Bottom-enriched Sn gradient and homogeneous NaF hybrid interfacial layer enabling mechanically stable SEI for sodium-metal batteries

A NaF-Sn hybrid interfacial layer with a bottom-enriched Sn gradient is constructed on sodium metal anodes. • The Sn gradient forms a sodiophilic nucleation zone that enables regulated bottom-up Na deposition. • A homogeneous NaF matrix provides a robust artificial SEI with suppressed side reactions. • The synergistic gradient alloy–fluoride architecture effectively suppresses dendrite growth. • Na 3 V 2 (PO 4 ) 3 ||NaF-Sn@Na full cells deliver ultra-long cycling stability (23,000 cycles at 20C, 83.5% capacity retention). Sodium metal represents a promising anode for high-energy batteries, yet it is plagued by an unstable solid-electrolyte interphase (SEI) and dendrite formation. Herein, we construct a hybrid interfacial layer on sodium metal that features a substrate-enriched Sn gradient and a homogeneous NaF matrix (denoted as NaF-Sn@Na). Within this architecture, the Sn concentration increases toward the Na metal substrate, forming a sodiophilic nucleation zone with a graded Sn distribution that drives uniform Na deposition outward from the substrate. Furthermore, homogeneously dispersed NaF endows the interfacial layer with robust mechanical strength and acts as a barrier between the electrolyte and sodium anode, thereby mitigating parasitic side reactions. The gradient Sn layer alleviates alloying-induced volume expansion and inhibits the fragmentation of nucleation sites, whereas the homogeneous NaF matrix sustains interfacial mechanical stability and enables fast Na + -ion transport. Benefiting from these structural advantages, NaF-Sn@Na symmetric cells achieve highly reversible Na plating/stripping with low overpotential and outstanding long-term cycling stability. More impressively, the full cell paired with a Na 3 V 2 (PO 4 ) 3 cathode delivers an ultra-long lifespan of 23,000 cycles at 20 C with a high capacity retention of 83.5%. Notably, it maintains excellent operational stability under low N/P ratio condition. This rationally designed gradient alloy/fluoride hybrid structure provides a feasible and practical pathway toward high-stability sodium metal batteries under harsh operating conditions. Synergistic gradient Sn alloying and homogeneous NaF interphase enable regulated bottom-up sodium nucleation and robust interfacial stability, effectively suppressing dendrite growth and parasitic reactions, achieving ultra-long cycling stability of 23,000 cycles in full cells. Download: Download high-res image (245KB) Download: Download full-size image