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The development of high-performance, sustainable sodium-ion batteries requires a mechanistic understanding of cathode redox processes and structural stability. Na₄Fe₃(PO₄)₂(P₂O₇) (NFPP) is a promising cathode material due to its non-toxicity, high average working voltage, and excellent structural and thermal stability. However, its practical application is hindered by impurity phase formation and low intrinsic electronic conductivity. To address these challenges, Na₄Fe_3-xM_x(PO₄)₂(P₂O₇) (M: Mn, Ni) composites are synthesized via a sol-gel method. A comprehensive characterization approach combining X-ray diffraction (XRD), X-ray absorption spectroscopy (XANES/EXAFS, soft XAS), and resonant inelastic X-ray scattering (RIXS) revealed that low-level substitution of Mn²⁺ and Ni²⁺ into Fe sites suppresses the formation of electrochemically inactive maricite NaFePO₄ and modifies the Fe-O coordination environment. These effects may result in lower ion migration energy barriers and better electrochemical reversibility. Among the doped samples, Mn-NFPP exhibited the best electrochemical performance, delivering a discharge capacity of ∼92 mAh g⁻¹ at 0.1 C and ∼80 mAh g⁻¹ at 2 C, with 99.5 % capacity retention after 100 cycles at 0.1 C. This work provides fundamental insights into the redox mechanism and atomic-scale structure–property relationship of NFPP, guiding the design of high-performance polyanionic cathodes for sodium-ion batteries.