Multi‐Scale Analysis Combined <i>Operando</i> Elemental/Spectroscopic Measurement Techniques in Oxide‐Type All‐Solid‐State Na Batteries

Understanding the charge/discharge mechanism of batteries plays an important role in the development of high‐performance systems, but extremely complicated reactions are involved. Because these complex phenomena are also bottlenecks for the establishment of all‐solid‐state batteries (ASSB), we conducted multi‐scale analysis using combined multi‐measurement techniques, to directly observe charge/discharge reactions at hierarchical scales for the oxide‐type ASSB using Na as the carrier cation. In particular, all of measurement techniques are applied to cross‐section ASSB in the same cell, to complementarily evaluate the elemental distributions and structural changes. From Operando scanning electron microscopy–energy‐dispersive X‐ray spectroscopy, the Na concentration in the electrode layers changes on the micrometer scale under charge/discharge reactions in the first cycle. Furthermore, Operando Raman spectroscopy reveal changes in the bonding states at the atomic scale in the active material, including changes in reversible structural changes. After cycling the ASSB, the elemental distributions are clearly observed along with the particle shapes and can reveal the Na migration mechanism at the nanometer scale, by time‐of‐flight secondary ion mass spectrometry. Therefore, this study can provide a fundamental and comprehensive understanding of the charge/discharge mechanism by observing reaction processes at multiple scales.