Alkali-Site Lithium Doping Enables a High-Performance Na3Fe2(PO4)(P2O7) Cathode for Sodium-Ion Batteries

Abstract

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 Na2.95Li0.05Fe2(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.