Rapid Low-Dimensional Li+ Ion Hopping Processes in Synthetic Hectorite-Type Li0.5[Mg2.5Li0.5]Si4O10F2.

Caroline Hiebl,Patrick Loch,Josef Breu,Maria Gombotz,Paul Heitjans,Marina Brinek,Bernhard Gadermaier,H Martin R Wilkening

doi:10.1021/acs.chemmater.0c02460

Abstract

Understanding the origins of fast ion transport in solids is important to develop new ionic conductors for batteries and sensors. Nature offers a rich assortment of rather inspiring structures to elucidate these origins. In particular, layer-structured materials are prone to show facile Li+ transport along their inner surfaces. Here, synthetic hectorite-type Li0.5[Mg2.5Li0.5]Si4O10F2, being a phyllosilicate, served as a model substance to investigate Li+ translational ion dynamics by both broadband conductivity spectroscopy and diffusion-induced 7Li nuclear magnetic resonance (NMR) spin–lattice relaxation experiments. It turned out that conductivity spectroscopy, electric modulus data, and NMR are indeed able to detect a rapid 2D Li+ exchange process governed by an activation energy as low as 0.35 eV. At room temperature, the bulk conductivity turned out to be in the order of 0.1 mS cm–1. Thus, the silicate represents a promising starting point for further improvements by crystal chemical engineering. To the best of our knowledge, such a high Li+ ionic conductivity has not been observed for any silicate yet.

Highlights

Anisotropic properties of ionic transport in polycrystalline samples can be probed by nuclear magnetic resonance (NMR) spin−lattice relaxation experiments.[5,30,32−35] The most prominent examples, whose Li+ diffusion properties were studied in this way,[36] include graphite[37,38] or transition metal chalcogenides such as TiS2,39−43 NbS2,44,45 and SnS2.46 Recently, two-dimensional (2D) Li+ diffusion has been determined in hexagonal LiBH4.29,47 Fast fluoride, F−, diffusion is observed in MeSnF4 (Me = Pb, Ba) and, as has been shown quite recently, in layer-structured RbSn2F5.48 In these materials, spatial constraints guide the ions over long distances
Li+ ion diffusion and electrical transport in the hectorite-type phyllosilicate Li0.5[Mg2.5Li0.5]Si4O10F2 was studied by broadband conductivity spectroscopy, modulus analysis, and 7Li NMR spin−lattice relaxation measurements
Conductivity spectroscopy points to an ionic conductivity as high as 0.14 mS cm−1, representing a favorable starting point for further improvements by crystal-chemical engineering

Summary

INTRODUCTION

The diffusion of small cations and anions plays an important role in many devices such as sensors[1−3] and batteries.[4−11] for some applications, e.g., in the semiconductor industry[12] or in the development of breeding materials[13−15] for fusion reactor blankets, diffusion is unwanted, in other branches, materials with extremely high diffusion coefficients are desired.[16−21] Finding the right material and tailoring its dynamic properties further requires an in-depth understanding of the origins that determine fast ion dynamics.[22−26]. To understand Li+ diffusion in structures offering 2D diffusion pathways, we chose hectorite-type Li0.5[Mg2.5Li0.5]Si4O10F2 as a model system, see Figure 1, and studied the Li+ self-diffusion properties and electrical ionic transport. While the latter is investigated by broadband conductivity spectroscopy,[52] we took advantage of 7Li NMR spin−lattice relaxation measurements[32,53] to shed light on Li+ translational dynamics. The present study contributes to this research field and is aimed at answering the question whether 2D silicate structures offer an assortment of materials with enhanced ion diffusion properties

EXPERIMENTAL SECTION

RESULTS AND DISCUSSION

CONCLUSIONS

■ ACKNOWLEDGMENTS

■ REFERENCES