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.