Preparation of high-performance Na_2+2xFe_2-x(SO₄)₃ cathode material by combining sodium dodecyl sulfate with carbon nanotubes

As a new cathode material for sodium-ion batteries, Na_2+2xFe_2-x(SO₄)₃ has attracted significant attention due to its high working voltage and low preparation cost. However, its inherently low conductivity limits its practical application. To address this issue, we synthesized a high-capacity Na_2+2xFe_2-x(SO₄)₃/C/CNTs-S cathode material using ball milling, spray drying, and low-temperature sintering, with ascorbic acid and carbon nanotubes as composite carbon sources, and sodium dodecyl sulfate (SDS) as milling aid. With the introduction of SDS into the ball milling process, the prepared cathode material has uniform primary particle size (40–50 nm in diameter) and regular particle morphology, which improves the dispersion of carbon nanotube powder in the precursor solution and reduces its loss during subsequent synthesis. After spray drying, NFS precursor particles are closely connected to carbon nanotubes, and the effective presence of carbon nanotubes during sintering inhibits the excessive growth of NFS particles and greatly improves the electrochemical performance and conductivity of NFS. The test results showed that the prepared Na_2+2xFe_2-x(SO₄)₃/C/CNTs-S cathode material had a discharge-specific capacity of 101.3 and 78.8 mAh g⁻¹ at 0.05C and 5C, respectively. After 500 cycles at 2C, the capacity retention rate was 94.8%.