Toward More Sustainable Solid-State Electrolytes: The Impact of Phosphorus Substitution on the Conductivity of Na₄SiS₄

Sulfide-based solid electrolytes for all-solid-state batteries (ASSBs) are known for their exceptional ionic conductivities and ease of manufacturing. In this study, the effects of aliovalent substitution of P for Si in the Na4-xSi1–xPxS4 series are investigated through a combination of structural characterization techniques (single crystal and powder X-ray diffraction, Raman spectroscopy), bond valence site energy (BVSE) analysis, and electrochemical performance assessments (electrochemical impedance spectroscopy and galvanostatic charge and discharge testing). A key contribution of this research is the determination of the optimal substitution level (x = 0.16) in the system that markedly improves conductivity of the doped samples up to 25 times compared to the parent Na4SiS4 phase. In an all-solid-state battery setup with TiS2 as the cathode and a Na–Sn alloy as the anode, this electrolyte maintains a steady performance, delivering a capacity of 100 mAh g–1. Modeling through the BVSE method highlights significant details of the mechanistic features of Na ion diffusion through Na vacancies in this system and suggests the potential of this quaternary solid electrolyte system for the design and optimization of more efficient, safer, and cost-effective ASSBs.