Properties of the Interphase Formed between Argyrodite-Type Li6PS5Cl and Polymer-Based PEO10:LiTFSI.

All-solid-state lithium metal batteries using thiophosphate solid electrolytes (SE) present a promising alternative to state-of-the-art lithium-ion batteries due to their potentially superior energy and power. However, reactions occurring at the lithium metal | SE interface result in an increasing internal resistance and limited cycle life. A stable solid polymer electrolyte (SPE) may be used as protective interlayer to prevent the SE from direct contact and reaction with lithium metal. This creates a new and rarely studied heteroionic interface between the inorganic SE and the SPE, which we investigate here. The interface resistance between argyrodite-type Li6PS5Cl and a poly(ethylene oxide)/LiTFSI-based SPE is quantified by four-point electrochemical impedance measurements using two wire-shaped reference electrodes (2.4 Ω cm2 at 80 °C). Two distinct processes are observed and attributed to lithium-ion conduction through a formed solid-polymer electrolyte interphase (SPEI) and an ionic charge-transfer (CT) process. The SPEI predominantly consists of polysulfides and lithium fluoride (LiF), as identified by X-ray photoelectron spectroscopy (XPS) analysis. A temperature-enhanced SPEI growth is observed using electrochemical impedance spectroscopy (EIS) and depth profiling combined with time-of-flight secondary ion mass spectrometry (ToF-SIMS). The results highlight the importance of four-point measurements to determine electrolyte-electrolyte interface properties. Overall, the low resistance and low activation energy of the SPEI makes the SPE interlayer an attractive candidate to protect Li6PS5Cl from decomposition at the lithium metal anode.