Extremely slow Li ion dynamics in monoclinic Li2TiO3—probing macroscopic jump diffusion via7Li NMR stimulated echoes

Abstract

A thorough understanding of ion dynamics in solids, which is a vital topic in modern materials and energy research, requires the investigation of diffusion properties on a preferably large dynamic range by complementary techniques. Here, a polycrystalline sample of Li₂TiO₃ was used as a model substance to study Li motion by both ⁷Li spin-alignment echo (SAE) nuclear magnetic resonance (NMR) and ac-conductivity measurements. Although the two methods do probe Li dynamics in quite different ways, good agreement was found so that the Li diffusion parameters, such as jump rates and the activation energy, could be precisely determined over a dynamic range of approximately eleven decades. For example, Li solid-state diffusion coefficients D_σ deduced from impedance spectroscopy range from 10⁻²³ m² s⁻¹ to 10⁻¹² m² s⁻¹ (240–835 K). These values are in perfect agreement with the coefficients D_SAE deduced from SAE NMR spectroscopy. As an example, D_SAE = 2 × 10⁻¹⁷ m² s⁻¹ at 433 K and the corresponding activation energy determined by NMR amounts to 0.77(2) eV (400–600 K). At room temperature D_σ takes a value of 3 × 10⁻²¹ m² s⁻¹.