A Stage-Coupled View of Interfacial Reconstruction in Anode-Free Sodium Metal Batteries

Anode-free sodium metal batteries (AFSMBs) remove the excess metal anode and increase projected cell-level energy density, but they place reversibility on a transient negative-electrode interface. In each cycle, Na is deposited on a foreign current collector and removed to a highly depleted state. Cell failure is governed by whether continuity can be preserved across a sequence of short-lived interfacial states, including bulk coordination, interfacial desolvation, heterogeneous nucleation, metallic coalescence, Na isolation, and recurrent solid electrolyte interphase (SEI) renewal. AFSMBs are therefore better understood as dynamically reconstructed interfacial systems, where non-uniformity introduced during interfacial entry can propagate through later plating and stripping. This review examines AFSMBs through three coupled stages: solvation-controlled interfacial entry, current collector regulated Na reconstruction, and SEI renewal controlled by reaction-pathway selectivity and mechanical coherence. Current density, stripping depth, pressure, temperature, sodium inventory, and cell architecture are treated as boundary conditions that determine whether local interfacial gains persist under practical operation. By linking coordination chemistry, metallic continuity, interphase renewal, and device-level constraints, this review provides a mechanistic framework for predictive AFSMB design.