Structural and spectroscopic insights of medium-entropy Na2-uNi2+2-xNi3+x(Cr3+1-yCr4+y)1-wFe3+w(PO4)3 cathodes with sodium-ion insertion

Medium-entropy phosphate cathode materials with an α-CrPO₄-type structure exhibit promising potential for Na-ion insertion. Fe-substituted Na_2-uNi²⁺_2-xNi³⁺_x(Cr³⁺_1-yCr⁴⁺_y)_1-wFe³⁺_w(PO₄)₃ (where u = x + y and w = 0, 0.5, and 1) were synthesized via a modified Pechini method. Rietveld refinement of the X-ray diffraction patterns confirmed the stoichiometries Na_1.7Ni²⁺_1.9Ni³⁺_0.1Cr³⁺_0.8Cr⁴⁺_0.2(PO₄)₃, Na_1.6Ni²⁺_1.7Ni³⁺_0.3Cr³⁺_0.4Cr⁴⁺_0.1Fe³⁺_0.5(PO₄)₃ and Na_1.8Ni²⁺_1.8Ni³⁺_0.2Fe³⁺(PO₄)₃. Although structurally similar to the α-Na₂Ni₂Fe(PO₄)₃, these materials display distinct atomic site coordinates and occupancies, with transition metals M(1) and M(2) (M = Fe, Ni, Cr) statistically disordered across 4a and 8g sites. Electrochemical performance was evaluated within a moderate voltage range of 1.00–3.75 V vs. Na⁺/Na. Due to the initially low sodium content, with partial occupation of 4b sites and empty 4c sites, the first discharge cycle yielded specific capacities of 165.8, 188.9, and 205.34 mA h g⁻¹, with reversibility rates of 80.1, 84.1, and 90 %, respectively. Ex-situ X-ray diffraction patterns, X-ray photoelectron spectroscopy, and Cr and Ni–K edge X-ray absorption near edge structure analyses confirmed the occurrence of Na-ion insertion reactions, ruling out conversion reaction. The materials demonstrated reversible multi-electron reactions involving moderate activation of Cr⁴⁺/Cr³⁺/Cr²⁺, Fe³⁺/Fe²⁺/Fe⁰, and Ni³⁺/Ni²⁺/Ni⁰ redox couples while preserving the structure. Ultimately, Fe substitution effectively suppressed phase transitions and enhanced structural reversibility.