Structural and thermodynamic properties of the Li$_6$PS$_5$Cl solid electrolyte using first-principles calculations

International audience ; We perform static and dynamic ab initio simulations to investigate the structural and the ther-modynamic properties of Li6 PS5 Cl, a solid electrolyte actively considered for solid-state batteries.Our simulations account for the disorder in the structure where the Li atoms can rotate eitheraround sulfur or chlorine atoms. Li6 PS5 Cl presents a non-uniform distribution of Li ions aroundS and Cl atoms, which tends to become more homogeneous at higher temperature. This specificshort-range order of Li has a significant impact on the stability of Li6 PS5 Cl. Comparing with re-cent X-Ray and neutron diffraction studies, we confirm one Li crystallographic site position (Li1)and amend the coordinates of a second one (Li2). We then address the calculation of the heatcapacity Cp with a combination of ab initio trajectories and a so-called temperature remappingapproximation. Indeed, the standard quasi-harmonic approximation is not able to capture thecomplex energy landscape experienced by the mobile lithium atoms. To the best of our knowledge,there exists no experimental or theoretical Cp value for Li$_6$ PS$_5$ Cl in the literature, despite theimportance of this thermodynamic quantity. Finally we use this more reliable Cp to investigate thethermodynamic stability of Li6 PS5 Cl against the decomposition reaction leading to Li$_2$ S, Li$_3$ PS$_4$and LiCl. We show that Li$_6$ PS$_5$ Cl is stable above 700 K, which is consistent with the high synthesistemperatures.