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

Abstract

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 C_p 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 C_p value for Li₆PS₅Cl in the literature, despite the importance of this thermodynamic quantity. Finally we use this more reliable C_p 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.