Visible inside the pressed and closed battery cells through optically transmitting ultra-thin electrode

Although the electrochemical reactions inside battery cells form the foundation of various energy storage and conversion processes, they are invisible because the electrodes facing the separator are pressed and closed in the cell. In this study, an optically transmitted ultra-thin Cu electrode was developed and the Li-metal plating behaviors inside the cells were observed, where a pressed planar interface existed between the electrodes and the electrolyte-soaked separator. The gas evolution at the pressed interface strongly disturbed the Li-metal plating, leading to a porous Li-plating morphology and unstable cycling with low Coulombic efficiency. Visual inspection of the pressed interface revealed the basic mechanisms of and resolved questions regarding Li-metal batteries, as well as various other devices based on closed-cell configurations.