Robust Sodium–Metal Batteries Through Electrolyte‐Mediated Interfacial Regulation: Fundamentals, Design Strategies, and Operando Characterization

Sodium–metal batteries (SMBs) exhibit high theoretical specific capacity and take advantage of sodium's natural abundance and low cost, showing great promise for broad next‐generation energy‐storage applications. In this review, we summarize the major practical bottlenecks restricting SMB development. The solid–electrolyte interphase (SEI) serves as a key mediator to overcome these challenges, and its formation mechanisms are systematically outlined. We comprehensively review diverse electrolyte strategies, including solvent engineering, salt chemistry optimization, additive utilization, high‐concentration, localized high‐concentration, and weakly solvating electrolytes, for modulating Na + transport and generating mechanically robust, ion‐conductive yet electronically insulating SEI layers. We further discuss advanced in‐situ/operando characterizations combined with multiscale computational modeling and machine‐learning techniques as powerful tools to decipher the dynamic evolution of sodium–metal interfaces. Particular focus is placed on the coupled effects of current density, temperature, electrolyte composition, interfacial layers on sodium nucleation, dendrite growth, dead‐sodium formation, and Coulombic efficiency (CE). Finally, future directions for SEI engineering have been outlook toward high‐performance, practical sodium–metal batteries.