Synergistic Impact of Vanadium Doping on the Structural and Electrochemical Performance of O3‐NaMnO ₂ as Sodium‐Ion Battery Cathode Material

ABSTRACT Vanadium‐doped sodium manganese oxides (O3‐NaMnO 2 ) are promising cathode materials for sodium‐ion batteries (SIBs) due to their structural stability and enhanced electrochemical performance. This study systematically investigates the effects of V doping ( x = 0.03–0.50) on the structural, morphological, and electrochemical properties of NaMnO 2 synthesized via a solid‐state method. X‐ray diffraction (XRD) confirms that low V doping ( x ≤ 0.10) stabilizes the layered O3 structure and suppresses Jahn–Teller (JT) distortions, while higher doping induces a secondary Na 0.5 VO 2 phase. Scanning and transmission electron microscopy (SEM/TEM) reveal that moderate V substitution ( x = 0.10) improves particle uniformity and Na‐ion transport. Electrochemical measurements show that both x = 0.03 and x = 0.10 exhibit good performance; however, x = 0.03 provides the best overall balance of high capacity, rate capability, and cycling stability, delivering initial charge and discharge capacities of 211 and 124 mAh/g, respectively. Redox and impedance analyses indicate reduced charge‐transfer resistance and enhanced Na‐ion kinetics at this composition. In contrast, excessive V doping ( x ≥ 0.30) causes structural degradation and capacity fading. These results highlight the importance of controlled V doping in optimizing NaMnO 2 ‐based cathodes for high‐performance SIBs.