In situ TOF-SIMS investigation of the dynamic metallic anode|solid electrolyte interface in solid-state sodium-ion batteries

<h2>Summary</h2> The metal anode|solid electrolyte interface in sodium-ion batteries has become a critical focus in advancing next-generation batteries. Current interface characterization relies on postmortem analysis, which can compromise the chemical integrity of the interface. In this study, an innovative technique is presented for <i>in situ</i> investigation of the interface under vacuum using time-of-flight secondary ion mass spectrometry (TOF-SIMS). Our findings demonstrate that metallic Na⁺ ions from the anode can diffuse, inducing charge/discharge cycling through the solid electrolyte, and mimic the behavior of conventional symmetric cells to form an <i>in situ</i> interface. Interface evolution and factors affecting the homogeneity of Na⁺ ion diffusion were observed, indicating an interphase formation, suggested to be Na₂ZrO₃, and homogeneity influenced by electrochemical age—inhomogeneous diffusion for non-aged and homogeneous diffusion for aged samples. This work offers an approach to probe interface evolution and sodium plating mechanisms at the metal anode|solid electrolyte interface under different electrochemical conditions.