Bulk SEI: Investigating the Instability of Li6PS5Cl at the Lithium Metal Anode

Abstract

Among the multitude of lithium ion conducting materials, thiophosphate-based solid electrolytes (thio-SEs) attract great attention from academia and industry alike. Due to their high ionic conductivities and increased safety, thio-SEs – such as lithium argyrodite Li6PS5Cl – are considered suitable to be employed in solid-state batteries (SSBs). Thus, by substituting liquid organic electrolytes of lithium-ion batteries (LIBs), higher energy and power densities (in combination with lithium metal anodes (LMAs)) as well as simplified cell designs are promoted.[1] Unfortunately, one major drawback of thio-SEs is the inadequate reduction stability in contact with LMAs,[2] which impedes device-level applications and has to be overcome.Due to its thermodynamic instability, Li6PS5Cl is reduced upon contact with highly reactive lithium metal to a mixture of compounds, i.e., Li2S, Li3P, and LiCl. Thus, a so-called solid electrolyte interphase (SEI) is formed, which is predominantly ion-conducting. Its electronically insulating nature is suspected to limit the interphase formation and further growth.[3] Additionally, the chemical and structural properties differ from that of pure SE. Unfortunately, the SEI only evolves at the Li | Li6PS5Cl interface and its thickness is in the nanometer range.[4] Suitable methods to examine the characteristics of SEI layers are therefore scarce and mostly limited to surface sensitive characterization techniques.Here, we employ a novel approach to gain further insights into the nature of SEI layers of thiophosphate-based SE in contact with lithium metal, demonstrated with Li6PS5Cl as model solid electrolyte. Bulk material corresponding to the composition of the SEI is synthesized from SE and Li-metal in a direct manner and hence, examined in respect to different properties. The successful synthesis of bulk-phase SEI is confirmed by X-ray photoelectron spectroscopy measurements, validating the chemical composition. This is complemented by structural analysis. Electrochemical characterization (electrochemical impedance spectroscopy and Wagner-Hebb) is utilized to quantify the ionic and electronic conductivity of the obtained bulk material, respectively, and to further determine transport properties and SEI growth at the Li | Li6PS5Cl interface. The combined results of our studies allow comprehensive understanding of the SEI nature and offer boundary conditions to approximate the nanoscale properties of the SEI at the Li | Li6PS5Cl interface from bulk material synthesis and analysis.