The sodium superionic conductor (NASICON)-type cathode, Na₃V₂(PO₄)₃ (NVP), is considered as a promising cathode material for sodium-ion batteries (SIBs), which offers stable sodium storage capability. However, hazardous and expensive vanadium (V) has limited its practical application. To reduce the V dependency in NASICON-type cathodes, two new NASICON-structured materials, Na₃VMg_0.5Ti_0.5(PO₄)₃ (N_3.0VMTP/C) and Na_3.5V_0.5MgTi_0.5(PO₄)₃ (N_3.5VMTP/C), were designed for cost-effectiveness as well as improvement of battery performance. N_3.0VMTP/C and N_3.5VMTP/C provided a sodium storage capacity of 155.84 mAh g⁻¹ and 105 mAh g⁻¹ at 12 mA g⁻¹ with 88 % and 84 % capacity retention after 500 cycles at 150 mA g⁻¹, respectively. In-situ XRD analysis revealed that both cathodes undergo a progressive solid solution reaction in the lower voltage region and two-phase reaction at higher voltages during (de)sodiation, with only minor difference in the degree of lattice displacement, confirming their high potential for the SIBs with sustainable and cheaper Mg for grid-scale utilization.

In this study, a pseudo-layered Na super-ionic conductor of Na₃V₂(PO₄)₂F₃ (NVPF)/C cathode for sodium-ion batteries is prepared successfully using a facile polyol refluxing process without any impurity phases. The X-ray diffraction and Rietveld refinement results confirm that NVPF possesses tetragonal NASICON-type lattice with a space group of P₄₂/mnm. In this preparative method, polyol is utilized as a solvent as well as a carbon source. The presence of nanosized NVPF particles in the carbon network is confirmed by field-emission scanning electron microscopy (FE-SEM) and high-resolution transmission electron microscopy (HR-TEM). The existence of carbon is analyzed by Raman scattering and elemental analysis. When applied as a Na-storage material in a potential window of 2.0–4.3 V, the electrode exhibits two flat voltage plateaus at 3.7 and 4.2 V with an electrochemically active V³⁺/V⁴⁺ redox couple. In addition, Na₃V₂(PO₄)₂F₃/C composite achieved a retention capacity of ~ 88% even after 1, 500 cycles at 15 C. Moreover, at high current densities of 30 and 50 C, Na₃V₂(PO₄)₂F₃/C cathode retains the specific discharge capacities of 108.4 and 105.9 mAh·g^–1, respectively, revealing the structural stability of the material prepared through a facile polyol refluxing method.