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 being same conduction mechanism of correlated barrier hopping (CBH) and 2D conduction pathways, a wide variation in the conductivity value has been found from NNTO to LCTO. Strikingly, NNTO exhibits the highest ionic conductivity (∼0.03 Sm−1 at 423 K) among the studied compounds, accompanied by the fastest relaxation dynamics (τm = 2.57×10−15 sec at 453 K) and lowest activation energy (Ea = 0.280(6) eV) in comparison to LCTO having a conductivity of ∼10-6 Sm−1 and τm = 2.11×10−7 sec at 553 K with Ea = 0.576(6) eV. Our neutron scattering study highlights the critical role of local crystal structural environment around the alkali Li/Na-ions in enhancing the ionic conduction. Specifically, the wider bottleneck radii governed by the triagonal 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, include the 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 the design and synthesis of high-efficient battery materials.