Abstract
Solid electrolytes are key elements for next-generation energy storage systems. To design powerful electrolytes with high ionic conductivity, we need to improve our understanding of the mechanisms that are at the heart of the rapid ion exchange processes in solids. Such an understanding also requires evaluation and testing of methods not routinely used to characterize ion conductors. Here, the ternary Li4MCh4 system (M = Ge, Sn; Ch = Se, S) provides model compounds to study the applicability of 7Li nuclear magnetic resonance (NMR) spin-alignment echo (SAE) spectroscopy to probe slow Li+ exchange processes. Whereas the exact interpretation of conventional spin–lattice relaxation data depends on models, SAE NMR offers a model-independent, direct access to motional correlation rates. Indeed, the jump rates and activation energies deduced from time-domain relaxometry data perfectly agree with results from 7Li SAE NMR. In particular, long-range Li+ diffusion in polycrystalline Li4SnS4 as seen by NMR in a dynamic range covering 6 orders of magnitude is determined by an activation energy of Ea = 0.55 eV and a pre-exponential factor of 3 × 1013 s–1. The variation in Ea and 1/τ0 is related to the LiCh4 volume that changes within the four Li4MCh4 compounds studied. The corresponding volume of Li4SnS4 seems to be close to optimum for Li+ diffusivity.
Highlights
The diffusive motion of small ions such as H+, Li+, and Na+ as well as F− and O2− plays pivotal roles in many branches of materials science.[1]
Li4GeS4 was first synthesized by Matsushita et al.;[82] in the past few years numerous studies investigated the effect of elemental substitutions on ion dynamics.[36,83−88] In a very recent study[35] we investigated the differences in structure, macroscopic, and microscopic transport in the two-dimensional substitution series Li4MCh4 (M = Sn, Ge; Ch = S, Se)
The investigation of static nuclear magnetic resonance (NMR) lines recorded at different temperatures entails information about ion dynamics that can average dipolar Li−Li couplings.[92]
Summary
The diffusive motion of small ions such as H+, Li+, and Na+ as well as F− and O2− plays pivotal roles in many branches of materials science.[1]. Besides other methods,[1] nuclear magnetic resonance (NMR) offers a large set of techniques[1,7−21] that can probe Li+ hopping processes on different length scales and time scales in a variety of materials including amorphous (glassy) and crystalline materials.[14] While some techniques, such as (pulsed or static) field gradient NMR,[13,22−28] can probe macroscopic, that is, tracer diffusion coefficients, others, such as high-resolution 1D or 2D exchange NMR, are sensitive to site-specific Li+ hopping processes.[9−11,29−33] Time-domain NMR methods,[1,12,15] including especially spin−lattice relaxation techniques,[1,15,34] probe short- and long-range ion dynamics,[7] depending on the temperature range used to sample the diffusion-induced relaxation rates. Stimulated echo NMR is, applicable to amorphous solids.[12,37,45−47] If it is combined with methods being sensitive to motional (or rotational) processes taking place at shorter length scales, in ideal cases the dynamic range accessible by NMR methods can cover up to 10 decades.[43]
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