Tian, JiabeiGuo, SiguangGao, BiaoLiu, MinZhou, YiRen, JianweiJavanbakht, MehranOmidvar, HamidLi, ZhuoSong, HaoHuo, Kaifu2025-10-012025-09Tian, J., Guo, S., Gao, B. et al. Defect-free Bi-Sn@C composites with high capacity and long cycle life for superior sodium storage. Science China Materials (2025). https://doi.org/10.1007/s40843-025-3491-6.2199-4501 (online)10.1007/s40843-025-3491-6http://hdl.handle.net/2263/104550Binary alloys have garnered significant attention for the development of the sodium-ion battery due to their ability to combine the advantages of single-phase alloys. However, these materials often demonstrate limited electrochemical performance, and the relationship between their crystallization states and their sodium storage properties remains poorly understood. Here, we synthesize Bi-Sn binary alloys with various compositions via phase-separation metallurgy to explore the sodium storage properties of different crystalline structures. The results indicate that hypo- and hyper-eutectic Bi-Sn alloys readily form a “dendritic” primary phase at the non-eutectic interface, which aggravates structural degradation and increases internal resistance. In contrast, Bi-Sn alloys with optimized eutectic interfaces effectively control dendritic growth and reduce defects, resulting in enhanced microstructural stability and superior electrochemical performance. As results, the eutectic p-Bi57Sn43@C anode achieves a record-high specific capacity of 470.3 mAh g−1 at 1 C and exhibits remarkable long-term cycling stability, retaining 95.2% of its capacity after 1000 cycles at 20 C. The defect-free eutectic concept presented here establishes a valuable foundation for future studies of binary and polycrystalline eutectic alloys in electrochemical applications.en© 2025, Science China Press. The original publication is available at : https://link.springer.com/journal/40843.Binary alloySodium-ion batteriesTwo-phase eutectic interfaceDendriticLong cycling lifePostprint Article