Interfaces have been a key region of interest in solid-state batteries for many years. The dynamic interplay between the solid-electrolyte and the respective anode defines crucial characteristics of the resulting Solid-Electrolyte Interphase (SEI), particularly in terms of chemical states and acts as a key indicator of the solid-electrolyte’s suitability for a specific battery system. Understanding the fundamentals of the SEI formation is paramount in developing next-generation batteries. Previous studies of interface characterisation have been confined to ex-situ analyses, which require numerous repetitions to yield reproducible and representative data for comparison with industrial systems. In this work, we present an in situ characterisation method for interfaces, that is simple and adaptable to various sample conditions such as, as-prepared material or electrochemically tested samples.

In this work, the solid-electrolyte used was based on the NASICON composition with a stochiometry of Na_3.4Zr₂Si_2.4P_0.6O₁₂ synthesised through solution-assisted solid state reaction. In situ metallic anode|NASICON interface was formed by inducing negative charge across the analysis area of the sample, to form a ‘negative’ electrode, through the applied combination of flood gun and primary-ion gun within a Time-of-Flight Secondary Ion Mass Spectrometer (ToF-SIMS). This created charge imbalance between the ‘negative’ electrode and metallic anode, promoting the diffusivity of Na⁺ ions. To facilitate the mobility of charge carriers, and provide optimal connection to both negative and positive electrode, samples were setup in an in-house metallic casing. Ion maps and profiles were obtained using the same ToF-SIMS instrument, and cross-validated to results obtained via ex situ X-Ray Photoelectron Spectroscopy.

Evolution in the chemical states of the sodium anode|NASICON interface was observed indicating an interphase formation, suggested to be Na₂ZrO₃. The condition of the sample, as-prepared or electrochemically tested, was also found to influence the homogeneity of the Na⁺ plating, thus resulting in direct observation of metal filament/dendrite propagation. Our findings demonstrate that Na⁺ mobility can be controlled to induce galvanostatic charge/discharge cycling, that effectively mimick a battery’s mode of operation and the SEI formation that would occur in a conventional symmetric cell. This work not only advances our understanding of solid-electrolyte interfaces using NASICON in sodium-ion batteries but also presents a practical and versatile approach to in situ characterisation, laying the foundation for the development of more efficient solid-electrolytes for next-generation batteries.