Understanding the Formation of a Solid-Polymer Interphase between Argyrodite-Type Li6PS5Cl and Polymer-Based PEO:LiTFSI
Authors: Simon Fabian Johannes , Hanauer Matthias , Henss Anja , Richter Felix , Janek Jürgen 
Published: February 2020

Lithium-ion batteries (LIBs) play a crucial role in today’s consumer goods and transportation market due to their superior ability to store energy. While the energy and power density has been strongly increased since its commercialization in 1991, LIBs will soon reach a limit, which cannot be overcome with conventional liquid electrolyte based systems [1]. However, solid-state batteries (SSB) based on solid electrolytes (SE) offer the possibility to further enhance the energy and power density [2].
Sulfide-based SEs show superior conductivities of several mS/cm at room temperature. Their ductile nature allows for cold pressing and results in good electrode contacting and hence lower interfacial resistance compared to oxide-based SEs [3]. However, sulfide-based SEs are not stable against lithium and the formation of an interphase has been shown in theory and experiment. The degradation of argyrodite-type Li6PS5X (X = Cl, Br, I) in contact with lithium has been investigated by Wenzel et al. using in-situ XPS. They reported Li3P, Li2S and LiX as decomposition products, which is in accordance to a thermodynamic analysis based on first-principle calculations carried out by Zhu et al. [4,5].
A promising approach to prevent the lithium/Li6PS5X interface from decomposition is the introduction of an interlayer. While many examples about sputtering thin interlayers onto the SE or lithium have been reported, only few reports can be found about thin polymer interlayers. However, the latter could be more attractive for commercial applications as they can be applied by less expensive techniques than sputtering.
By the introduction of a polymer interlayer between lithium and Li6PS5X, an extra electrolyte/electrolyte interface in addition to the lithium/electrolyte interface is formed. While the lithium/electrolyte interface has already been intensively studied for several electrolytes, only little is known about the electrolyte/electrolyte interface especially in case of the polymer/solid electrolyte interfaces. In previous studies, the specific resistances for the La0.55Li0.35TiO3\PEO20:LiCF3SO3 (2200 Ωcm2, 80°C) [6], the Ohara\PEO16:LiCF3SO3 (32 Ωcm2, 80°C) [7], the Ohara\PEO10:LiTFSI (47 Ωcm2, 40°C) [7], and the Li7La3Zr2O12/PEO20:LiClO4 (9000 Ωcm2, 70°C) [8] interfaces have been reported. However, these studies use oxide-based SEs while to the best of our knowledge no comprehensive study on the interface between polymer electrolyte and sulfide-based SE exists.
In the present study, we electrochemically and analytically investigate the interface between argyrodite-type Li6PS5Cl and poly(ethylene oxide): lithium bis(trifluoromethanesulfonyl)imide (PEO:LiTFSI). The PEO:LiTFSI electrolyte is prepared solvent-free in order to exclude any solvent-related decomposition reactions at the Li6PS5Cl/PEO:LiTFSI interface. In the first part, the Li6PS5Cl/PEO:LiTFSI interface is studied via electrochemical impedance spectroscopy (EIS). Two separated processes, which occur at different characteristic frequencies, are identified using an in-house developed four-point measurement setup (Figure 1a). The development of the cell impedance is monitored over 10 days at 80°C and reveals the rise of a medium-frequency process over time. In order to study the interface analytically, the polymer is removed and X-ray Photoelectron Spectroscopy (XPS) measurements are carried out after different ageing times. A clear evolution of decomposition products at the Li6PS5Cl/PEO:LiTFSI interface is observed (Figure 1b). These observations are confirmed via Time-of-Flight secondary ion mass spectrometry (ToF-SIMS). Possible decomposition pathways are discussed.

References

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Figure 1