Composite polymer electrolytes (CPEs), consisting of solid electrolyte particles embedded within a solid polymer electrolyte matrix, are promising materials for all-solid-state batteries because of their mechanical properties and scalable production processes. In this study, CPEs consisting of PEO₂₀:LiTFSI blended with 1, 10, and 40 wt % (CPE40) of the Li₆PS₅Cl electrolyte filler are prepared by a slurry-based process. The incorporation of Li₆PS₅Cl improves the lithium-ion conductivity from 0.84 mS cm^-1 (PEO₂₀:LiTFSI) to 3.6 mS cm^-1 (CPE40) at 80 °C. Surface-sensitive X-ray photoelectron spectroscopy (XPS) reveals LiF, polysulfides, and Li₃PO₄ on the CPE surface, originating from decomposition reactions between PEO₂₀:LiTFSI and Li₆PS₅Cl. The decomposition products influence the formation of the solid electrolyte interphase (SEI) at the lithium metal | CPE interface, resulting in a reduced SEI resistance of 3.3 Ω cm² (CPE40) compared to 5.8 Ω cm² (PEO₂₀:LiTFSI) at 80 °C. The SEI growth follows a parabolic rate law and the growth rate declines from 1.2 Ω cm² h^-0.5 (PEO₂₀:LiTFSI) to 0.57 Ω cm² h^-0.5 (CPE40) during thermal aging at 80 °C. By substituting CPEs for PEO₂₀:LiTFSI in lithium plating and stripping experiments, the increase in SEI resistance was reduced by more than 75%. In order to get a deeper understanding of the SEI formation process, in situ XPS measurements were carried out where the lithium metal is successively deposited on the CPE sample and XPS is measured after each deposition step. On the basis of these measurements, a multistep decomposition mechanism is postulated, including the formation of LiF and Li₂S as key components of the SEI.

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