Electrochemical Quantification of Lithium Oxide Impurity in Lithium Sulfide via Lithium–Sulfur Battery Operation

A novel electrochemical quantification method is proposed for determining low contents of lithium oxide (Li₂O) impurities in lithium sulfide (Li₂S), based on the operating principles of lithium-sulfur battery systems. Li₂O is a critical impurity in Li₂S used for next-generation battery applications, including sulfide solid-state electrolytes and high-energy cathodes. It promotes electrolyte decomposition and induces parasitic side reactions during solid electrolyte synthesis. In addition, due to its intrinsically low ionic and electronic conductivity, Li₂O increases the overall electrode resistance. Accurate quantification of Li₂O is therefore essential for achieving high-purity Li₂S and a predictable performance. However, conventional techniques such as X-ray diffraction (XRD) and titration suffer from poor accuracy and reproducibility, especially at low concentrations or in amorphous forms. To address these challenges, we designed Li₂S electrodes with controlled amounts of Li₂O and employed linear sweep voltammetry (LSV) to detect and quantify the oxidation peak associated with Li₂O around ∼3.7 V (vs Li/Li⁺). A strong linear correlation (<i>R</i>² > 0.992) was observed between the LSV peak area and Li₂O content below 5 wt %. Cross-validation with industrial Li₂S samples using XRD, SEM-EDS, and oxygen analysis further confirmed the reliability of this approach. This methodology enables direct quantification of a low Li₂O content through a simple electrode fabrication and LSV measurement process. Its high accuracy, low cost, repeatability, and fast analysis speed make it a promising tool for the quality assessment of Li₂S materials and process control in commercial production lines.