Effect of vanadium doping on electrochemical properties of carbon coated nano lithium iron phosphate for high rate applications

Lithium iron phosphate (LiFePO₄) is a promising electrode material for the lithium ion battery technology as it has the potential to meet the requirements of the high energy density and power density applications. However, its limitations such as low conductivity and a low diffusion coefficient lead to high impedance due to which its application is restricted. The cationic substitution of vanadium in the native LiFePO₄/C is one of the methods to enhance the conductivity. Here the work is focused on obtaining a cathode material with improved electrical performance by doping it with different compositions of vanadium in the lithium site of LiFePO₄/C. Nano powders of LiFePO₄/C and Li_1–xV_xFePO₄/C (x = 0.05, 0.1, 0.15–0.2) were synthesized from precursors via the solid state vacuum synthesis. The solid solution formation up to Li_1–xV_xFePO₄/C (x = 0.2) without impurity phases is confirmed by the XRD and FTIR results. The particles distribution of synthesized powders was followed by the SEM. Also, dc conductivities are measured on sintered pellets and activation energies are calculated using the Arrhenius equation. The dependence of conductivity and activation energy of LiFePO₄/C on variation of vanadium doping is also investigated in this study. The CR2032 cells were fabricated and subjected to cyclic voltammetry studies, charge-discharges at different C-rates and cycle life. The electrochemical impedance spectroscopy was used to obtain the ac impedance and diffusion coefficients, and the effect of doping on these parameters is discussed. It was observed that the room temperature dc conductivity was improved by vanadium doping when compared to LiFePO₄/C (2.141 × 10^–2 S cm^–1) and it was maximum for Li0.85V0.15FePO₄/C (22.201 × 10^–2 S cm^–1). It was also observed that the diffusion coefficient of Li+ in Li0.85V0.15FePO₄/C (12.89 × 10^–9 cm² s^–1) improved by two orders of magnitude as compared to that of pure LiFePO₄ (10^–12 cm2 s–1) and carbon coated nano LiFePO₄/C (0.626 × 10^–11 cm² s^–1). Cells with Li_0.85V_0.15FePO₄/C are able to deliver useful capacity of around 101 mAh g^–1 at 10C rate. More than 1000 cycles were achieved with Li_0.85V_0.15FePO₄/C at 20C rate.