Uncovering the Dynamics of Li-Au Alloying and Li Nucleation at the Au/LiPON Interface with In Situ Contrast and Surface Roughness Contrast Imaging via Scanning Electron Microscopy.

Controlling the nucleation, growth, and dissolution of Li is crucial for the high cycling stability in rechargeable Li metal batteries. The overpotential for Li nucleation (η) on Li alloys such as Li-Au is generally lower than that on metal current collectors (CCs) with very limited Li solubility like Cu. However, the alloying process of CC and its impact on the Li nucleation kinetics remain unclear. Herein, we experimentally investigate the Li nucleation mechanism on Li-Au alloys using scanning electron microscopy with multiple detectors to distinguish the atomic number and topography contrast. Nonequilibrium processes near the Au CC film/lithium phosphorus oxynitride (LiPON) glass electrolyte interface were observed. Local dissolution of Li occurs concurrently with the Li plating process. This phenomenon indicates that Li nucleates as a metastable phase in Li-Au alloy CC films during Li plating due to the substantially lower interfacial energy of Li with the alloy phase. This low interfacial energy also leads to a low η. Moreover, the local Li redissolution process results in a nonrandom spatial distribution of Li growth sites. Consequently, the alloy layer acts as a buffer, facilitating a more uniform distribution of the reaction sites.