Reversibility‐Driven Degradation Mechanisms and Targeted Modulation Strategies for Anode‐Free Sodium Metal Batteries

ABSTRACT Anode‐free sodium metal batteries (AFSMBs) forgo the excess metallic sodium anode, thus offering a compelling route to simultaneously enhance energy density and simplify manufacturing. However, their practical deployment is fundamentally limited by the irreversible depletion of active sodium, a consequence of inherently unstable interfacial chemistry and the stringent constraint of finite sodium inventory. In this review, we introduce theoretical reversibility as a unifying criterion to categorize the dominant sodium loss pathways. By integrating thermodynamic principles with kinetic analysis, we systematically deconvolute the intricate mechanisms of capacity degradation by tracking the migration and deposition behavior of Na + ions. Furthermore, we present a comprehensive taxonomy of state‐of‐the‐art strategies for mitigating capacity fade, which are anchored in the dual pillars of uniform Na nucleation/growth and regulated solid electrolyte interphase (SEI) formation/evolution. Special emphasis is placed on stacking stress, a critical yet underappreciated factor that dictates the morphological evolution of sodium deposits. Finally, we delineate the current fundamental bottlenecks and outline promising avenues for future research to accelerate the development of high‐performance AFSMBs.