Alkali-site lithium doping enables a high-performance Na 3 Fe 2 (PO 4 )(P 2 O 7 ) cathode for sodium-ion batteries

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.