<p>Due to the depletion and rising cost of lithium resources, sodium-ion batteries (SIBs) have emerged as a promising alternative for large-scale energy storage. However, layered oxide cathodes for SIBs suffer from the structural instability followed by rapid capacity fading. In this study, Ni_0.85Fe_0.10Mn_0.05(OH)₂ precursors, synthesized via co-precipitation, were sodiated at various temperature range from 600 °C to 750 °C to fabricate NaNi_0.85Fe_0.10Mn_0.05O₂ cathodes. The effects of sodiation temperature on structural stability and electrochemical performance were systematically investigated. The cathode sodiated at 700 °C exhibited uniform and spherical secondary particles with an optimal porous structure, leading to the highest initial discharge capacity of 197 mAh g^-1 and excellent rate capability. In contrast, the cathode sodiated at 650 °C delivered superior capacity retention albeit a slightly lower initial capacity, highlighting the importance of a balanced microstructure and crystallinity for the battery performance. Our work underscores that deliberate control of sodiation temperature is required to optimize the phase stability and electrochemical properties of layer-structured cathodes, thereby offering insights for the development of next-generation SIB cathode materials.</p> Graphical Abstract <p></p>