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 due to their mechanical properties and scalable production process. In this study, CPEs consisting of PEO20:LiTFSI blended with 1 wt%, 10 wt% and 40 wt% (CPE40) of 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 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 SPEs 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 lithium metal is successively deposited on the CPE sample and X-ray photoelectron spectroscopy 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.