Diffusion Pathways and Activation Energies in Crystalline Lithium-Ion Conductors

Abstract

Geometric information about ion migration (diffusion pathways) and knowledge about the associated energy landscape (migration activation barriers) are essential cornerstones for a comprehensive understanding of lithium transport in solids. Although many lithium-ion conductors are discussed, developed, and already used as energy-storage materials, fundamental knowledge is often still lacking. In this microreview, we give an introduction to the experimental and computational methods used in our subproject within the research unit FOR 1277, "Mobility of Lithium Ions in Solids (molife)". These comprise, amongst others, neutron diffraction, topological analyses (procrystal-void analysis and VoronoiDirichlet partitioning), examination of scattering-length density maps reconstructed via maximum-entropy methods (MEM), analysis of probability-density functions (PDFs) and one-particle potentials (OPPs), as well as climbing-image nudged-elastic-band (cNEB) computations at density-functional theory (DFT) level. The results of our studies using these approaches on ternary lithium oxides and sulfides with different conduction characteristics (fast/slow) and dimensionalities (one-/two-/three-dimensional) are summarized, focusing on the close orbit of the research unit. Not only did the investigations elucidate the lithiumdiffusion pathways and migration activation energies in the studied compounds, but we also established a versatile set of methods for the evaluation of data of differing quality.

And Yet It Moves: A High‐Temperature Neutron Diffraction Study of Ion Diffusion in the Inverse Perovskites BaLiX3 (X = F, H, D)

“…Alternatively, it can be mapped directly using reconstruction via maximum‐entropy methods (MEM) to minimize artefacts. (For a detailed discussion, see ref one can use both methods to derive the effective one‐particle potential (OPP) describing the energy landscape experienced by the ion in motion .…”

The inverse perovskite BaLiF3: single-crystal neutron diffraction and analyses of potential ion pathways

“…the neutron analogue of the electron density probed with X-ray diffraction) that we reconstructed using MEM. These methods yield the maximum variance of calculated structure factors within standard deviations of observed structure factors, in this way producing density maps, which contain less artefacts and are less prone to misinterpretation (Wiedemann et al, 2016;Wiedemann, Islam et al, 2017). These maps (see Fig.…”

Structural complexities and sodium-ion diffusion in the intercalates NaxTiS2: move it, change it, re-diffract it

“…A more detailed overview has already been published. 47 The pathways refer to thermally activated ion diffusion at temperatures high enough to warrant ample ion mobility.…”