Vanadium-doped hexagonal MoO3: Structural and electrochemical characterization for aluminium-ion battery applications

Vanadium-doped hexagonal molybdenum trioxide (h-MoO3) has been systematically investigated as a cathode material for aluminium-ion batteries (AIBs). The evolution of the structural, morphological, compositional, optical, and electrochemical properties of h-MoO3 doped with different vanadium concentrations were analysed by X-ray diffraction (XRD), micro-Raman spectroscopy, Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), high resolution transmission microscopy (HRTEM), SEM and TEM-energy-dispersive X-ray microanalysis (EDS), X-ray photoelectron spectroscopy (XPS), UV-Vis optical absorption and electrochemical techniques. Moderate vanadium doping maintains the hexagonal structure of the oxide host and does not adversely affect the crystallinity of the samples, while inducing morphological changes and local lattice distortions. Optical measurements revealed a significant reduction in the band gap by increasing the dopant concentration, suggesting enhanced electronic conductivity. Electrochemical studies demonstrated that vanadium incorporation improves charge transfer kinetics and cycling stability, with an optimal doping level corresponding to a V/Mo atomic ratio of 0.16, yielding a high specific capacity of ∼240 mA h g⁻¹ at 100 mA g⁻¹ over 100 cycles. However, an excessive vanadium content led to secondary phase formation, structural degradation, non-homogeneous dopant spatial distribution, and decreased electrochemical performance. Ex-situ SEM-EDS and Raman analysis confirmed the excellent structural stability of vanadium-doped h-MoO3 upon cycling, with uniform chloroaluminate species intercalation. These findings establish vanadium doping as an effective strategy to enhance h-MoO3 for AIB applications, providing a balance between enhanced conductivity, electrochemical stability, and structural integrity.