Tunnel-Structured Na4Mn7.2Cr1.8O18 Metal Oxide as a Potential Cathode for Energy Storage Application in Sodium-Ion Batteries

Abstract

This study presents the partial Cr-doping in Na4Mn9O18 to synthesize tunnel-structured Na4Mn7.2Cr1.8O18 through a conventional solid-state method, followed by comprehensive physicochemical and electrochemical characterizations as a cathode for sodium-ion batteries (SIBs). The partial substitution of Mn3+ with Cr effectively reduced the Jahn-Teller distortions and stabilized the material for efficient Na+ ion diffusion. Detailed physicochemical investigations, including thermogravimetric analysis (TGA), derivative thermogravimetry (DTG), X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), inductively coupled plasma optical emission spectroscopy (ICP OES), Raman spectroscopy, energy-dispersive spectroscopy (EDS), transmission electron microscopy (TEM), selected area electron diffraction (SAED), high-resolution TEM (HRTEM), and X-ray photoelectron spectroscopy (XPS) studies, confirmed the phase purity, uniform rod-like morphology, desired stoichiometry, and favorable bonding environment of Na4Mn7.2Cr1.8O18. Its electrochemical characterizations in a sodium half-cell configuration revealed charge and discharge specific capacities of ∼105 and 74 mAh/g, respectively, at 0.05C within 2.0-4.0 V and about 76% capacity retention after 200 cycles at 0.2C. Cyclic voltammetry (CV) demonstrated a high redox activity, Na+ ion diffusion, and pseudocapacitive behavior. Electrochemical impedance spectroscopy (EIS) and distribution of relaxation times (DRT) studies revealed low resistances due to various physical processes and stable charge transfer resistance before and after cycle life testing, alongside a low double-layer capacitance, representing robust interfacial charge migration. These results support Na4Mn7.2Cr1.8O18 as a potential cathode for the fabrication of sustainable SIBs. © 2025 American Chemical Society.