Statistically Optimized Microwave Hydrothermal Na ₃ V ₂ (

ABSTRACT

Scalable and lower‐severity synthesis of Na ₃ V ₂ (PO ₄ ) ₃ /C (NVP/C) cathodes for sodium‐ion batteries (SIBs) remains limited by trial‐and‐error optimization of hydrothermal processing conditions. This practice is known to cause variations in crystal quality and poor reproducibility in electrochemical behavior. Microwave‐assisted hydrothermal synthesis has emerged as a faster and more efficient route, but the combined effects of synthesis temperature (T) and holding time (t) on phase evolution and electrochemical performance of NVP remain insufficiently understood from a statistical perspective. In this study, central composite design coupled with response surface methodology (CCD‐RSM) is used to systematically examine the T–t parameter space for NVP/C synthesis. The statistically identified optimum at 140°C for 20 min produces a mixed granular‐porous morphology encased in a semi‐graphitic carbon shell (~4.22 nm), achieved without dopants or conductive additives, at substantially lower thermal severity (140°C/20 min vs. 180°C–200°C/12–24 h for conventional hydrothermal routes). The optimized NVP (M‐140‐20✶) delivers 105.65 mAh g ⁻¹ at 0.1C, retains 99.7% capacity after 100cycles at 1C, and sustains 85.4% retention after 3500 cycles at 3C, competitive with chemically modified NVP systems requiring significantly higher synthesis energy. The NVP‖HC full cell achieves approximately 311 Wh kg ⁻¹ at 0.5C, while retaining a high energy efficiency of 93.7% at 1C. Kinetic analysis reveals mixed Na ⁺ storage, with b ‐values of 0.62–0.67 and increasing surface contribution at higher scan rates, while post‐mortem XRD, SEM, and HR‐TEM show degradation through micro‐cracking, carbon‐shell thinning, and separator‐fiber intrusion. These findings clarify synthesis‐structure‐performance‐degradation relationships in polyanionic cathodes.

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