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

This study systematically investigates an Mn-Fe-Ni pseudo-ternary system for Na(Mn-Fe-Ni)O₂ 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi>c</mml:mi></mml:math> 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_0.500Fe_0.125Ni_0.375O₂ delivers a high specific capacity of ~149 mAh g^-1 in the 2.0-4.0 V range. Galvanostatic charge-discharge and <i>dQ/dV</i> versus V curves suggest that a Ni/Fe ratio > 1 enhances specific capacity and lowers voltage polarization in the materials. NaMn_0.500-Fe_0.250Ni_0.250O₂ demonstrated the best rate performance, exhibiting 85.7% capacity at 1C and 69.7% at 3C, compared to 0.1C, while biphasic NaMn_0.625Fe_0.125Ni_0.250O₂ (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. <i>Operando</i> 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.