Doping, Alloying And Carbon-Coating Of LiFePO₄ Cathode Materials

LiFePO₄ has been extensively studied due to its safety, environmental compatibility, low cost, relatively high specific capacity and long cycle life. However, low electric conductivity and volumetric energy density are two main obstacles for the use of LiFePO₄ as a commercial cathode material. In the present work, conductivity of LiFePO₄ is improved by doping, alloying, carbon coating with different carbon sources.Li_1-xM_xFePO₄/C (M = Nb, Mg; x =0, 0.005, 0.01, 0.015, 0.02) were synthesized by one step solid state reaction at 600 °C for 10 h using LiOH·H₂O, FePO₄·4H₂O, polypropylene, Nb₂O₅ and MgO as reactants. It is found from high resolution transmission electron microscopy (HRTEM) observations that carbon films were homogeneously coated on the surface of the Li_1-xM_xFePO₄ particles and carbon webs between the particles were formed during the synthesis. Elemental mapping by HRTEM demonstrates that Nb is homogeneously distributed in particles. Electrochemical tests show that the capacity of Li_0.99Nb_0.01FePO₄/C reaches 140 mAh/g at 1 C, which is 20 mAh/g higher than that of the undoped sample. But the capacity decreases as more than 1.0 mol% Nb was doped. The capacity of LiFePO₄ could be also improved by doping with Mg²⁺, but less significantly than Nb⁵⁺ doping. The discharge capacity of Li_0.99Nb_0.01FePO₄/C is 10 mAh/g higher than that of Li_0.99Mg_0.01FePO₄/C at 1C. It is considered that the occupancy of the doped high valent ions at the Li⁺-sites leads to Li⁺-vacancies in the crystals to sustain the electronic neutrality, which improves the electric conductivity of LiFePO₄ consequently.LiFe_1-xV_xPO₄/C was synthesized by alloying solid state reaction. X ray diffraction (XRD) analysis indicates that the substitution of vanadium for iron decreases the cell parameter in a-axis when x= 0.02, 0.04, 0.06. It is found that Li₃V₂(PO₄)₃ phase occurs when the amount of vanadium exceeds 10 mol%, which forms a LiFePO₄/Li₃V₂(PO₄)₃ composite cathode material. Images of TEM show that the composites are of spherical or quadrate shapes with an average size less than 200 nm. Electrochemical measurements indicate that the discharge capacity of LiFe_0.94V_0.06PO₄/C is 20 mAh/g higher than that of LiFePO₄. LiFe_0.8V_0.2PO₄/C synthesized using Fe₂O₃ as iron source has a discharge capacity of 158 mAh/g at 0.1 C. However, LiFe_1-xV_xPO₄/C has low discharge capacities when FePO₄ was used as the Fe²⁺ and PO₄^3- sources due to the complicated reactions. Both LiFe_0.7V_0.3PO₄ and LiFe_0.6V_0.4PO₄ reach an initial discharge capacity of 170 mAh/g at 0.1 C when FeC₂O₄ was used as Fe²⁺ source. Cyclic voltammogram (CV) shows that LiFe_1-xV_xPO₄/C has 4 couples of redox peaks, whichbelongs to iron and vanadium. The redox peaks at high voltage become obvious as amount of vanadium increase.LiFePO_/C was synthesized at 600°C for 10 h by one step solid state reaction with polypropylene, glucose, citric acid and mixed precursors as reductive agents. The results of TD/TG A analyses demonstrate that the decomposing temperature of polypropylene or glucose and the formation of LiFePO₄ are nearly at the same temperature range between 380 °C to 470 °C. But citric acid is decomposed at 200°C which is much lower than that of LiFePO₄ formed. Elemental analysis shows that LiFePO₄/C synthesized at 600 °C using glucose and citric acid as carbon source contain 7.7 wt.% and 1.5 wt.% carbon, respectively. SEM observation shows that the particles of LiFePO₄/C synthesized at 600 °C using polypropylene, glucose and the hybrid precursors as carbon sources are smaller than 300 nm and coated with carbon webs. LiFePO₄/C particles synthesized at 600 °C using citric acid as carbon source are spherical and larger than 500 nm. The particles sizes increase to larger than 1 μm for the sample synthesized at 800 °C. Raman spectra verifies that increasing the sintering temperature could be beneficial to the decomposition of carbon procursors and the graphitization of soft carbon. Electrochemical tests prove that LiFePO₄/C synthesized at 600 °C using glucose and hybrid precursors as carbon source have the best electrochemical properties among the samples investigated in the present work, with a discharge capacity of 150 mAh/g at 0.5 C. The capacity of LiFePO₄/C decreases with increasing the reacting temperature due to particle coarsening.