Yolk‐Shell Sodium Iron Sulfate@Carbon for Advanced Sodium Storage with Enhanced Capacity and Stability

Abstract Sodium iron sulfates (NFS) have garnered immense scientific and industrial interest as the cost‐effective and high‐voltage cathode materials of sodium‐ion batteries. However, the sodium storage properties of NFS are far beyond the theoretical threshold, which remains a formidable challenge. Herein, a yolk‐shell NFS@carbon hollow spheres (NFS@CHS) architecture for effectively expediting the electrochemical kinetics and stability is demonstrated. Hierarchically porous carbon hollow spheres are screened out to sufficiently encapsulate NFS nanosized particles by strong capillary confinement, thus constructing highly conductive electron/ion transportation expressways for enhanced electrode reactivity and stability. Impressively, the yolk‐shell composite achieves a record‐high reversible capacity of 120.2 mAh g⁻¹ at 0.1 C, excellent rate retention of 65 mAh g⁻¹ at 20 C, along with a remarkable working lifespan over 12 000 cycles. The kinetic‐enhanced reaction mechanism and its potential application of NFS@CHS in full cells are systematically studied. This work will afford an insightful perspective for rational design and synthesis of high‐performance NFS‐based materials toward commercial practice.