Precursor Size Governs Thermally Activated Solid State Conversion Into the Redox Active Framework Formation in Na 4 Fe 3 (PO 4 ) 2 P 2 O 7

ABSTRACT Na 4 Fe 3 (PO 4 ) 2 P 2 O 7 is an attractive Fe‐based polyanionic cathode for low‐cost sodium‐ion batteries, but scalable synthesis remains constrained by the trade‐off between precursor homogeneity and process compatibility. Soluble Fe salts enable intimate mixing, whereas Fe (II)‐phosphate precursors avoid nitrate/oxalate burdens but react as insoluble domains. Here we identify precursor defined solid state reaction length as the key variable governing NFPP formation from Fe 3 (PO 4 ) 2 . Reaction–diffusion simulations predict that oversized Fe 3 (PO 4 ) 2 domains retain under converted interiors, while low submicrometre domains enter a conversion‐saturated regime. Experiments using size‐controlled precursors confirm that this length scale regulates competing NaFePO 4 and Na 2 FeP 2 O 7 formation, Fe 2+ /Fe 3+ local chemistry and Na + transport kinetics. A CV precursor captures most of the kinetic and electrochemical benefit of stronger refinement, delivering 104.1 mAh g −1 at 0.1 C and 61.8 mAh g −1 at 20 C, together with stable long‐term and full‐cell operation. These findings establish reaction length scale control as a practical design rule for phosphate compatible NFPP synthesis.