New prospects in studying Li diffusion—two-time stimulated echo NMR of spin-3/2 nuclei

Abstract

The measurement of diffusion parameters like activation energies and translational jump rates of small cations plays a key role in materials science. Especially the in-depth investigation of Li diffusion in ionic conductors is of great interest, because suitable ionic conductors are needed for, e. g., the development of new secondary ion battery systems. As the standard tracer method is not applicable to study Li diffusion due to the lack of a suitable radioactive isotope, Li diffusion is alternatively probed by solid state NMR techniques. With the different NMR methods being available, diffusion processes can be studied on different length-and time-scales. In the present paper we use two-time spin-alignment echo (SAE) NMR for the direct, i. e., model independent, measurement of extremely small translational Li jump rates. To this end, different crystalline and glassy ion conductors like Li x TiS 2 , Li 4 SiO 4 as well as LiNbO 3 served as model substances to reveal the special features of this technique. SAE-NMR, which was originally developed for deuterons, has also been applied in a few cases to spin-3/2 nuclei, like 7 Li, before. The corresponding correlation functions yield not only information about diffusion parameters but also about geometric properties of the diffusion pathways, making SAE NMR a powerful method which complements well-established NMR techniques.

Atomic-scale measurement of ultraslow Li motions in glassyLiAlSi2O6by two-timeL6Wilkening1, Kuhn2, Heitjans3 2008Phys. Rev. B594500View full textAdd to dashboardCite

“…Ϸ 375 K. At this temperature the Li jump rate is approximately given by MN −1 =2 ϫ rl where rl is the rigidlattice line width at T → 0 K. Inserting rl Ϸ 3.1 kHz yields a hopping rate of about 1.95ϫ 10 4 s −1 at 375 K. Thus, below this temperature Li correlation rates are equal or smaller than about 10 4 s −1 so that they fall in the time window which is accessible by stimulated echo NMR being sensitive to extremely small jump rates. [21][22][23] Examples of 6 Li spinalignment echoes are displayed in Fig. 1͑c͒.…”

“…Advantageous over already well-established Li NMR methods [15][16][17][18][19] is the use of 7 Li stimulated echo, or more precisely, spin-alignment echo ͑SAE͒ NMR which has so far been applied mainly to crystalline Li conductors. 17,[20][21][22][23][24][25][26][27][28][29][30][31][32] Similar to modern twodimensional 6 Li and 7 Li exchange NMR spectroscopy, 33-38 7 Li SAE NMR is sensitive to extremely slow cation exchange processes and gives access to diffusion parameters from a microscopic point of view. Thus, it is possible to record single-particle motional correlation functions, 39 from which the atomic jump rate can be directly obtained, i.e., without the help of a diffusion model.…”

“…Thus, it is possible to record single-particle motional correlation functions, 39 from which the atomic jump rate can be directly obtained, i.e., without the help of a diffusion model. [21][22][23][24][25][26][27] In some cases the use of 6 Li ͑spin quantum number I =1͒ is advantageous over 7 Li ͑I =3/ 2͒ SAE NMR. In particular, this is true when ͑i͒ strong 7 Li homonuclear dipole-dipole interactions 30 largely influence the generation of pure spinalignment order necessary to record a single-particle twotime correlation function or ͑ii͒ if three-or four-time phase correlation functions have to be measured ͑see, e.g., Refs.…”

Slow Lithium Transport in Metal Oxides on the Nanoscale

“…In combination, diffusivities between 10 −15 and 10 −25 m 2 /s at diffusion lengths of 0.5 nm to several mm can be determined [1]. A method which can reveal diffusion processes on the microscopic length scale and which can also very well be used to study slow diffusion processes is spin-alignment echo-NMR (SAE-NMR) [2][3][4]. Here, diffusivities down to 10 −20 m 2 /s at diffusion lengths of 1 nm can be detected [5][6][7].…”

“…Investigations on self-diffusion in Li containing solids were done in recent years nearly exclusively by NMR methods including beta-NMR and muon-spin spectroscopies [2,[9][10][11][12][13][14][15] which allow a characterization of fast as well as slow, generally short-range diffusion processes. In addition, conductivity measurements were done for an indirect characterization of diffusion [13,16].…”