Solid Electrolytes Based on Fast Fluorine Ion Motions — 19F T1ρ NMR Relaxation vs Conductivity Measurements of Mechanosynthesized Nanocrystalline Ba0.6La0.4F2.4

Wednesday, 8 October 2014
Expo Center, 1st Floor, Center and Right Foyers (Moon Palace Resort)
F. Preishuber-Pflügl (Institute for Chemistry and Technology of Materials, DFG Priority Program (SPP) 1415, Graz University of Technology) and M. Wilkening (Christian-Doppler Laboratory for Lithium Batteries, Institute for Chemistry and Technology of Materials, DFG Priority Program (SPP) 1415, Graz University of Technology)
The use of mechanosynthesis for the preparation of ion conductors provides access to new compounds whose properties are governed by chemical metastability, defects introduced and size effects present. The benefits of this valuable synthesis method manifest in enhanced ion transport properties of the prepared nanocrystalline ceramics. Such materials are useful candidates to act as solid electrolytes, e.g., in fluorine ion batteries [1]. Here, the introduction of large amounts of La3+ into the cubic structure of BaF2 was carried out by using high-energy ball-milling. The replacement of Ba2+ with a trivalent cation leads to the formation of an increased number fraction of point defects in the metastable crystal structure, which resulted in an increase of the dc-conductivity when compared to pure, nano-sized BaF2 [2]. Until now, only little is known about activation energies and jump rates of the elementary hopping processes of such materials. In the present study, we used broadband impedance spectroscopy and 19F NMR relaxometry to get to the bottom of ion jump diffusion proceeding on short-range and long-range length scales in Ba0.6La0.4F2.4. The dielectric measurements revealed an activation energy of 0.57 eV for macroscopic ion transport, whilst NMR, which is sensitive to both long-range motion and localized jumps, gave much smaller activation energies [3]. This was confirmed by high-frequency impedance measurements carried out at frequencies as high as 3 GHz that enabled us to compare the results from complementary methods applied in the same time window.

[1] C. Rongeat, M. A. Reddy, R. Witter, and M. Fichtner, J. Phys. Chem. C, 117, 4943 (2013).

[2] A. Düvel, J. Bednarcik, V. Šepelák, and P. Heitjans, J. Phys. Chem. C, in press (2014), DOI: 10.1021/jp410018t.

[3] F. Preishuber-Pflügl, and M. Wilkening, Phys. Chem. Chem. Phys., in press (2014), DOI: 10.1039/C4CP00422A.