Alkali-site lithium doping enables a high-performance Na₃Fe₂(PO₄)(P₂O₇) cathode for sodium-ion batteries

Iron-based polyanionic Na₃Fe₂(PO₄)(P₂O₇) (N3F2PP) has emerged as a compelling cathode candidate for sodium-ion batteries (SIBs) owing to its cost-effectiveness and structural stability. Nevertheless, its practical application is constrained by intrinsically sluggish Na⁺ diffusion kinetics and limited accessible capacity. Herein, a trace Li substitution strategy at alkali sites was proposed to address these limitations. Partial substitution of Na⁺ with smaller Li⁺ induces localized lattice contraction and optimizes the local structural environment, thereby promoting Na⁺ transport. Moreover, Li incorporation subtly modifies the local Fe-O coordination environment, alleviating the lattice strain associated with the Fe²⁺/Fe³⁺ redox reaction. As a result, the optimized Na_2.95Li_0.05Fe₂(PO₄)(P₂O₇)@C (NLF2PP@C-0.05) cathode delivered a remarkable discharge capacity of 113.2 mAh g^-1 at 0.1 C, along with outstanding rate capability (84.1 mAh g^-1 at 20 C) and cycling stability (97.5% capacity retention after 2000 cycles at 20 C). This work highlights alkali-site substitution as a novel design paradigm for polyanionic cathodes, offering an effective complement to conventional Fe-site engineering strategies.