Interphase Formation of PEO20:LiTFSI–Li6PS5Cl Composite Electrolytes with Lithium Metal

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.