<p>Binary 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.</p>