Leveraging the Amorphous Nature of Sn–P Alloys for Improved Stability and Energy Density in Na‐Ion Batteries

ABSTRACT Developing alternative anode materials is a key route to improve the volumetric energy density of sodium‐ion batteries (SIBs). Here, Sn–P alloys with varying Sn/P atomic ratios are systematically evaluated as Na‐alloy anodes. Among them, SnP ₃ outperforms all other alloys and offers the best balance between cost, volumetric capacity and cycling stability. SnP ₃ can be made in either amorphous or crystalline forms. Crystalline SnP ₃ becomes amorphous on sodiation and remains amorphous during cycling. Amorphous Na _x SnP ₃ shows a solid‐solution‐type and highly reversible sodiation/desodiation behavior, demonstrating exceptional cycling performance and structural stability. As comparison, the crystalline Sn ₄ P ₃ phase dealloys to Sn and a phosphorus containing phase upon repeated sodiation/desodiation, showing rapid capacity decay and severe particle pulverization. These findings highlight amorphous SnP ₃ as a promising high‐energy‐density anode material for SIBs and provide valuable insights into how an amorphous structure influences Na‐alloy behavior.