Dichroic-Like Interfacial Optical Response Reveals Asymmetric Interfacial Evolution during Li-Metal Plating and Stripping

Abstract The development of high-energy-density lithium-metal batteries requires a detailed understanding of Li-metal plating and stripping at buried electrochemical interfaces. Here, we employ visible-light hyperspectral imaging in combination with a visibly transparent Li/Cu cell based on an optically transmitting Cu working electrode to visualize Li-metal plating and stripping under operando conditions. During the early stages of Li plating, a distinct dichroic-like optical response emerges at the electrode interface. Under oblique illumination, the interface exhibits a reddish-brown appearance due to preferential transmission of longer-wavelength components, whereas, under coaxial epi‑illumination, enhanced specular reflection of blue-green wavelengths produces a characteristic cyan coloration. Spectral analysis reveals complementary wavelength-dependent changes in transmission-related and reflection-related optical signals. Importantly, Li stripping exhibits optical responses markedly different from those observed during Li plating at comparable nominal Li-metal amount states. These observations reveal a wavelength-dependent optical hysteresis between the plating and stripping pathways and are consistent with asymmetric interfacial evolution. The present approach provides a new platform for investigating buried electrochemical interfaces and uncovering previously inaccessible interfacial phenomena in advanced battery systems.