It is commonly accepted that today’s Li-ion technologies might have issues addressing future long term targets.¹ Hence there is an increasing focus on novel cell designs. Solid electrolytes (SE) have attracted growing interest as they could enable the use of lithium metal. They feature several advantages over liquid electrolytes, such as a rigid and non-leaking structure, non-flammability as well as a larger temperature operation range.² All-solid-state batte­ries thus have great potential to enhance safety, lifetime and energy density compared to established Li-ion cell technologies.³

Sulfide-based SEs like crystalline Li₁₀GeP₂S₁₂ (LGPS) or glass-ceramic 70Li₂S∙30P₂S₅ (LPS), feature high ionic conductivity exceeding that of current liquid electrolytes. Moreover, their softness allows for good contacting, thus providing lower grain-boundary and interfacial resistance compared to oxide-based SEs.¹ However, chemical insta­bility towards air and moisture as well as Li metal remains as critical issue.³ Moreover, recent studies revealed that their electrochemical stability has generally been overesti­mated so far.^4–6 The rather narrow stability demands for protection on both cathode and anode side.¹

While coating of the active material is a common strategy to prevent degradation of the SE at the cathode side, diffe­rent approaches aim for overcoming decomposition issues at the Li metal anode. Numerous examples of sputtering thin interlayers onto the SE and lithium metal, respective­ly, have been reported in the literature.^7–11 However, most sputtering techniques are expensive and thus less attractive for large-scale applications. An alternative approach con­sists in application of a thin polymer interlayer, which has already been successfully tested for Li_1+xAl_xTi_2-x(PO₄)₃, an oxide-based SE that is known to rapidly decompose in contact with lithium metal.¹²

The chemical stability of Li₁₀SnP₂S₁₂ (LSPS), the tin-deri­vate of LGPS, towards PEO has been investigated in a different context. Blanga et al. demonstrated a composite electrolyte of PEO and LSPS with improved cycling beha­vior and safety in lithium/sulfur batteries.¹³ The authors stated that addition of the salt LiI results in the formation of a novel Li_10+xI_xSnP₂S₁₂/P(EO)₃/LiI electrolyte.

In the present study, we analyzed the compatibility of LSPS with PEO-LiTFSI membranes by both looking into electrochemical properties and performing XPS as well as further analytics of the aged interfaces. We will give experimental evidence for the chemical reactivity of LSPS in contact with PEO. In particular, the formation of poly­sulfides as well as sulfite will be proven by a combination of various techniques. Potential reasons for the observed decomposition will be critically discussed and a possible degradation mechanism will be proposed.

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