Rapid Low-Dimensional Li⁺ Ion Hopping Processes in Synthetic Hectorite-Type Li_0.5[Mg_2.5Li_0.5]Si₄O₁₀F₂

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 Li_0.5[Mg_2.5Li_0.5]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.