Nanoarchitectonic\nApproach to Zinc-Substituted NaSICON\nNa<sub>4</sub>VMn<sub>1–<i>x</i></sub>Zn<sub><i>x</i></sub>(PO<sub>4</sub>)<sub>3</sub>/C as High-Rate Cathod

The NASICON-structured Na<sub>4</sub>VMn(PO<sub>4</sub>)<sub>3</sub> (NVMP) cathode material, known for its three-dimensional\nframework\nand high theoretical capacity, faces limitations due to its poor electronic\nconductivity. To enhance its practicality for sodium-ion battery (SIB)\napplications, we employed strategies such as coating with a thin conducting\ncarbon layer and doping Zn<sup>2+</sup> ions into the Mn<sup>2+</sup> sites of NVMP. These modifications aimed to improve the intrinsic\nproperties and reduce Jahn–Teller distortion effects. Zn or\nZn-based oxides, typically anode materials for SIBs, serve as structural\nsupport when doped into cathodes. We synthesized Na<sub>4</sub>VMn<sub>1–<i>x</i></sub>Zn<sub><i>x</i></sub>(PO<sub>4</sub>)<sub>3</sub>/C (Zn(x)-NVMP/C, where x = 0.00, 0.05, 0.10,\n0.15, 0.20) via a citric acid-assisted sol–gel method, followed\nby calcination at 750 °C under an Ar atmosphere. Characterizations\nusing XRD, XPS, HRTEM, and Raman spectroscopy confirmed the high purity\nof these materials. Among them, Zn(0.15)-NVMP/C exhibited the best\nperformance, with lower internal resistance and enhanced diffusion\nkinetics, achieving a stable capacity of 72 mAh g<sup>–1</sup> at 10C with 83% retention after 3000 cycles, and high rate tolerance\nup to 50C. Operando XRD studies revealed highly reversible two-phase\ncharge storage mechanisms, and <i>ex situ</i> XPS analysis\nconfirmed the charge storage mechanism of the cathode, while electrochemical\nimpedance spectroscopy (EIS) and cyclic voltammetry (CV) analyses\nconfirmed reduced polarization of the NVMP cathode after Zn doping.\nThese findings indicate that Zn(0.15)-NVMP/C is a promising cathode\nmaterial for SIB applications.