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

• Published in: Surface engineering and applied electrochemistry Vol. 53; no. 6; pp. 515 - 524
• Main Authors: Satyavani, T. V. S. L., Srinivas Kumar, A., Subba Rao, P. S. V.
• Format: Journal Article
• Language: English
• Published: Moscow Pleiades Publishing 01.11.2017; Springer Nature B.V
• Subjects: Activation energy; Coating effects; Conductivity; Diffusion coefficient; Diffusion effects; Doping; Electrical resistivity; Electrochemical analysis; Electrochemical impedance spectroscopy; Electrode materials; Engineering; Flux density; High impedance; Iron; Lithium; Lithium-ion batteries; Low conductivity; Machines; Manufacturing; Mathematical analysis; Processes; Sintering (powder metallurgy); Synthesis; Vanadium; lithium iron phosphate; high rate discharge; high rate applications; solid state vacuum synthesis; dc conductivity; ac impedance; diffusion coefficient
• ISSN: 1068-3755; 1934-8002
• DOI: 10.3103/S1068375517060102

Lithium iron phosphate (LiFePO 4 ) 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 LiFePO 4 /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 LiFePO 4 /C. Nano powders of LiFePO 4 /C and Li 1– x V x FePO 4 /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– x V x FePO 4 /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 LiFePO 4 /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 LiFePO 4 /C (2.141 × 10 –2 S cm –1 ) and it was maximum for Li0.85V0.15FePO 4 /C (22.201 × 10 –2 S cm –1 ). It was also observed that the diffusion coefficient of Li+ in Li0.85V0.15FePO 4 /C (12.89 × 10 –9 cm 2 s –1 ) improved by two orders of magnitude as compared to that of pure LiFePO 4 (10 –12 cm2 s–1) and carbon coated nano LiFePO 4 /C (0.626 × 10 –11 cm 2 s –1 ). Cells with Li 0.85 V 0.15 FePO 4 /C are able to deliver useful capacity of around 101 mAh g –1 at 10C rate. More than 1000 cycles were achieved with Li 0.85 V 0.15 FePO 4 /C at 20C rate.