Electrochemical and Mechanical Evolution of Sulfide-Based Solid Electrolytes: Insights from Operando XPS and Cell Pressure Measurements

Abstract

Understanding the electrochemical and mechanical behavior of solid electrolytes beyond their electrochemical stability window is crucial for enabling high energy density all-solid-state batteries. Accordingly, this work systematically studies a model working electrode of Li₃PS₄, ball milled with vapor grown carbon fiber (VGCF). Operando X-ray photoelectron spectroscopy can identify and quantify the potential-dependent redox byproducts, their reversibility, and electrical properties, while operando cell pressure measurements correlate these with volume changes and mechanical instability. The study examines voltages up to 5.0 V and down to −0.05 V versus Li/Li⁺, mimicking cathode and anode cycling. It demonstrates that within the 2.4–5.0 V region, Li₃PS₄ oxidation byproducts are primarily polysulfides composed of bridging sulfurs (P-S-S-P) between PS₄^3- units, free of elemental sulfur (S⁰), and electrically conductive. The Li₃PS₄ oxidation process occurs at 2.8 V during first charge and ends at 3.4 V, with volume shrinkage at the VGCF interface. During reduction (2.4 to −0.05 V), polysulfides convert reversibly to Li₃PS₄ between 1.9 and 1.7 V, then to Li₂S and Li_nP (0 ≤ n ≤ 3) between 1.9 and 0.6 V, causing volume expansion and the transition to an electrically insulating interphase. Below 0.6 V, Li₂O formation dominates without further evolution of Li₂S or Li_nP.