Résumé
Monitoring the chemical and physical processes during the initial charge of commercial‐type cells is crucial for accelerating their optimization. In this study, operando optical calorimetry, pressure sensing, and infrared fiber evanescent wave spectroscopy (IR‐FEWS) are harnessed as powerful diagnostic tools to investigate the first charge of the formation cycle of layered oxide‐based sodium‐ion cells composed of P2 or O3 cathode material and hard carbon (HC) anode. It is first revealed that the cathode composition significantly influences the initial charge behavior, showing that the O3 cathode triggers larger electrolyte decomposition than P2, which is associated with significant heat and gas generation at high states of charge. Then, the use of succinonitrile (SN) and prop‐1‐ene‐1,3‐sultone (PES) is explored as additives in the electrolyte, proving that while both additives raise the heat generation in P2/HC and O3/HC cells, they effectively suppress solvent and salt decomposition. These observations are further corroborated by online electrochemical mass spectrometry (OEMS) and X‐ray photoelectron spectroscopy (XPS) analyses. Overall, this work underlines the importance of combining operando calorimetric and chemical studies in optimizing the cell chemistry and highlights the effectiveness of optical sensing techniques for investigating the interphase formation in commercial‐type cells.