Sustainable Upcycling of Industrial By‐Products Into High‐Rate and Stable Sulfate‐Based Cathode for Sodium‐Ion Batteries

ABSTRACT Transforming industrial by‐products into high‐value materials is crucial for sustainable development. The high‐value conversion of industrial sodium sulfate (Na ₂ SO ₄ ) by‐product into a high‐performance Na _2.56 Fe _1.72 (SO ₄ ) ₃ /carbon (NFSO/KB) composite cathode material for sodium‐ion batteries (SIBs) is studied. The cathode with a robust skeleton composed of [Fe ₂ O ₁₀ ] dimers and [SO ₄ ] tetrahedra ensures excellent structural stability, and the unique three‐dimensional (3D) framework facilitates rapid Na ⁺ diffusion. Theoretical and structural analyses reveal that NFSO/KB possesses an ultralow Na ⁺ migration barrier and a highly reversible reaction mechanism with minimal volume change (1.21%) during cycling. As a result, the cathode delivers a reversible capacity of 84.26 mAh g ⁻¹ at 10 mA g ⁻¹ , with a high operating voltage of 3.8 V (vs. Na/Na ⁺ ). Moreover, it demonstrates a remarkable rate capability of 55.77 mAh g ⁻¹ at 1000 mA g ⁻¹ and exceptionally long cycle life (89.13% capacity retention after 3000 cycles). This work establishes a new paradigm for industrial by‐products upcycling and provides a promising design strategy for low‐cost, high‐rate, and long‐lifespan cathodes for SIBs.