sustainability grant [Mat4Sus-739.017.009]
- Austrian Federal Ministry of Science, Research and Economy (BMWFW)
- National Foundation for Research, Technology and Development (CD-Laboratory of Lithium Batteries: Ageing Effects, Technology and New Materials)
- European H2020 Framework program [769929]
- FFG comet project safe battery
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Synthesizing Li-ion-conducting solid electrolytes with application-relevant properties for new energy storage devices is challenging and often requires a combination of strategies to achieve high ionic conductivity. Introducing conductor-insulator interfacial regions in nanostructured materials is an important design strategy, facilitating rapid Li+ jump processes.
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 Li-7 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 Li+ NMR longitudinal magnetization. The fact that almost no difference is seen between LiBH4-LiI/Al2O3 and LiBH4/Al2O3 unequivocally reveals that the overall Li-7 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). Li-7 (and H-1) 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.
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