Probing the Compositional and Structural Effects on the Electrochemical Performance of Na(Mn‐Fe‐Ni)O2 Cathodes in Sodium‐Ion Batteries

ABSTRACT This study systematically investigates an Mn‐Fe‐Ni pseudo‐ternary system for Na(Mn‐Fe‐Ni)O 2 cathodes, focusing on the effects of varying transition metal fractions on structural and electrochemical properties. X‐ray diffraction reveals that increasing Mn content induces biphasic behavior. A higher Ni content reduces the c parameter, while higher Mn and Fe concentrations expand the lattice. Average particle size increases with an increase in Mn content and Fe/Ni ratio. NaMn 0.500 Fe 0.125 Ni 0.375 O 2 delivers a high specific capacity of ~149 mAh g⁻¹ in the 2.0–4.0 V range. Galvanostatic charge‐discharge and dQ/dV versus V curves suggest that a Ni/Fe ratio > 1 enhances specific capacity and lowers voltage polarization in the materials. NaMn 0.500 Fe 0.250 Ni 0.250 O 2 demonstrated the best rate performance, exhibiting 85.7% capacity at 1C and 69.7% at 3C, compared to 0.1C, while biphasic NaMn 0.625 Fe 0.125 Ni 0.250 O 2 (MFN‐512) excelled in cyclic stability, retaining 93% of capacity after 100 cycles. The performance of MFN‐512 in a full cell configuration was studied with hard carbon as the anode, resulting in a specific capacity of ~92 mAh g −1 and a nominal voltage of ~2.9 V at a 0.1C rate, demonstrating its potential in practical applications. Transmission electron microscopy confirmed the biphasic nature of MFN‐512, with columnar growth of P2 and O3 phases. Electrochemical impedance spectroscopy revealed that better‐performing samples have lower charge transfer resistance. Operando Synchrotron XRD reveals reversible phase transformations in MFN‐512, driven by its optimized transition metal ratios and phase fraction. This work outlines a systematic approach to optimizing low‐cost, high‐performance Mn‐Fe‐Ni layered oxides.