Dendritic conductive carbon networks enhance Na⁺ transport in Na_2+2δFe_2-δ(SO₄)₃@C cathode for fast charging and wide temperature sodium-ion batteries

Iron-based polyanionic cathode materials for sodium-ion batteries are cost-effective alternatives to lithium iron phosphate due to their similar electrochemical mechanisms. However, previously reported materials often suffer from limited rate capacity, poor cycle life, suboptimal low-temperature performance, and inadequate gravimetric energy density. In this study, a Na_2.6Fe_1.7(SO₄)₃@C composite cathode with uniform dendritic conductive carbon networks was fabricated through a unique liquid-solid synergistic strategy. On the one hand, the composite cathode demonstrated enhanced electron conductivity, and stress-buffering capability, resulting in a high reversible discharge capacity (108.29 mAh g⁻¹), excellent cycling stability (∼80 % retention after 10,000 cycles), ultrafast-charging capability (up to 100 C), and gravimetric energy density exceeding 400 Wh kg⁻¹. On the other hand, enhanced Na⁺ diffusion dynamics enable more Na⁺ extraction from Na3 sites through structure-adaptive reconstruction mechanisms, further increasing the specific capacity. Notably, the composite cathode also showed stable performance across a broad temperature range (-25°C to 60°C), highlighting its environmental adaptability. Corresponding kilogram-scale Na_2.6Fe_1.7(SO₄)₃@C achieves 80.2 % retention after 8000 cycles at 20 C and delivers a high-rate capacity of 55.26 mAh g⁻¹ at 50 C. The Na_2.6Fe_1.7(SO₄)₃@C//HC full cell maintains 93.7 % capacity retention after 100 cycles at 1 C, highlighting its suitability for large-scale energy storage applications.