Tracking Interphase Growth at Alloy Anode Interfaces in Sulfide Solid-State Batteries

The chemical stability of solid-state electrolytes (SSEs) in contact with negative electrode materials is essential to enable high performance and safety of solid-state batteries (SSBs). While interphase layers are known to form between Li metal and various sulfide SSEs, there is a lack of understanding of interphase growth in contact with other promising anode materials, such as silicon and aluminum alloys. Here, we track and quantify interphase growth rate, thickness, and composition of various alloy anode thin films in contact with the widely used argyrodite Li₆PS₅Cl SSE. Using coulometric titration time analysis (CTTA), we find that the average interphase thickness on four alloy anode materials (Ag, Al, Si, and Ge) is less than half that of pure Li metal after 400 h of growth. Furthermore, the interphase growth rate is strongly dependent on the applied stack pressure and varies among the different alloy materials. The interfacial contact area, which is governed by alloy mechanical properties and deformation under stack pressure, is found to be a critical factor in determining interphase growth rate. Time-of-flight secondary-ion mass spectrometry further confirmed thinner and uniform interphase growth on alloy anodes compared to Li metal. This study bolsters our understanding of interfacial stability of various alloy anode materials married with Li₆PS₅Cl SSE, and it suggests that alloy anodes could exhibit enhanced stability compared to Li in sulfide SSB applications.