Direct operand visualization of Li-metal plating and stripping inside pressed, closed battery cells using optically transmitting ultra-thin electrode

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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 transmitting ultra-thin Cu electrode was developed and the Li-metal plating/stripping behaviors inside cells, where a pressed planar interface existed between the electrodes and the electrolyte-soaked separator, were observed. Gas evolution at the pressed interface strongly disturbed the Li-metal plating, leading to a porous Li-metal plating morphology and unstable cycling with low Coulombic efficiency. Visual inspection of the pressed interface reveals certain underlying mechanisms of Li-metal plating including the influence of gas evolution, which are not readily captured by indirect approaches, while offering a general framework applicable to various other devices based on closed-cell configurations. Briefs Visible inside the pressed, closed Li-metal batteries cells via optically transmitting ultra-thin Cu electrode. • Optically transmitting ultra-thin Cu electrode was developed for transparent cell. • Li plating/stripping at pressed interface was visualized without destruction. • Gas evolution was captured in real-time by operand transparent observation. • Gas evolution on electrode surface disturbs Li metal nucleation and growth. • Different cell properties derived from electrolytes visually explained.