Origin of Outstanding Phase and Moisture Stability in a Na₃P_1-xAs_xS₄ Superionic Conductor

Sodium ion (Na) solid-state electrolytes (SSEs) are critical to address notorious safety issues associated with liquid electrolytes used in the current Na ion batteries. Fulfilling multiple innovations is a grand challenge but is imperative for advanced Na ion SSEs, such as a combination of high ionic conductivity and excellent chemical stability. Here, our first-principles and phonon calculations reveal that Na₃P_1-xAs_xS₄ (0 ≤ x ≤ 1) is a solid-state superionic conductor stabilized at 0 K by zero-point vibrational energy and at finite temperatures by vibrational and configurational entropies. Especially, our integrated first-principles and experimental approach indicates that Na₃P_1-xAs_xS₄ is dry-air stable. Additionally, the alloying element arsenic greatly enhances the moisture (i.e., H₂O) stability of Na₃P_1-xAs_xS₄ by shifting the reaction products from the easy-forming oxysulfides (such as Na₃POS₃ and Na₃PO₂S₂ with H₂S release) to the difficult-forming hydrates (such as Na₃P_1-xAs_xS₄·nH₂O with n = 8 and/or 9) due mainly to a weaker As-O affinity compared to that of P-O. The present work demonstrates that alloying is able to achieve multiple innovations for solid-state electrolytes, such as a desirable superionic conductor with not only a high ionic conductivity (for example, 1.46 mS/cm at room temperature achieved in Na₃P_0.62As_0.38S₄) but also an excellent chemical stability with respect to temperature, composition, and moisture.