Effect of Niobium Doping on Electrochemical Properties of Microwave Synthesized Carbon Coated Nanolithium Iron Phosphate for High Rate Underwater Applications

Lithium iron phosphate (LiFePO₄) for lithium-ion batteries is considered as perfect cathode material for various military applications, especially underwater combat vehicles. For deployment at high rate applications, the low conductivity of LiFePO₄ needs to be improved. Cationic substitution of niobium in the native carbon coated LiFePO₄ is one of the methods to enhance the conductivity. In the present work, how the niobium doped solid solution could be formed is studied. Nanopowders of LiFePO₄/C and Li_1−xNb_xFePO₄/C (x = 0.05, 0.1, 0.15, 0.16) are synthesized from precursors using microwave synthesis. The solid solution formation up to (x = 0.15) Li_1−xNb_xFePO₄/C without impurity phases is confirmed by X-ray diffraction (XRD) pattern and Fourier transform infrared spectroscopic (FTIR) results. Particle distribution is obtained by scanning electron microscope from the synthesized powders. Energy dispersive X-ray spectrometer (EDS) results qualitatively confirmed the presence of niobium. Also, direct current (dc) conductivities are measured using sintered pellets and activation energies are calculated using Arrhenius equation. The dependence of conductivity and activation energy of LiFePO₄/C on variation of niobium doping is investigated in this study. CR2032 type coin cells are fabricated with the synthesized materials and subjected to cyclic voltammetry studies, rate capability and cycle life studies. Diffusion coefficients are obtained from electrochemical impedance spectroscopy studies. It is observed that room temperature dc conductivity improved by niobium doping when compared to LiFePO₄/C (0.379 × 10⁻² S/cm) and is maximum for Li_0.9Nb_0.1FePO₄/C (40.58 × 10⁻² S/cm). It is also observed that diffusion coefficient of Li⁺ in Li_0.9Nb_0.1FePO₄/C (13.306 × 10⁻⁹ cm² s⁻¹) improved by two orders of magnitude in comparison with the pure LiFePO₄ (10 − 12 cm² s⁻¹) and carbon-coated nano LiFePO₄/C (0.632 × 10⁻¹¹ cm² s⁻¹). Cells with Li_0.9Nb_0.1FePO₄/C are able to deliver useful capacity of around 104 mAh/g at 10 C rate. More than 500 cycles are achieved with Li_0.9Nb_0.1FePO₄/C at 20 C rate.