Elucidating the Reductive Decomposition Mechanism in Sulfide Solid Electrolyte Li₄SnS₄

The sulfide solid electrolyte Li 4 SnS 4 has garnered considerable interest due to its exceptional moisture durability, which is attributed to its stable hydrated state. However, a major limitation of certain sulfide solid electrolytes, including Li 4 SnS 4, is their low reduction durability, which limits their application in the negative electrodes of all-solid-state batteries and impedes qualitative material development assessments. In this study, we introduced a quantitative and straightforward method for evaluating the reductive decomposition of Li 4 SnS 4 to better understand its degradation mechanism. The configuration of the electrochemical evaluation cell was modified from SUS|Li 4 SnS 4 |Li to SUS|Li 4 SnS 4 |Li 3 PS 4 |Li, allowing for stabilization of the reference potential of the counter electrode. The reductive decomposition potential of Li 4 SnS 4 was quantitatively assessed by using cyclic voltammetry in a two-layer electrochemical evaluation cell. We observed a minor irreversible reduction current below +1.2 V and a pronounced decomposition peak at +1.0 V. Notably, reductive decomposition continued below 0 V, which is typically the onset point for Li electrodeposition. Postreduction, the solid electrolyte was comprehensively analyzed through optical microscopy, X-ray diffraction, and X-ray absorption spectroscopy. These analyzes revealed the following: (i) The SnS 4 4– unit in Li 4 SnS 4 initially decomposes into Li 2 S and β-Sn with the dissociation of the Sn–S bond; (ii) the resulting β-Sn forms Li x Sn alloys such as Li 0.4 Sn; and (iii) the ongoing reductive decomposition reaction is facilitated by the electronic conductivity of these Li x Sn alloys. These findings offer crucial methodological and mechanistic insights into the development of higher-performance solid electrolyte materials.