Capillary-confined sodium iron sulfate by porous carbon skeleton for reinforced sodiation kinetics and longevity

Alluaudite-type sodium iron sulfate (Na_2+2xFe_2−x(SO₄)₃, NFS) is known as a commercially promising cathode material of sodium-ion batteries due to its high working voltage and low-price production. However, the poor electrical conductance and structural instability of NFS significantly retard its scale-up practical pace. Herein, we propose a design strategy to reinforce the sodiation kinetics and longevity of NFS by capillary-confining with a porous tube-like carbon skeleton. The carbon functions as a fishing network for anchoring NFS particles, which remarkably promotes the electrical conductivity and mechanical stability of NFS. The unique capillary-confined composite establishes fast electron/ion transport highways for boosted reaction kinetics and a resilient architecture for electrode integrity. Impressively, the composite delivers an exceptional capacity of 105.2 mAh g⁻¹ at 0.1 C and sustains excellent rate capability and cycling stability (96.1% capacity retention after 1000 cycles). The dynamic impedance evolution across the electrode is first systematically elucidated by in situ analyzing distribution of relaxation/capacitive times. This study affords an insightful electrochemical diagnostics for designing advanced NFS-based cathodes toward high-efficiency sodium-ion storage.