Analysis of the Interphase Formation of Thiophosphate Solid Electrolytes and the Lithium Metal Anode in Solid-State Batteries

Riegger, Luise

Due to their high ionic conductivities and better safety, thiophosphate solid electrolytes (SEs) such as Li₆PS₅Cl or Li₇SiPS₈ are of interest for applications in solid-state batteries (SSBs). However, drawbacks such as their inherent reduction instability, which impede their application with a lithium metal anode (LMA) [1] as well as low gravimetric and volumetric energy densities of the resulting full cells [2] still have to be overcome.

If the decomposition products of SEs formed upon contact with lithium metal are sufficiently electronically conductive, the decomposition layer forms a so-called mixed conducting interphase (MCI). A (semi)metal dispersed in the MCI should decrease the internal resistance and lead to short circuits [3]. However, this is not the case for all metal ion containing SEs [4]. For example, the resistance of a Li|Li₇SiPS₈|Li symmetric cell constantly increases. Thus, knowledge about the composition of the interphase between lithium metal and SEs as well as the resistance evolution of a cell containing the respective SE is necessary.

We report our latest findings of the stability of several thiophosphate SEs in contact with lithium metal. We show the kinetics of the heterogeneous solid-state reaction and the chemical composition of the resulting interphase layer. Impedance spectroscopy measurements were conducted to investigate the temporal evolution of the interphase resistance between the lithium metal anode and the SE. In situ-sputter-deposition of lithium metal onto the surface of the SE pellets and subsequent X-ray photoelectron spectroscopy (XPS) measurements are performed to study the chemical composition upon contact. Multiple repetitions of these steps allow a detailed characterization and identification of the decomposition products within the forming interphase. The combined results of our studies allow us to distinguish between the possible interphases of metal ion containing electrolytes and promotes the understanding of the SE stability towards LMAs.

References:

[1] X. Li, J. Liang, X. Yang, K. Adair, C. Wang, F. Zhao, X. Sun, Energy Environ. Sci. (2020) 10.1039/C9EE03828K.

[2] P. Braun, C. Uhlmann, M. Weiss, A. Weber, E. Ivers-Tiffée, J. Power Sources 393 (2018) 119-127.

[3] S. Wenzel, T. Leichtweiss, D. Krüger, J. Sann, J. Janek, Solid State Ion. 278 (2015) 98-105.

[4] L. Riegger, R. Schlem, J. Sann, W. Zier, J. Janek, Angew. Chem. (2020) 10.1002/ange.202015238.

208 Analysis of the Interphase Formation of Thiophosphate Solid Electrolytes and the Lithium Metal Anode in Solid-State Batteries

208
Analysis of the Interphase Formation of Thiophosphate Solid Electrolytes and the Lithium Metal Anode in Solid-State Batteries