Deciphering Interfacial Reactions via Optical Sensing to Tune the Interphase Chemistry for Optimized Na‐Ion Electrolyte Formulation

Abstract Interphases, solid‐electrolyte interphase (SEI), and cathode‐electrolyte interphase (CEI) are the key influencers in determining battery life and performance. Especially, for technologies such as sodium‐ion batteries that are in the development stage, it is crucial to tune the interphase chemistry without which it suffers at present from poor performance metrics for reaching real‐life applications. In this study, optical sensors are utilized as a tool to follow operando, the thermal events of interfacial reactions, and establish the role of different electrolyte additives during the SEI/CEI formation. Using the acquired knowledge from sensing, together with complementary studies in Na‐ion full‐cells, a new electrolyte formulation is proposed that shows stable cycling performance at 0–55 °C with very low self‐discharge and improves safety due to mitigated gassing during cycling. Finally, the studies are nurtured by transferring the know‐how to prototype cylindrical 18 650 cells. It is hoped that such findings will accelerate the practical development of Na‐ion batteries.