Galvanostatic cycling of a micron-sized solid-state battery: Visually linking void evolution to electrochemistry.

The formation of interface voids, peculiar to the solid-solid contact between metal anodes and solid electrolytes (SEs), has become a fundamental obstacle for developing practical lithium metal solid-state batteries (SSBs). Addressing this issue requires the operando observation of void evolution with high spatio-temporal resolution and the direct linkage of voids to solid-state electrochemistry. Here, we present such an attempt by visualizing both the stripping and plating interfaces of a micron-sized SSB cycled in galvanostatic mode in a transmission electron microscope. Various voltage responses in the charge/discharge curves are well correlated to the nucleation, growth, and refilling of single voids. Notably, two distinct modes of Li stripping, namely, void-growth stripping and void-free stripping, are experimentally identified. We unveil the roles of stack pressure and current density on void evolutions, which suggests a mechanism of void suppression without involving plastic deformation of Li metal. Furthermore, Li|SE|Li symmetric SSBs enabling repeated void-free cycling without stack pressure are in situ demonstrated.