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

Abstract

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.