Unveiling Solid Electrolyte Interphase Formation at the Molecular Level: Computational Insights into Bare Li-Metal Anode and Li₆PS_5–xSe_xCl Argyrodite Solid Electrolyte

Abstract

The development of high-energy-dense, sustainable all-solid-state batteries faces a major challenge in achieving compatibility between the anode and electrolyte. A promising solution lies in the use of highly ion-conductive solid electrolytes, such as those from the argyrodite family. Previous studies have shown that the ionic conductivity of the argyrodite Li6PS5Cl can be significantly enhanced by partially substituting S with Se. However, there remains a lack of fundamental knowledge regarding the effect of doping on the interfacial stability. In this study, we employ long-scale ab initio molecular dynamics simulations, which allowed us to gain unprecedented insights into the process of solid electrolyte interface (SEI) formation. The study focuses on the stage of nucleation of crystalline products, enabling us to investigate in silico the SEI formation process of Se-substituted Li6PS5Cl. Our results demonstrate that kinetic factors play a crucial role in this process. Importantly, we discovered that selective anionic substitution can accelerate the formation of a stable interface, thus potentially resolving anode–electrolyte compatibility issues.

Two-Step Growth Mechanism of the Solid Electrolyte Interphase in Argyrodyte/Li-Metal Contacts

“…As previously shown using AIMD, our results also find that the crystalline products are not a phase-separated mixture of LiCl, Li 2 S, and Li 3 P but a 5Li 2 S·Li 3 P·LiCl solid solution, which corresponds to the antifluorite structure type . This is not incompatible with experimental evidence: XPS measurements have been employed to identify the three products; however, this experimental method, based on characterizing the electronic binding energies of individual chemical species, faces difficulties in distinguishing between phase-separated domains or a singular solid solution, where the S sublattice comprises individual substitutions by P and Cl.…”

“…Direct visualization of the MD trajectories shows the SEI to be a mixture of amorphous and crystalline regions, the latter increasingly dominating over the former as time goes by (Figures and ). To identify and quantify the extent of the crystalline regions, we employ a methodology developed in ref (see below). In turn, we define the amorphous region as the part of the reduced electrolyte that is not crystalline.…”

Operando Photoelectron Spectroscopy Analysis of Li6PS5Cl Electrochemical Decomposition Reactions in Solid-State Batteries

“…The overall intensity trends suggest that the SEI has a heterogeneous/layered microstructure with Li 2 O preferentially located near the current collector, followed by Li 2 S. Additionally, with decreasing potential, regions near the current collector become increasingly deficient in chlorine and even more so in phosphorus. Theoretical work is already run to model the underlying reaction mechanisms, ,− and we are confident that our approach will be a highly valuable source of experimental data.…”