A 2D Metallic KCu4S3 Anode for Fast‐Charging Sodium‐Ion Batteries

Abstract The search for advanced electrode materials to solve slow ion diffusion and poor conductivity issues has spurred the development of fast‐charging sodium‐ion batteries (SIBs). Herein, a 2D metallic anode, KCu 4 S 3 , is reported expertly crafted using a KSCN molten salt approach, laying the foundation for fast‐charging SIBs. It is found that the mixed metal‐valence states within this compound provide substantial advantages, particularly in enhancing the high‐rate capability and ensuring long‐term durability. The mechanism that appears to facilitate these benefits can be traced to the formation of NaCu 2 S 2 intermediate, which assist in electron transfer during Na + (de)intercalation. In situ observations confirm the sodiation products of NaCu 2 S 2 and sodium polysulfide can recover to the original phase upon desodiation. Such distinctive characteristics endow KCu 4 S 3 with remarkable electrochemical performances, including an impressive capacity of 355 mAh g −1 at 20 A g −1 and 100% capacity retention within 3000 cycles. Moreover, the full cell exhibits a high energy density of 332 Wh kg −1 and retains 92% of its capacity across 150 cycles at 1 A g −1 . This work opens new horizons in the field of fast‐charging materials, making a significant step forward in shaping the future of SIBs.