Nonaqueous Liquid Electrolytes for Sodium‐Ion Batteries: Fundamentals, Progress and Perspectives

Sodium‐ion batteries (SIBs), driven by sustainability and cost advantage, have been recognized as one of the most promising electrochemical energy storage devices. Electrolytes, as the most unique component that not only ionically connect while insulating electronically electrodes but also determine the eventual improvements in performance mainly regarding cycle life, Coulombic efficiency, energy density, and safety, hold the key to the practical implementation of SIBs. In this review, the fundamental design principles of Na + ‐ion electrolytes and the chemical properties of the Na + cation over the Li + cation in terms of ion transport, salt dissolution, and solvation structure are first discussed. Then, a sequence of crucial experimental discoveries and strategical achievements in the field of electrolytes for SIBs are presented, with focuses on the ether‐based electrolytes for co‐intercalation into graphite, diluted and highly concentrated electrolytes, wide temperature range electrolytes, nonflammable electrolytes, indispensable electrolyte components (functional additives and new sodium salts). Finally, a detailed analysis of research trends of practically feasible Na + ‐ion electrolytes is presented to aid in the ongoing quest for better SIBs of the future.