Structural and thermodynamic properties of the Li6PS5Cl solid electrolyte using first-principles calculations

We perform static and dynamic ab initio simulations to investigate the structural and the thermodynamic properties of Li6PS5Cl, a solid electrolyte actively considered for solid-state batteries. Our simulations account for the disorder in the structure where the Li atoms can rotate either around sulfur or chlorine atoms. Li6PS5Cl presents a non-uniform distribution of Li ions around S and Cl atoms, which tends to become more homogeneous at higher temperature. This specific short-range order of Li has a significant impact on the stability of Li6PS5Cl. Comparing with recent 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 heat capacity Cp with a combination of ab initio trajectories and a so-called temperature remapping approximation. Indeed, the standard quasi-harmonic approximation is not able to capture the complex energy landscape experienced by the mobile lithium atoms. To the best of our knowledge, there exists no experimental or theoretical Cp value for Li6PS5Cl in the literature, despite the importance of this thermodynamic quantity. Finally we use this more reliable Cp to investigate the thermodynamic stability of Li6PS5Cl against the decomposition reaction leading to Li2S, Li3PS4 and LiCl. We show that Li6PS5Cl is stable above 700 K, which is consistent with the high synthesis temperatures.