Structural and Electrochemical Evolution of Alloy Interfacial Layers in Anode-Free Solid-State Batteries

“Anode-free” solid-state batteries feature high energy density since there is no anode active material upon assembly. While beneficial effects of interfacial layers have previously been demonstrated, the mechanisms through which they influence lithium plating/stripping are unclear. Here, we reveal the evolution of 100-nm silver and gold interfacial layers during anode-free lithium plating/stripping using electrochemical methods, electron microscopy, and modeling. The alloy layers significantly improve Coulombic efficiency and resistance to short circuiting, even though the alloys form solute regions or particulates that detach from the current collector as lithium grows. In-situ electrochemical impedance spectroscopy shows that the alloy layers return to the interface and mitigate contact loss at the end of stripping, avoiding a critical vulnerability of anode-free cells. The enhanced contact retention is driven by increased Li uniformity that promotes spatially even stripping, as well as local alloy delithiation in response to current concentrations to homogenize current and diminish voiding.