Study On Preparation And Modification Of LiFePO₄ As Cathode Materials For Lithium-ion Battery

The study of power lithium-ion battery has already been extremely urgent since the shortage of energy resources. LiFePO4 as lithium-ion battery has broad application foreground for its high thermal stability, high security, high specific capacity, environmental benign and low cost etc. However, the defects of the low conductivity and ion diffusion speed of LiFePO4 must be resolved to improve its properties. This paper aims at those defects to do the following researches: optimization of preparation process of LiFePO4 by high-temperature solid-state reaction; performance influence of LiFePO4 anode materials doped with Ni2+; performance influence of LiFePO4 anode materials Cu particles; performance influence of LiFePO4 anode materials Coated with carbon addition of PVA (polyvinyl alcohol). The main conclusions as follows:(1) LiFePO4 materials prepared in different synthesis processes usually show great differences in property. High-temperature solid-phase reaction method is suitable for industrial production for its simple operation and low cost. Based on the thermal analysis of precursor, orthogonal testing method with three factors and three levels was introduced to research the influences of presintering temperature, synthesis temperature and holding time on the electrochemical properties of materials. Results showed that the most available process was to presinter at 350℃and hold for 12 hours at 650℃, in the condition of which the first discharge capacity of LiFePO4 got to 151.7mAh/g, and remained 140.9mAh/g after 30 cycles.(2) LiFePO4 materials have poor electronic conductivity and low ion diffusion speed, but its electrochemical properties could be improve efficiently by doping metal ions, which could produce lattice defects in materials. Optimized High-temperature solid-phase reaction was introduced into this experiment to prepare a series of LiFe1-xNixPO4 (x=0, 0.03, 0.05,0.07, 0.1, 0.15) with Ni doped by In-situ method. The XRD and SEM showed that Ni replaced Fe and material grains were refined. The charge-discharge results indicated that materials performed best when the content of Ni equals 0.05. Under the charge ratio of 0.1C, the first discharge capacity of LiFe0.95Ni0.05PO4 was 154.5mAh/g, showing improvement compared to that of pure LiFePO4 samples (151.7mAh/g). The same results happened under higher ratio, the doped samples showed 132mAh/g for 0.5C and 122mAh/g for 1C, compared to 125.4mAh/g (0.5C) and 117.7mAh/g (1C) for pure ones respectively. Doped Ni2+ improved the cycle performance effectively, as the capacity hold retention (R30/1) of LiFe0.95Ni0.05PO4 samples reached up to 0.9955 (0.1C) while that of the pure LiFePO4 samples was 0.9288 (0.1C) after 30 cycles.(3) It is common to improve the conductivity of LiFePO4 by coating nano Cu or Ag particles. However, these LiFePO4/Cu (Cu/Li=0, 1/20, 1/15, 1/10) composite materials coated with different proportion of nano Cu particles didn't show better electrochemical properties. The XRD indicated that Cu didn't get into lattice of LiFePO4 but just mechanically mixed with LiFePO4, and the SEM diplayed stronger particle dispersion for the composite samples. The capacity of Cu-coated composite materials showed the tendency of falling after rising with the increasement of the amount of Cu. Of all samples, the best electrochemical performance is only 142.8mAh/g for the first discharge capacity (Cu/Li=1/15), decreasing greatly compared to pure LiFePO4 (151.7mAh/g). The slow scanning cyclic voltammetry and electrochemical impedance spectroscopy analysis showed that, although Cu particles could improve the electronic conductivity in some ways, they oxidized during the first charge cycle irreversiblly, causing lower discharge capacity and larger initial irreversible capacity loss.(4) It helped to inhibit oxidation of Fe2+ efficiently during high-temperature solid phase reaction if polyvinyl alcohol (PVA) had been added when preparing precursor. At the same time, the pyrolytic carbon was coated on the surface of LiFePO4, preventing the grains from growing up. Experimental composite samples of LiFePO4/C (C/Li=0, 1/15, 1/12, 1/10, 1/7, 1/5) had delicate particles and good dispersion. However, XRD showed that: when C/Li>1/10, excessive C would reduce Fe and then caused reaction with P generating Fe2P, which had great influence on the electrochemical properties of LiFePO4/C. The discharge capacity of composite materials showed the tendency of falling after rising with the increasement of the amount of carbon, and the best proportion is C/Li=1/10. The discharge capacities of this sample are 166.3mAh/g for 0.1C, 154.2mAh/g for 0.5C and 150.3mAh/g for 1C, showing great improvements compared to respective discharge ratio of pure LiFePO4 (151.7mAh/g for 0.1C, 125.4mAh/g for 0.5C, 117.7mAh/g for 1C). Moreover, composites had better circle performance, the best capacity hold retention for 20cycles (R20/1) was 0.9657 (0.1C), But 0.9578 for pure LiFePO4 (0.1C).