Conversion of Industrial by‐Product Sulfates into Na _2.6 Fe _1.7 (SO ₄ )

ABSTRACT

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 _2.6 Fe _1.7 (SO ₄ ) 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. Mg ²⁺ adjusts the local coordination environment and electronic structure. Together, these factors lower the Na ⁺ diffusion barrier, suppress excessive contraction and relaxation of [Fe ₂ O ₁₀ ] 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.