Divalent ion exchange in Na2MgCl4 double chloride enabling new effective solid-state electrolytes for sodium ion batteries

Abstract

The search for new solid-state electrolytes for sodium-ion batteries is essential to enhance energy storage performance and stability. This study explores the potential of Na₂MCl₄ (M²⁺ = Mg²⁺, Zn²⁺, Ca²⁺ and Sr²⁺) as battery materials using advanced atomistic simulations. Structural analysis reveals that Na₂SrCl₄ has the largest unit cell due to its larger cationic radius, followed by Na₂CaCl₄, both introducing subtle octahedral distortions. Energy gap trends highlight the impact of cation selection on electronic properties, while polyhedral volume variations influence lattice stability. Defect analysis confirms the NaCl Schottky defect as the most favourable, with decreasing solution energy as the cation size increases, reinforcing ambient stability. Na₂ZnCl₄ emerges as the most stable compound based on defect formation energies. Mechanical properties show a decrease of both bulk and shear modulus with respect to an increase of cation size, where most compounds meet the shear modulus threshold for dendrite suppression, except for Na₂SrCl₄. Pugh's ratio confirms their ductile nature, supporting their viability as solid-state electrolytes. Transport property evaluations reveal that smaller cations such as Mg²⁺ and Zn²⁺ enhance Na⁺ mobility by reducing activation barriers and improving ionic conductivity. Specifically, Na₂MgCl₄ exhibits the lowest activation energy for conductivity at 0.17 eV, while Na₂ZnCl₄ shows the highest conductivity at 0.215 mScm⁻¹, reinforcing their efficiency for Na⁺ transport. Optimizations of lattice interactions can further enhance the sodium ion diffusion, making these materials strong candidates for solid-state battery applications.