Fuzzy logic: about the origins of fast ion dynamics in crystalline solids

Abstract

Nuclear magnetic resonance offers a wide range of tools to analyse ionic jump processes in crystalline and amorphous solids. Both high-resolution and time-domain 1 , 2 H , 6 , 7 Li , 19 F , 23 Na NMR helps throw light on the origins of rapid self-diffusion in materials being relevant for energy storage. It is well accepted that Li + ions are subjected to extremely slow exchange processes in compounds with strong site preferences. The loss of this site preference may lead to rapid cation diffusion, as is also well known for glassy materials. Further examples that benefit from this effect include, e.g. cation-mixed, high-entropy fluorides ( Ba, Ca) F 2 , Li-bearing garnets ( Li 7 La 3 Zr 2 O 12 ) and thiophosphates such as LiTi 2 ( PS 4 ) 3 . In non-equilibrium phases site disorder, polyhedra distortions, strain and the various types of defects will affect both the activation energy and the corresponding attempt frequencies. Whereas in ( Me, Ca ) F 2 ( Me = Ba , Pb ) cation mixing influences F anion dynamics, in Li 6 PS 5 X ( X = Br , Cl , I ) the potential landscape can be manipulated by anion site disorder. On the other hand, in the mixed conductor Li 4 + x Ti 5 O 12 cation-cation repulsions immediately lead to a boost in Li + diffusivity at the early stages of chemical lithiation. Finally, rapid diffusion is also expected for materials that are able to guide the ions along (macroscopic) pathways with confined (or low-dimensional) dimensions, as is the case in layer-structured RbSn 2 F 5 or MeSnF 4 . Diffusion on fractal systems complements this type of diffusion. This article is part of the Theo Murphy meeting issue ‘Understanding fast-ion conduction in solid electrolytes’.

Opening Diffusion Pathways through Site Disorder: The Interplay of Local Structure and Ion Dynamics in the Solid Electrolyte Li6+xP1–xGexS5I as Probed by Neutron Diffraction and NMR

“…We can think of several adjusting screws ,− to influence Li + cation translational dynamics on both the short- and long-range length scales. − Concepts include, for example, the discussion of structural parameters and polyhedra connections, polarization effects, and the change of Li–S bond strengths through both alio- and isovalent substitution. Anion site disorder might also influence the Li + sublattice as originally empty (interstitial) Li + sites become occupied in anion-mixed compounds.…”

Understanding fast ion dynamics in sodiated Li4Na x Ti5O12: from interfacial to extended Li+ and Na+ dynamics in its mixed-conducting solid solutions

“…Again, on the new position it senses strong Li 8a -Li 16c interactions. Altogether, these local repulsions result in an overall frustrated situation [35] that gives rise to both strongly enhanced local and long-range Li + ion dynamics, see below [22,50]. The same 8a-16c diffusion pathway would be opened if Na + kicks off the Li + ions on 16d.…”

“…We used 7 Li and 23 Na nuclear magnetic resonance (NMR) spectroscopy [33,34] to quantify the elementary steps of ion hopping in Li 4+x Ti 5 O 12 samples differing in Na composition (x = 0.1, 0.3 and 1.5). NMR is a uniquely positioned method to selectively study ion dynamics of the distinctly mobile ion charge carriers in LTO [35][36][37][38][39][40]. It helped us to identify any interrelations of cation dynamics in these mixed-conducting samples that also reveal clear electronic conduction properties.…”

Understanding fast-ion conduction in solid electrolytes

“…The first contributed article in this theme issue is ‘Fuzzy Logic: About the Origins of Fast Ion Dynamics in Crystalline Solids’ by Gombotz et al [ 46 ]. This Perspective article presents a series of case studies where nuclear magnetic resonance (NMR) techniques have been used to probe the dynamics of mobile ions in a variety of fast-ion conducting solid electrolytes, and uses these to discuss a number of concepts and schema that can be used to rationalise the fast-ion conduction of specific materials.…”