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

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.