Single-Atom Engineering for Synergistic Nucleation and Interfacial Regulation Enabling Durable Anode-Free Sodium Metal Batteries

Anode-free sodium metal batteries (AFSMBs) are highly promising candidates for low-cost, sustainable, and high-energy-density storage systems. However, their practical deployment is challenged by uncontrolled dendrite growth and unstable solid electrolyte interphase (SEI) formation. To address these issues, a highly reversible and robust Na metal host enabled by atomic Bi sites is devised, coordinated in a unique N₃-Bi-S₁ moiety anchored on interconnected carbon tubes (Bi-N₃S₁@CT). Crucially, this designed remarkably sodiophilic Bi single-atom promotes uniform Na nucleation with minimal Na⁺ consumption, enabling durable and highly reversible Na plating/stripping, while effectively suppressing electrolyte over-decomposition and fostering the formation of robust inorganic-rich SEI films, as supported by comprehensive theoretical calculations and experimental analyses. Consequently, Bi-N₃S₁@CT achieves an extraordinary average Coulombic efficiency (CE) of 99.6% over 900 cycles at 12 mA cm⁻² and 6 mAh cm⁻², along with long-term durability of 1000 h at 10 mA cm⁻² and 10 mAh cm⁻² in symmetric cells. Notably, an anode-free pouch cell paired with a high-loading Na₃V₂(PO₄)₃ cathode exhibits decent cyclability over 240 cycles at 1C while maintaining good rate capability. This work demonstrates a promising strategy to simultaneously enhance energy density and stability in AFSMBs via atomic-level sodiophilicity regulation and SEI engineering.