In-situ scanning electron microscopy observations of Li plating and stripping reactions at the lithium phosphorus oxynitride glass electrolyte/Cu interface

Morphology variations during electrochemical lithium plating-stripping reactions at the lithium phosphorus oxynitride glass electrolyte (LiPON)/copper current collector (Cu) interface are observed in-situ by scanning electron microscopy (SEM). This in-situ SEM observation shows dynamically that the plating reactions at 50 μA cm^-2 distribute initial lithium growth sites sparsely at the LiPON/Cu interface, later, local lithium growth occurs from the pre-plated sites through the Cu film, and finally, most of the precipitated lithium grows to be needle-shape with the height of micron order. This local growth rate attains to be 6.8 mA cm^-2, about 100 times higher value than applied one. When those precipitated lithium are stripped at 50 μA cm^-2, core region of each precipitate is mostly stripped but its degree depends on the length of the precipitate. This dependency will arise from the diffusivity of Li. When this stripping current density is increased to 500 μA cm^-2, the coulomb efficiency is further decreased. In-situ SEM observation shows that plated lithium around the interface becomes thin preferentially while that far away from the interface (upper side of plated lithium) remains unchanged. This will isolate most of precipitate lithium from LiPON film electrically, leading to further decreasing of the coulomb efficiency.