Interfacial Evolution of Surface Modified Li Metal Anodes in All-Solid-State Batteries

For decades, the pursuit of solidifying electrolytes using inorganic solid electrolytes (SEs) that display high mechanical strength has been regarded as a “Holy Grail” in the quest to enable Li metal anodes. Sulfide SE materials such as Li argyrodite (e.g., Li₆PS₅Cl) are particularly attractive due to their mechanical sinterability and high ionic conductivities, making them subjects of extensive research for practical ASSBs. However, these materials encounter a critical limitation: their electrochemical instability in contact with Li metal. This instability leads to the reductive decomposition of sulfide SEs, producing byproducts like Li₂S, which substantially escalate interfacial impedance. More critically, Li metal tends to grow through the SE layers, inducing catastrophic short circuits. To address these challenges, various surface modification techniques have been explored. Inorganic materials (e.g., LiF) and alloying elements (e.g., Ag) have received considerable attention. However, despite advances in enhancing the performance of Li metal ASSBs, there is a noticeable gap in our understanding on the interfacial evolution, particularly concerning factors associated with cell fabrication methods and/or operating conditions.<br/>In this presentation, we report our strategies for stabilizing Li metal anodes in ASSBs that use Li₆PS₅Cl, specifically through the incorporation of electroless-plated In or MgF₂ interlayers. Our findings indicate that neither intermetallic nor simple inorganic interlayers maintain stability throughout cell fabrication or operation. In contrast, MgF₂ interlayers exhibit substantial enhancements in these aspects. Furthermore, comprehensive analytical techniques, including operando electrochemical pressiometry, to probe the interfacial evolution in Li metal ASSBs, are also presented.