Mechanical Failure of Cu Current Collector Films Affecting Li Plating/Stripping Cycles at Cu/LiPON Interfaces

Abstract

Herein an electro-chemo-mechanical theory, which states that mechanical work to deform a Cu current collector (CC) film influences the nucleation overpotential (η nc) for Li nucleation at the Cu CC film/lithium phosphorus oxynitride (LiPON) electrolyte interface, is examined. The finite element method (FEM) simulated the mechanical pressure that the CC film exerted on the Li nuclei at the Cu/LiPON interface, and the results agreed with the trends in our previous study. In situ scanning electron microscopy (SEM) observations for cycling of Li plating/stripping showed that Li repeatedly nucleated and grew at positions where the CC film was locally fractured, and η nc decreased with repeated Li plating/stripping because the mechanical pressure to the Li nuclei was no longer applied at locations where the CC film was fractured. On the other hand, for thicker CC films that did not crack, η nc exhibited nearly consistent values in the Li plating/stripping cycles. Consequently, the experimental results in this study supported our nucleation theory for a metal/solid-state-electrolyte interfacial system.