| Lithium battery cathode material LiFePO4 has the advantages of rich raw materials,low price, high discharge capacity (170 mAh· g-1), good cycle performance and environmental friendly. Now,the particle size of LiFePO4 is general about 5μ m, the larger the paticle size is,the more difficultly the Li+ diffuses,the lower the diffusion coefficient is,and the electrochemical performance is not ideal.As the precursor of LiFePO4,only ultrafine (nano) FePO4 can prepare ultrafine (nano) LiFePO4. This thesis researched the preparation crafts of ultrafine (nano) FePO4.This thesis made a reasonable improvement on traditional homogeneous precipitation method, used FeSO4·7H2O as the iron source, H3PO4 as phosphorus source,prepared FePO4 in ethanol-water system. Explored feeding methods and feed speed,different type and amount of surfactant,P:Fe,the end of the reaction pH, drying method,the initial concentration of reactants,reaction temperature, reaction time, stirring speed,the amount of ethanol and calcination temperature,etc.on the particle size and morphology of the structure of FePO4 by single factor and orthogonal experiment.Ultimately determined the optimal conditions:firstly add H3PO4, Fe2(SO4)3 is dropped with the speed of 5mL.min-1 dispersant is 0.4‰ C,the P:Fe is 1.4:1,the end of the reaction pH is 1.9,use pray dried,concentration of reactants is 1.0 mol.L-1,the reaction temperature is 40°C, the reaction time is 2 h,the stirring speed is 2500r.min-1,the amount of ethanol is 25%,and the calcination temperature is 450° C.Finally,LiFePO4/C was prepared by high-temperature carbo thermal reduction with the homemade ultrafine (nano) FePO4.The products were characterized by TG-DSC,FT-IR,XRD,tap density,BET, SEM and the C content,etc. The LiFeO4/C was assembled into 2032-type button battery, then tested the electrochemical performance by land electric tester (CT-2001-A-5V20mA)The FT-IR and TG-DSC analysis shows that the homemade FePO4·xH2O has two H2O,that is FePO4·2H2O, and lose crystal water at 120-200°C.The SEM and XRD analysis shows that the amorphous state FePO4·2H2O calcined in high purity trigonal system structure FePO4·;The BET and tap density analysis shows that both the BET and tap density increase after calcined.The particle size distribution and SEM result indicates that the homemade FePO4 has perfect crystalline phases and less reunited, uniform particle size,d (0.1) as 110nm, d (0.5) as 192nm, d (0.9) as 454 nm.The electrochemical performance test under different discharges current density show that the analog battery has a high and stable operating voltage.The discharge capacity is 163.6 mAh·g-1 under 0.1C;The discharge capacity is 144.6 mAh· g-1 under 1C and the capacity rentention rate is 98.76% after 200 cycles.This thesis made a reasonable improvement on traditional homogeneous precipitation method.The prepared high purity ultrafine (nano) FePO4,which disperses well, has uniform particle size and perfect crystalline phases.The homemade FePO4 can prepared LiFePO4/C,which has high discharge capacity,stable operating voltage and excellent cycle performance. |
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