Expanding the Fe–Fe distance for superior electrochemical performance of Na2.6Fe1.7(SO4)3 cathodes for sodium-ion batteries

Abstract

The alluaudite-type Na_2+2xFe_2−x(SO₄)₃ (NFS) has attracted attention as a promising cathode material for sodium-ion batteries, due to its remarkable working voltage and cost-effectiveness among various cathode candidates. However, the short Fe–Fe distance in the structure induces Fe–Na exchange that leads to local structural distortions and detrimental effects on electrochemical performance. In this work, we synthesized a series of Cu substituted Na_2.6Fe_1.7−xCu_x (SO₄)₃ (x = 0–0.6) via a spray-drying technique. Compared to the pristine sample (Fe–Fe distance: 3.19 Å), the optimized composition exhibits an extended Fe–Fe distance of 3.30 Å as confirmed by X-ray absorption fine structure (XAFS) characterization. Benefiting from this enlarged Fe–Fe distance, the optimized sample shows a high discharge capacity (100.49 mAh g⁻¹ at 0.1 C), excellent rate performance (∼76.4 mAh g⁻¹ at 30 C) and cycle stability over 5000 cycles (capacity retention of 80%). In situ XRD results demonstrated that the Cu doping strategy effectively mitigates the variation in lattice volume. This work provides a reference for improving the electrochemical properties of NFS and demonstrates the potential of NFS materials.