Zn‐Na Alloy Interphase Engineering for Fast Kinetics and High Performance in Sodium‐Ion Batteries

ABSTRACT Poor compatibility and sluggish kinetics at the hard carbon electrode‐electrolyte interface represent a critical bottleneck limiting the development of high‐performance sodium‐ion batteries. To address this issue, this study introduces Zn(OTf) ₂ and NaSO ₂ CF ₃ as multifunctional composite additives, constructing an electrolyte system capable of both interface modification and capacity compensation. Experimental results demonstrate that Zn(OTf) ₂ preferentially decomposes and in situ forms an SEI film rich in NaZn ₁₃ on the hard carbon surface. This NaZn ₁₃ component exhibits excellent stability and sodium‐ion conductivity, promoting uniform Na ⁺ distribution and rapid migration, thereby enhancing interfacial kinetics. Concurrently, NaSO ₂ CF ₃ acts as a sodium reservoir that can fully decompose during cycling, effectively compensating for the irreversible capacity loss in the first cycle. Based on this synergistic strategy, the initial Coulombic efficiency of the Na ₄ Fe ₃ (PO ₄ )P ₂ O ₇ ||HC full cell increases from 67.62% to 80.78%, and the capacity retention after 1000 cycles improves from 56.08% to 86.93%. This design not only modifies the interface but also compensates for the active sodium loss incurred during the alloying process, significantly enhancing the interfacial performance and cycling stability of the battery and offering promising application prospects.