Crystal structure driven electrical properties and mobile ion-dynamics of the layered compounds: A2M2TeO6 (A = Li/Na and M = Cu/Ni) family

Abstract

We present a comprehensive investigation of the ionic conduction mechanisms and their correlation with the crystal structure by employing a combined approach of impedance spectroscopy and neutron scattering. Our study includes a series of iso-formula 2D layered compounds—Li₂Cu₂TeO₆ (LCTO), Na₂Cu₂TeO₆ (NCTO), Li₂Ni₂TeO₆ (LNTO) and Na₂Ni₂TeO₆ (NNTO). Our study reveals that, despite having the same conduction mechanism of correlated barrier hopping (CBH) and 2D conduction pathways, a wide variation in the conductivity value is found from NNTO to LCTO. Strikingly, NNTO exhibits the highest ionic conductivity (∼2 × 10⁻² S m⁻¹ at 413 K) among the studied compounds, accompanied by the fastest relaxation dynamics (τ_m = 2.57 × 10⁻¹⁵ s at 453 K) and the lowest activation energy (E_a = 0.280(6) eV) in comparison to LCTO having a conductivity of ∼10⁻⁶ S m⁻¹ and τ_m = 2.11 × 10⁻⁷ s at 553 K with E_a = 0.576(6) eV. Our neutron scattering study highlights the critical role of the local crystal structural environment around the alkali Li/Na ions in enhancing the ionic conduction. Specifically, the wider bottleneck radii governed by the trigonal prismatic local crystal structural environment of alkali-ions and the partial site occupancies of Na⁺ ions contribute to high ionic conduction in NNTO. The present comprehensive study, based on varying crystal structures, includes analyses of dc conductivity, ac conductivity, electric modulus, dielectric constant, diffusivity, and hopping time of conducting alkali-metal ions, providing an in-depth understanding of the microscopic ionic conduction mechanism. These insights are expected to be significant for the progress of future battery technology, especially for the design and synthesis of highly efficient battery materials.