Understanding the Origin of Lithiophilicity Toward Molten Li-Metal Using <i>In-situ</i> Scanning Electron Microscopy (SEM)

Maintaining a uniform Li deposition at the anode electrode/electrolyte interface is a fundamental challenge during extended cycling of solid-state Li batteries. The reactivity of the Li metal at the current collector surface remains a principal parameter for battery optimization. It is known that a bare Cu foil current collector substrate does not have a good lithiophilic surface to deposit molten Li metal because the Cu surface’s wettability by a Li metal melt is very poor, requiring a metal or metal oxide interlayer to improve wetting [1, 2]. Zn is considered one of the most promising lithiophilic elements. In this work, we used in-situ scanning electron microscopy (SEM) to study the interfacial reactions between molten Li metal and Cu foil current collector modified by a thin Zn film (Cu@Zn) to understand the origin of lithiophilicity. We performed in-situ heating tests on a Cu foil substrate coated with a 50 nm Zn interlayer (Cu@Zn) in contact with a Li metal foil in SEM. We present experimental heating and cooling set ups for in-situ Li study inside a high vacuum SEM. We present our observations of the reaction between the molten Li metal and Cu surface during Li solidification in real time as well as post-mortem energy dispersive spectroscopy (EDS) and grazing incidence X-ray diffraction (GIXRD) on the coatings to analyze the chemistry of the reaction products. For the first time in literature, our in-situ study shows the existence of a metastable ternary Li-Cu-Zn alloy at 300 °C after the reaction of Cu@Zn with the molten Li metal. We report the segregation of Cu and Zn by formation of Cu-Zn intermetallic compounds during the cooling down step. The schematic of the experimental set up for in-situ heating tests inside the SEM in Figure 1 shows the in-situ SEM observation (top view) of Li metal wettability of a Cu@50nm (nominal thickness) Zn foil substrate in contact with a molten Li metal at 300 °C. We observed the wettability of the Cu@Zn foil near the edges of the Li metal foil where the molten Li metal contacted the substrate. Subsequently, the in-situ SEM observation (top view) of Li metastability showed formation of secondary phase particles on the Li surface during Li solidification. Figure 2A-B show the top view snapshots of the Cu@Zn and metallic Li surface observed in in-situ settings. The Li surface microstructure of the same area (indicated by the red circle in Figure 2B) at 300 °C and after cool down to RT are shown in Figure 2C-D. We will discuss these results in the context of the understanding the chemical reactivity of molten Li toward metal foils. Utilizing in-situ SEM is pivotal to clarify the interfacial reactions occurring between a lithiophilic current collector and molten Li metal and have utmost importance for designing advanced new anode materials for future solid-state battery applications. Schematic of the in-situ heating set up inside the scanning electron microscope (SEM). Top view snapshot of the Cu@Zn foil substrate (A), top view snapshot of the metallic Li foil (B), the Li surface microstructure at 300 °C (C) and after cool down to RT (D). The backscattered electron (BSE) images in (C-D) were taken from the same area indicated in red in (B) at the same magnification.

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