The Impact of Li Surface Heterogeneity and Morphology on Performance in Solid-State Batteries

Solid-state Li metal batteries have the potential to enable both fast charging and more than a 30% increase in energy density. Two of the most pervasive and limiting challenges in these materials are the interface between the Li metal and the solid electrolyte and the formation of Li filaments that penetrate the solid electrolyte leading to cell shorting. Currently, most efforts to solve these problems are focused on solid electrolytes. Here we present our work investigating the Li side of the equation. This work compares four thin Li sources (&lt;40 µm), three rolled Li from commercial suppliers, and a fourth prepared in-house using thermal evaporation. Using X-ray photoelectron spectroscopy (XPS) and attenuated total internal reflection infrared spectroscopy (ATIR), we determined that the surface layers on these different Li films varied significantly in composition and thickness. Subsequent electrochemical testing of these films in liquid, polymer, and ceramic solid electrolytes, showed that variation in the surface thickness and chemistry significantly impacted the interfacial resistance. Furthermore, an analysis of cell failure due to shorting led to the clear conclusion that in addition to the surface contamination, the Li microstructure also had a significant impact in determining when failure will occur.<b>Acknowledgments:</b> Simon Thompson and the DOE EERE Vehicle Technology office Battery Material Research Program for funding supporting this work.