Synthesis And Modification Of Cathode Materials LiFePO₄ And Li₂FeSiO₄ For Lithium Ion Battery

The development of rechargeable lithium-ion batteries and cathode materials are reviewed in detail. With the advantages of abundant recource, non-toxicity, high safety, low cost, LiFePO4 and Li2FeSi04 have been considered as a promising cathode material on lithium-ion batteries. Synthesis of materials, modification process, structure characterization, electrochemical behaviors of LiFePO4 and Li2FeSiO4 has been involved in this study.Based on the disadvantages of unstable properties of FePO4·χH2O which is the raw materials of LiFePO4, the pure FePO4·χH2O was prepared by liquid precipitation method. The effect of reactants' concentration on the physical performance and electrochemical behavior of FePO4·χH2O precorsor and LiFePO4 was studied. The results showed that LiFePO4 synthesised by homogeneous FePO4·2H2O precorsor had the best electrochemical properties when the concentration was 1.0 mol·L-1.Based on the disadvantages of poor ion diffusion and low electronic conductivity of the pure phase Li2FeSi04, the Li2FeSi04 was prepared by sol-gel technique using citric acid as a complexing agent. Comparison research on performance of two samples which prepared by different ways by using citric acid or not have been studied, the result the sample by using citric acid with better crystallinity, uniform particles and excellent electrochemical properties. The initial discharge capacity of Li2FeSi04 synthesized is 135.4 mAh·g-1 at C/16 rate and the capacity fading is only 15.7% after 30 cycles. The effect of complex ratio on the physical structure and electrochemical properties of Li2FeSi04 was researched. The results indicated that the electrochemical properties of Li2FeSiO4 increased then decreased with the changing of complex ratio and the best sample obtained when the complex ratio is 1.0.According to preliminary studies by our group, adding only a kind of organic carbon would improve the electrochemical properties of Li2FeSi04 limited, therefore, the influence of surface modification by combined organic carbon coating on the properties of Li2FeSiO4/C was investigated. It was found that the Li2FeSi04/C by adding 35% citric acid+5%glucose+5%pyromellitic anhydride as carbon source has the outstanding electrochemical properties.Meanwhile, the effect of synthesis conditions including sintering the temperature and time on the physical performance and electrochemical behavior of Li2FeSiO4/C was studied and the synthesis conditions were optimized. The results show that the sintering temperature and time play an important role in the crystal structure and morphology of the material. The optimized sintering temperature and time were 700℃and 20h. The initial discharge capacity of Li2FeSiO4/C(F3) synthesized on the optimized condition is 157.5 mAh·g-1 at C/16 rate and the capacity retention is 92.06% after 30 cycles.The lithium deintercalation-intercalation kinetics of Li2FeSiO4/C(F3) was investigated by cyclic voltammetry and electrochemical impedance spectroscopy methods. The results further proved that the modified Li2FeSi04/C with the decreasing of electrode polarization and the improvement of reversibility and electrochemical properties.