From insulator to semiconductor：carbon nanotubes enhancing the electrochemical performance of Na_(2.5) Fe_(1.75) (SO_4)_3 cathode materials for sodium ion batteries

Abstract

Sodium iron sulfate (Na_(2+2x) Fe_(2-x) (SO_4 )_3, NFS) is a promising cathode material for sodium-ion batteries (SIBs) for large-scale energy storage systems due to its low cost, and long life. However, NFS is the insulator and cannot reversibly store sodium ions largely. To address this problem, NFS material integrated with carbon nanotubes (CNTs) is developed in this work. The incorporation of CNTs effectively converts NFS from an insulator to a semiconductor, thereby significantly enhancing the electrochemical performance. The NFS@CNTs incorporates an alluaudite-type framework that improves electronic conductivity, therefore enhanced rate capability and cycling life. By introducing 10 wt% CNTs, the NFS@CNTs composite achieves a high working voltage of 3.8 V (vs. Na/Na⁺) and impressive high-rate specific capacities of 80 mAh g⁻¹ at a rate of 20 C. Furthermore, it can retain 81.4% of the initial specific capacity after 20,000 cycles at a rate of 50 C, demonstrating an outstanding cycling stability. This work not only provides an in-depth understanding of NFS’s potential to be used as a high-rate, long-lasting cathode material for SIBs, but also lays the foundation for SIBs in large-scale energy storage applications.