From insulator to semiconductor: carbon nanotubes enhancing the electrochemical performance of Na2.5Fe1.75(SO4)3 cathode materials for sodium ion batteries

Abstract

Sodium iron sulfate (Na_2+2xFe_2−x(SO₄)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 an insulator and cannot reversibly store sodium ions to a large extent. To address this problem, an 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 material incorporates an alluaudite-type framework that improves electronic conductivity, thereby enhancing 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 an impressive high-rate specific capacity of 80 mA h g⁻¹ at a rate of 20C. Furthermore, it can retain 81.4% of the initial specific capacity after 20 000 cycles at a rate of 50C, demonstrating an outstanding cycling stability. This work not only provides an in-depth understanding of the potential of NES 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.