Conversion of Industrial by-Product Sulfates into Na_2.6Fe_1.7(SO₄)₃ Cathode Material Suitable for Sustainable...

Green and high-value use of industrial by-products is crucial for sustainable development. However, impurity elements in these by-products severely limit their high-value applications. Traditional trial-and-error methods for screening Na_2.6Fe_1.7(SO₄)₃ (NFS) dopants to improve conductivity and structural stability are challenging and inefficient. This paper proposes a first-principles (DFT) driven strategy for precise doping design with experimental closed-loop verification. By calculating the doping formation energy and bandgap of element-doped NFS, and combining these with experimental data, K and Mg in by-product sulfates were quickly identified as the optimal dopants. The resulting NFS-K/Mg cathode delivers a reversible capacity of 91.8 mAh·g^-1 at 0.1C, a high-rate capacity of 85.0 mAh·g^-1 at 10C, and 84.3% capacity retention after 500 cycles at 1C in a full cell. Theoretical calculations and in situ XRD indicate that K⁺ occupies Na⁺ sites and widens ion diffusion channels. Mg²⁺ adjusts the local coordination environment and electronic structure. Together, these factors lower the Na⁺ diffusion barrier, suppress excessive contraction and relaxation of [Fe₂O₁₀] dimers under high voltage, and keep cell volume change to only 2.95% during charge and discharge, greatly improving crystal structure stability. This work provides a new approach for the high-value utilization of two industrial by-product sulfates.