Harmonized Na-Site Occupancy and Fe Local Structure in a Mixed-Phosphate Cathode for All-Climate Sodium-Ion Batteries.

Abstract

Na4Fe3(PO4)2P2O7 (NFPP) has attracted considerable attention as a low-cost, structurally stable cathode for sodium-ion batteries. However, its commercialization is severely hampered by electrochemically inert impurities from conventional synthesis and insufficient energy density. Herein, we report a heteroatom-free strategy combining precursor pathway engineering with Na-site regulation and local Fe-environment modulation to synthesize a phase-pure NFPP-type cathode using a nominal Na3.67Fe2.67(PO4)1.67(P2O7) precursor stoichiometry. The resulting N3.67F2.67PP sample exhibits distinct Na-site occupancies from conventional NFPP. Theoretical calculations and experimental characterizations demonstrate that increased occupation ratios of Na4 and Na2 sites collectively elevate the discharge voltage plateau and enhance cycling stability. Meanwhile, precise regulation of the Na/Fe/PO4/P2O7 ratio modulates the electronic structure of Fe and increases Fe-O coordination disorder, markedly boosting Na+ migration kinetics and electronic conductivity. Consequently, the optimized material delivers a high specific capacity of 110.4 mAh g-1 at 0.1 C, retains 98.6% of its capacity after 1700 cycles at 1 C, and exhibits an impressive 103.5 mAh g-1 at 10 C. It also exhibits superior performance over a wide temperature range from -40 to 60 °C. Notably, Ah-level pouch and cylindrical batteries assembled with kilogram-scale materials achieve outstanding performance, highlighting a practical route toward commercialization. This work clarifies the structure-property relationship of NFPP-based materials and provides valuable guidance for designing high-performance mixed-phosphate cathodes.

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