Nanoarchitectonic Approach to Zinc-Substituted NaSICON Na₄VMn_1–<i>x</i>Zn_<i>x</i>(PO₄)₃/C as High-Rate Cathodes for Sodium-Ion Batteries

The NASICON-structured Na4VMn(PO4)3 (NVMP) cathode material, known for its three-dimensional framework and high theoretical capacity, faces limitations due to its poor electronic conductivity. To enhance its practicality for sodium-ion battery (SIB) applications, we employed strategies such as coating with a thin conducting carbon layer and doping Zn2+ ions into the Mn2+ sites of NVMP. These modifications aimed to improve the intrinsic properties and reduce Jahn–Teller distortion effects. Zn or Zn-based oxides, typically anode materials for SIBs, serve as structural support when doped into cathodes. We synthesized Na4VMn1–xZnx(PO4)3/C (Zn(x)-NVMP/C, where x = 0.00, 0.05, 0.10, 0.15, 0.20) via a citric acid-assisted sol–gel method, followed by calcination at 750 °C under an Ar atmosphere. Characterizations using XRD, XPS, HRTEM, and Raman spectroscopy confirmed the high purity of these materials. Among them, Zn(0.15)-NVMP/C exhibited the best performance, with lower internal resistance and enhanced diffusion kinetics, achieving a stable capacity of 72 mAh g–1 at 10C with 83% retention after 3000 cycles, and high rate tolerance up to 50C. Operando XRD studies revealed highly reversible two-phase charge storage mechanisms, and ex situ XPS analysis confirmed the charge storage mechanism of the cathode, while electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) analyses confirmed reduced polarization of the NVMP cathode after Zn doping. These findings indicate that Zn(0.15)-NVMP/C is a promising cathode material for SIB applications.