Preparation, Characterization And Properties Of Phosphate Cathode Materials For Lithium-ion Batteries

Olivine-type lithium iron phosphate (LiFePO4) and lithium manganese phosphate (LiMnP04) have received much attention as the new generation cathode materials for lithium ion batteries because of their high safty, low cost, flat voltage plateau, stable cycle performance and good environmental compatibility. However, their low tap density, poor electronic conductivity and low ionic diffusivity greatly hinder their application. The major objective of this work is to improve their electrochemical properties by carbon coating and ion doping. LiMPO4 (M=Fe and Mn) were synthesized by modified solid state reaction, co-precipitation and sol-gel processing. The as-prepared materials were characterized by means of X-ray diffraction, scanning electron microcopy, energy-dispersive spectroscopy, atomic absorption spectrometry, Raman spectroscopy, electrochemical impedance spectroscopy, cyclic voltammetry and charge/discharge test, etc.In order to investigate the correlation between doping effect and electrochemical properties, Mg-doped LiFePO4/C composite materials were prepared via high temperature solid-state method and co-precipitation process, respectively. The mechanism and process of solid-state reaction was first studied. The results show that the electrochemical performance of LiFePO4 strongly depended on the reaction temperature and the optimum temperature was 750℃. Then LiFe1-xMgxPO4/C composites were synthesized by the optimum route. Among of the as-prepared materials, LiFe0.98Mg0.02PO4/C exhibited the highest discharge capacity and fairly good rate performance and the reversible capacity could reach 150 mAh g-1 at 0.1 C. Finally, the doping effect of Mg-doped LiFePO4/C composites prepared by co-precipitation process was investigated. The introduction of Mg suppressed the crystal growth of ferrous oxalate, leading to the particle size decrease of the final products. However, only negligible Mg was detected in the as-prepared materials because magnesium oxalate has a higher solubility than that of ferrous oxalate with the result that Mg was removed from the precipitate by repeated washing with deionized water. The powders with fine particle size showed a small polarization and better electrochemical performance.When LiFePO4 was coated with carbon, an appreciable amount of Li3PO4 impurity would generate along with Fe2P via carbothermal reduction because LiFePO4 was often synthesized from starting materials in a stoichiometric ratio. Although Fe2P is able to enhance the electronic conductivity of LiFePO4, the presence of inactive Li3PO4 would result in capacity degradation and impedance increase. By suppressing Li3PO4 formation, the discharge capacity of LiFePO4 produced by non-stoichiometry synthesis could be increased by 10—44 mAh g-1. Studies on the structure, morphology and electrochemical performance of the non-stoichiometry samples revealed that the LiFePO4 with an excess of 3 wt.% Fe and P exhibited excellent electrochemical performance when it was heated at 400℃for 10 h and then at 750℃for 10 h.LiFe1±xPO4 were prepared by non-stoichiometry synthesis in order to further enhance the electrochemical properties. The kinetics of LiFePO4 could be improved by adding or reducing appropriate amount of Fe. Fe-deficient sample, LiFeo.99P04, was synthesized by three different methods, and the results showed that its capacity is higher than that of stoichiometry LiFePO4 by 4—16 mAh g-1.The material prepared by sol-gel method formed 3D pore structure due to citrate pyrolysis and the reversible capacity could reach 150 mAh g-1 at 0.1 C. Fe-rich samples, LiFe1+xPO4 (x=0.01,0.03,0.06 and 0.13), were also prepared by sol-gel method. The results of charge/discharge test displayed that the best performance was obtained at x=0.03 with the discharge capacity of 152 mAh g-1 at 0.1 C and 103 mAh g-1 at 5 C. The electrochemical performance of LiFe0.99PO4 and LiFe1.01PO4 at 25℃was compared, and the result showed that the two samples both have good rate capability and outstanding cyclability, but the LiFeo.99P04 demonstrated a higher capacity of 158 mAh g-1 at 0.1 C and 120 mAh g-1 at 5 C.has a theoretical capacity of 171 mAh g-1 and a redox potential around 4.1 V vs Li/Li+. Compared with LiFePO4, LiMnPO4 provides a higher energy density. In this work, the thermal behavior of the precursor was studied and then carbon coated LiMnPO4 composites were prepared by solid state method at various temperatures. The optimum temperature was 800℃. In order to increase the lithium insertion/extraction rate in LiMnPO4 crystal, Mn-site co-substitution with Fe-Mg was proposed and a synergistie effect was observed. Compared to the unsubsitituted and Fe substituted samples, LiMn0.9Fe0.05Mg0.05PO4/C showed an excellent rate capability and the reversible capacity could reach 140 mAh g-1 at 0.1 C,117 mAh g-1 at 1 C and 62 mAh g-1 at 5 C.