Unveiling the Electrolyte and Solid Electrolyte Interphase in Sodium Ion Batteries: Mechanisms, Progress, and Perspectives

Abstract Sodium‐ion batteries (SIBs) are regarded as a promising alternative to lithium‐ion batteries due to the low cost and abundant availability of sodium. Electrolyte, as the medium for ion transport, plays a crucial role in determining the electrochemical performance. Currently, SIBs employ mainly organic electrolytes, aqueous electrolytes, ionic liquids, gel electrolytes, and solid electrolytes. These electrolytes have made significant progress according to the needs of various application scenarios. Notably, the solid electrolyte interphase (SEI) formed by decomposition of electrolytes on the electrode surface has a decisive influence on the performance of SIBs, and its composition and formation mechanism are closely related to the chemical nature of the electrolyte. Therefore, a deep understanding of the structure and interfacial chemistry of the SEI is essential for developing high‐performance SIBs, preferably through the simple and effective modulation of electrolyte composition. However, the fragmented and insufficient mechanistic summary on this connection results in poor guidance on future research, especially for the co‐design of electrolyte and solid electrolyte interphase. This review summarizes and compares the research progress of various electrolyte systems, discusses the formation and aging mechanisms of SEI, and presents the perspectives on the integrated design of electrolyte and SEI.