Mechanism of Carbon Nanotubes in Enhancing the Electrical Conductivity and High-Rate Performance of Na2.6Fe1.7(SO4)3/CNTs Cathode Materials

The Alluaudite-type Na2.6Fe1.7(SO4)3 cathode material used in sodium-ion batteries is limited by poor electronic conductivity and slow ionic migration, which restricts its high-rate charge-discharge performance. To confront these challenges, this study investigated various Na2.6Fe1.7(SO4)3/CNTs composite materials to elucidate how carbon nanotube composites modulate the local crystal structure, sodium storage mechanisms, and electrochemical performance. Kinetic analysis shows that introducing CNTs substantially reduces charge-transfer impedance and the apparent activation energy of Na+, thereby shifting the reaction from diffusion-controlled to pseudocapacitive behavior and greatly accelerating ion transport. In situ XRD confirms that highly reversible, single-phase, solid-solution reactions occur in all samples. The Na2.6Fe1.7(SO4)3/SWCNTs composite, containing only 1.2 wt % SWCNTs, exhibits the smallest change in unit cell volume and the greatest structural elasticity. This composition achieves a reversible specific capacity of 95.01 mAh·g-1 at 0.1C and a rate performance of 82.37 mAh·g-1 at 50C. Overall, this study reveals the key role of CNTs in facilitating synergistic electron and ion transport and stabilizing the crystal structure, providing guidance for the design of high-power sodium-ion battery cathodes.