A Family of Sodium Solid‐State Electrolytes Based on the NaGa _x Al _1‐x Cl ₄ Solid Solution

ABSTRACT Sodium‐based metal chloride solid electrolytes are promising for sodium solid‐state batteries due to their excellent oxidation stability, which, as shown for Li halides, can coexist with high ionic conductivity. To explore cationic substitution effects, we synthesized NaGa _x Al1 _‐x Cl ₄ (0 ≤ x ≤ 1) via ball milling and investigated structural and transport properties. X‐ray diffraction and Rietveld refinement confirmed complete solubility between NaAlCl ₄ and NaGaCl ₄ within the orthorhombic P 2 ₁ 2 ₁ 2 ₁ space group. Electrochemical impedance spectroscopy revealed a minimal conductivity decrease with increasing Ga ³⁺ content at first (≈ 1‐3 x 10 ⁻⁶ S cm ⁻¹ for 0 ≤ x ≤ 0.5), but down to 5 × 10 ⁻⁷ S cm ⁻¹ for NaGaCl ₄ . Structural analysis linked this trend to bottleneck closure in Na ⁺ pathways, increasing activation energy. Despite the reduced conductivity, Ga substitution significantly broadened the electrochemical stability window to 2.12 ‐ 4.86 V vs. Na ⁺ /Na, surpassing most sodium halide electrolytes. Ex situ XAS and XPS confirmed the excellent reduction stability. This work improves our understanding of solid electrolytes and of the structure‐property relationship in halide materials. Thereby, it expands the accessible range of cation choices for sodium metal chloride materials with new candidates of interest in light of their excellent electrochemical stability.