Iron‐Based Sodium‐Salt Composite Cathode for Sodium‐Ion Batteries

ABSTRACT The use of iron metal powder directly as the cathode active mass is a transformative opportunity to increase energy density and reduce costs for rechargeable batteries. Realizing this potential, however, requires creating a chemical environment that can activate multiple‐electron transfer of iron redox chemistry. Here, we report that the composite synthesized by ball milling Na ₂ SO ₄ , Fe metal, and CuS can be directly desodiated as a sodium‐ion battery cathode, which delivers a reversible discharge capacity of 149 mAh g ⁻¹ at ~2.6 V with stable cycling performance. Comprehensive spectroscopic characterization reveals that Fe serves as the primary redox center, while CuS functions as a redox mediator. The results demonstrate that the heterointerfaces formed by ball milling are critical for charge transfer and high iron utilization. This work establishes iron metal sodium‐salt composites as viable sodium‐ion battery cathodes by unlocking multi‐electron Fe redox within a composite architecture.