Theoretical evaluation of Na2MgCl4 double chlorite as an electrolyte for all-solid-state sodium-ion batteries†

Abstract

The quest for effective and exceptional solid-state electrolytes for metal ion batteries has to address two primary challenges: overcoming the interfacial resistance between the solid-state electrolyte and the electrodes and improving their relatively low dc conductivity at operating temperatures. This study presents advanced atomistic simulations of the primary properties of the Na₂MgCl₄ double chlorite compound. Calculated results revealed that Na₂MgCl₄ possessed key properties suitable for a solid-state electrolyte in energy storage applications. Influenced by the hybridizations of [MgCl₆] and [NaCl₆], Na₂MgCl₄ exhibited insulator properties with an energy band gap of 4.7 eV. Mechanical properties suggested that Na₂MgCl₄ was a stable and ductile compound with favorable bulk, shear, and Young's moduli, thereby ensuring compatibility and stability with potential electrodes. Defect energetics highlighted the NaCl Schottky defects as the most abundant, with Zn²⁺ and Ga³⁺ as effective dopants that enhanced Na-vacancy concentration, impacting large-scale transport properties. From the evaluation using the bond valence site energy method, Na₂MgCl₄ possesses excellent Na activation energies for diffusion (0.20 eV) and conduction (0.17 eV) along with high diffusivity of 0.17 mS cm⁻¹ and conductivity of 1.65 × 10⁻⁹ cm² s⁻¹ at 300 K. These attributes were competitive with those of the current solid-state electrolytes, underscoring the potential of Na₂MgCl₄ for high-performance battery applications. Overall, Na₂MgCl₄ meets the essential criteria to be used as a solid-state electrolyte in Na-ion batteries.