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

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, KCu₄S₃, 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 NaCu₂S₂ intermediate, which assist in electron transfer during Na⁺ (de)intercalation. In situ observations confirm the sodiation products of NaCu₂S₂ and sodium polysulfide can recover to the original phase upon desodiation. Such distinctive characteristics endow KCu₄S₃ 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.