Effect of Al substitution on the enhanced electrochemical performance and strong structure stability of Na₃V₂(PO₄)₃/C composite cathode for sodium-ion batteries

In this study, a promising cathode material in Na-ion batteries, Al-doped NASICON-type Na₃V_2-xAl_x(PO₄)₃/C (0≤ × ≤0.03) samples are synthesized and characterized. The doping effects on the crystal structure are investigated by XRD and XPS, indicating that low dose of Al³⁺ doping generates no damage on the structure of the material, and aluminum is substituted for the vanadium site successfully. Electron microscopy and Raman data show that amorphous carbon coated on the matrix can enhance the electron conductivity. The electrochemical kinetic response of Al³⁺ doping are tested based on “slow-charge and rapid-discharge” electrochemical mode, results from before and after the cycles show that the doping samples have strong structure stability and excellent electrochemical performance because of low internal resistances and high ion conductivity. Thus, Na₃V_1.98Al_0.02(PO₄)₃/C exhibits an initial reversible capacity of 102.7 mAh g⁻¹ at 10 mA g⁻¹ in the potential range between 2.3 and 3.8 V and delivers a discharge value of 95 mAh g⁻¹ vs. 59.9 mAh g⁻¹ of NVP/C at current density of 70 mA g⁻¹ discharge after 50 cycles. The ionic conductivity of Na₃V_1.98Al_0.02(PO₄)₃/C sample at 3.4 V after 50 cycles at 10 mA g⁻¹ charge 200 mA g⁻¹ discharge is 1.31 × 10⁻¹² cm²s⁻¹, four orders of magnitude higher than the undoped one(7.79 × 10⁻¹⁷ cm²s⁻¹).