Electronic Orchestration of Iron-Based Sulfate Cathodes Toward Exceptional Sodium-Storage Longevity Beyond 20000 Cycles

Sodium iron sulfate (Na_2+2xFe_2-x(SO₄)₃, NFS) has garnered industrial attention as a cheap and high-voltage cathode material for sodium-ion batteries. However, the practical application has been severely hindered by its sluggish reaction kinetics and poor working/air stability. In this study, we demonstrate an electronic orchestration strategy via inner cation doping to construct NFS composites with carbon encapsulation toward boosted sodiation kinetics and durability. Ni²⁺ incorporation is theoretically and experimentally revealed to increase the electron conductivity and ion diffusivity of NFS by modulating charge redistribution and promote the formation of a uniform, stable, and NaF-rich interphase. A hollow carbon scaffold is of dual benefits for expedited electron-transfer kinetics by building intimate electrical contacts to NFS particles and enhanced electrode robustness by cushioning volume change upon cycling test. These synergistic advantages enable the optimized Ni-NFS/C cathode to deliver an exceptional capacity of 116.8 mAh g⁻¹ at 0.1 C, a high-rate retention of 76.6 mAh g⁻¹ at 20 C, and an unprecedented lifetime with 83.1% capacity preserved after 20 000 cycles. The practical viability of the composite is demonstrated by its excellent on-shelf air stability and superior performance in full cell.