Understanding the Structural Properties and the Stability of the Solid Electrolyte Material Li₄PS₄I with First-Principles Simulations

Abstract

In this study, first-principles simulations were utilized to investigate the structural characteristics and address some of the stability issues of the solid electrolyte material Li$_4$PS$_4$I. The computational results indicate that, unlike most ion-conducting systems, Li$_4$PS$_4$I does not exhibit a fully ordered phase that is stable at ambient conditions. Instead, the material prefers a disordered configuration at room temperature, and the configurational entropy arising from the disordered nature of the material plays a critical role in stabilizing the material. Additionally, a plausible metastable fully ordered phase of the material was identified, which was used to examine the structural properties and interface stability with Li anode of the material. Our analysis of the interface stability revealed that Li$_4$PS$_4$I is likely to react with Li anode, but the reaction is limited to the surface layers of the material. Furthermore, the comparison of the idealized interfaces with Li anode of Li$_4$PS$_4$I with the analogue $\gamma-\text{Li}_3\text{PS}_4$ suggests that less P-S bonds are broken for Li$_4$PS$_4$I, which is attributed to the presence of Iodine on the surface. This finding supports the argument that the incorporation of Lithium iodine into sulfide-based electrolytes enhances their stability when in contact with a Li anode.