Elucidating the Reductive Decomposition Mechanism in Sulfide Solid Electrolyte Li₄SnS₄

Area of Science:

• Materials Science
• Electrochemistry
• Solid-State Batteries

Background:

• Lithium thiophosphate (Li₄SnS₄) is a promising solid electrolyte due to its moisture stability.
• However, its low reduction durability hinders its use in negative electrodes of all-solid-state batteries.

Purpose of the Study:

• To develop a quantitative method for assessing the reductive decomposition of Li₄SnS₄.
• To elucidate the degradation mechanism of Li₄SnS₄ under reductive conditions.

Main Methods:

• Modified electrochemical cell (SUS|Li₄SnS₄|Li₃PS₄|Li) for stabilized reference potential.
• Cyclic voltammetry to determine reductive decomposition potential.
• Post-reduction analysis using microscopy, XRD, and XAS.

Main Results:

• Identified reductive decomposition of Li₄SnS₄ starting below +1.2 V, with a peak at +1.0 V.
• Decomposition continues below 0 V, the typical lithium deposition potential.
• Observed decomposition into Li₂S and β-Sn, followed by Li-Sn alloy formation (e.g., Li_0.4Sn).

Conclusions:

• The Sn-S bond dissociation initiates decomposition.
• Formed Li-Sn alloys enhance electronic conductivity, facilitating further decomposition.
• Provides critical insights for designing improved solid electrolyte materials.