Elucidating the Reductive Decomposition Mechanism in Sulfide Solid Electrolyte Li₄SnS₄

The sulfide solid electrolyte Li₄SnS₄ 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 Li₄SnS₄, 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 Li₄SnS₄ to better understand its degradation mechanism. The configuration of the electrochemical evaluation cell was modified from SUS|Li₄SnS₄|Li to SUS|Li₄SnS₄|Li₃PS₄|Li, allowing for stabilization of the reference potential of the counter electrode. The reductive decomposition potential of Li₄SnS₄ 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 SnS₄^4- unit in Li₄SnS₄ initially decomposes into Li₂S and β-Sn with the dissociation of the Sn-S bond; (ii) the resulting β-Sn forms Li_xSn alloys such as Li_0.4Sn; and (iii) the ongoing reductive decomposition reaction is facilitated by the electronic conductivity of these Li_xSn alloys. These findings offer crucial methodological and mechanistic insights into the development of higher-performance solid electrolyte materials.