Interface Reconstruction Reviving Dead Na Enables 3,800-Cycle Ah‑Level Anode‑Free Sodium Batteries.

The accumulation of inactivated or 'dead' Na is the primary cause of rapid capacity decay in anode-free sodium batteries (AFSBs). These dead Na is typically encapsulated by a passivated solid electrolyte interphase (SEI), which renders it electrochemically inactive. Current strategies have primarily focused on suppressing dead Na formation, yet its reactivation has rarely been explored. Here, we reactivate dead Na by electrochemically reconstructing the interface through a tailored overdischarge method, thereby recovering the lost capacity and extending cycle life. Specifically, this reactivation process selectively dissolves the passivating SEI surrounding dead Na to restore its activity, while simultaneously rebuilding a thin, homogeneous, and NaF‑rich interphase that ensures subsequent stable cycling. Collectively, by periodically using the method, an Ah-level anode-free pouch cell realizes 3,800 cycles at 2C with 83.5% capacity retention, representing a 343% enhancement over the best cyclability in the previous report. Besides, a practical 5 Ah anode-free pouch cell (181.1 Wh kg-1) also exhibits stable cycling over 2,500 cycles with 93.7% capacity retention, comparable to the performance of commercial LiFePO4 batteries, demonstrating great application potential. Moreover, with proven effectiveness across diverse electrolytes and cathodes, this dead Na reactivation method offers a universal and effective pathway toward practical realization of AFSBs.