| In this dissertation, carbon-coated LiFePO4 were synthesized by a modified solid-state reaction method, which improve the state of precursor. FePO4 was used as Fe source and P source, the inexpensive compound containing carbon element was used as carbon source. The influences of the synthetic condition and the precursor state on the performance of LiFePO4/C composite were mainly studied. At the same time, the method which improves the tap-density of LiFePO4/C composite was discussed.1. FePO4 was prepared from FeSO4·7H2O and Fe(NO3)3·9H2O by the co-precipitation method, respectively. The FePO4 prepared from FeSO4·7H2O was characterized by XRD, SEM, TG-DTA, XRF, IR and so on. The results show that the prepared FePO4 is completely amorphous, the particle size is about 100 nm, the formula of it should be written as FePO4-2H2O and the mol ratio of Fe and P is 0.95:1.2. Five classes LiFePO4/C composites were synthesized in this dissertation. Class A and Class B were synthesized by a slurry-state method. FePO4·4H2O available on market and homemade FePO4·2H2O was used as starting material, respectively. The soluble starch serves as carbon source and reducing agent. The influence of the calcination temperature, calcination time and the content of starch on the properties of LiFePO4/C composite were studied. The results show that the calcination temperature is the most important factor, and the influence of calcination time is secondary, while content of carbon source also has some influence. For A-class LiFePO4/C composites, the results show that the optimal calcination temperature is 550℃and calcination time is 4 h. The LiFePO4/C composite synthesized at this condition serves as cathode for lithium ion batteries, which has the specific discharge capacity of 95.3 and 112.5 mAh g-1 at 1C for the first cycle and 50th cycle. For the Class B, it was found out that the optimal calcination temperature is 600℃and calcination time is 10 h. The LiFePO4/C composite synthesized at this condition shows discharge capacity of 154.5 mAh g-1 for the 50th cycle at 1C. Remarkably, even at a high current density of 30C, the cell still displays discharge capacity of 72 mAh g-1 and shows good cycle retention. The difference on capacity performance for the two classes LiFePO4/C composites mainly come from the difference of the particle size. The reduced particle size is favorable to the capacity performance of the composite.3. The LiFePO4/C composites of Class C were synthesized by a slurry-state method, the homemade FePO4·2H2O was used as Fe and P source, stearic acid serves as carbon source. The influence of calcination time on the properties of LiFePO4/C composite was studied. The results show that the composite synthesized at 600℃for 4 h displays a optimal electrochemical performance. The sample of 2.2 wt.% carbon content shows a discharge capacity of 154.8 mAh g-1 at 1C. Even at a high current density of 30C, the material still presents a discharge capacity of 93.2 mAh g-1 and exhibits an excellent cycling performance. As a surfactant, stearic acid plays the dispersant role in the process of grinding and limited agglomeration of the FePO4·2H2O particles, it is very important in improved performance of materials.4. The LiFePO4/C composites of Class D were synthesized by a pellet-state method. The homemade FePO4·2H2O was used as starting material and stearic acid serves as carbon source. A powder-state method was employed to synthesize LiFePO4/C composites of Class E. The homemade FePO4·xH2O was used as starting material and stearic acid serves as carbon source. A paste-state precursor was calcined after drying, grinding and manual pelletizing in pellet-state method and powder-state method. This two methods can be employed to synthesized LiFePO4/C composite with close-packed structure, which possess improved electrical conductivity. As a result, both tap density and electrochemical performance can be significantly improved by the two methods. The highest tap density of Class D and Class E is 1.56 g cm-3 and 1.71 g cm-3, respectively. For the composite with tap density of 1.36 g cm-3 in class D, it shows a discharge capacity of 149.1 mAh g-1 after 700 cycles at 1C and exhibits an excellent cycling performance. Even at a high rate of 20C, the composite still presents a discharge capacity of 111.7 mAh g-1. For the material with tap density of 1.35 g cm-3 in class E, it shows a discharge capacity of 154.6 mAh g-1 at 1C. Even at a high rate of 20C, the composite still presents a discharge capacity of 109.2 mAh g-1 and exhibits a well cycling performance. |
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