Statistically Optimized Microwave Hydrothermal Na3V2(PO4)3/C: Process-Structure-Performance Mapping and Post-Mortem Degradation Diagnostics

Abstract

Scalable and lower-severity synthesis of Na₃V₂(PO₄)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.