In situ regeneration engineering enables low-cost and high-rate Na2+2xFe2−x(SO4)3 cathodes from industrial byproducts

Alluaudite-type Na 2+2x Fe 2−x (SO 4 ) 3 is considered as a promising cathode for large-scale energy storage due to its high structural stability and cost-effectiveness. However, due to low theoretical specific capacity, NFS still exhibits insufficient overall cost competitiveness, and it also suffers from low intrinsic electrical conductivity. Here, we propose a direct regeneration strategy based on industrial byproducts to prepare a low-cost and high-conductivity Na 2.56 Fe 1.65 Mg 0.05 Mn 0.014 Al 0.006 (SO 4 ) 3 cathode. Compared to traditional synthetic routes for pure compounds, the direct degeneration of industrial byproducts can reduce iron source costs by approximately 83%. In addition, Mg, Mn and Al impurities from industrial byproducts are in situ doped into the polyionic framework, achieving functionalized utilization. Mg/Mn/Al co-doping not only induces strong interactions with oxygen to stabilize the crystal structure, but also decreases Na + migration energy barriers and enhances the electrical conductivity. Na 2.56 Fe 1.65 Mg 0.05 Mn 0.014 Al 0.006 (SO 4 ) 3 exhibits low lattice strain and uniform ion concentration distribution during electrochemical processes, demonstrating excellent rate capability and cycling stability. It shows 100% capacity retention after 1500 cycles at 3 C, and delivers a high discharge capacity of 50.16 mAh g −1 at 30 C. This study provides a scalable route for the valorization of industrial waste and the development of low-cost, high-performance polyanionic cathodes for sodium-ion batteries.