Rare-earth gradient lattice engineering enables ultra-fast charging sodium-ion batteries

Alluaudite-type iron-based sulfates are promising low-cost cathodes for sodium-ion batteries but suffer from poor ionic/electronic transport kinetics and interfacial degradation. Herein, we innovatively propose a rare-earth gradient lattice engineering strategy, synergistically improving both interfacial stability and bulk electrochemical behavior. By synthesizing the rare-earth (RE = Y, La, Ce, Pr, Nd, Gd, and Yb) modified Na_2.5Fe_1.75(SO₄)₃ (RE-NFS), an ingenious substitution strategy is proposed. We systematically examine the effect of RE ionic radius on lattice strain and structural stability. A systematic screening of rare-earth elements reveals an inverse correlation between dopant ionic radius and cycling stability, enabling the rational design of a rare-earth-ion gradient architecture that forms a Ce-enriched surface layer while preserving active sites in the bulk. Mechanistic studies show that Ce acts as a structural pillar to improve lattice reversibility and weakens the Na-O bond to accelerate charge transfer kinetics. The designed RE-NFS cathode delivers ultra-stable cycling performance (over 10,000 cycles at 5C) and an ultra-fast charge capability at 100C. An Ah-level pouch cell achieves a capacity retention of 87.5% after 1200 cycles at 1C. This gradient lattice engineering offers a new design paradigm for ultra-fast charging and long-life cathode materials in advanced sodium-ion batteries.