| Abstract:In this paper, on the base of reviewing the development of lithium ion batteries and cathode materials in detail, olivine LiFePO4/C and xLiFePO4·yLi3V2(PO4)3/C were chosen and their synthesis, electrochemical performances, physic properties and the kinetics behaviors were studied by using various electrochemical methods.Nanosized spherical FePO4·2H2O was prepared by ultrasonic aided precipitation. The optimal process conditions were:solution concentration of0.1mol/L, addition amount of H2O2of5.5mL, stirring speed of800r/min and pH of2.2. And at this optimum operating condition, sample small than80nm was well distributed. The n(Fe)/n(P) molar ratio was1.01and the yield reached99%. FePO4can be obtained via calcining at485℃.Three oblique type FeVO4precursor of2LiFePO4·Li3V2(PO4)3was prepared by ultrasonic aided precipitation combined heat treatment. Fe4(VO4)4·5H2O had five crystal water at the optimal process conditions of pH of6, solution concentration of0.1mol/L and the stirring time of2h. The evenly distributed particles were small. The n(V)/n(Fe) molar ratio was1.002. And the FeVO4particles with good crystallinity were obtained via calcining at600℃for6h and the surface of particles close to150nm are smooth.Nanosized spherical FePO4·2H2O was as the source of iron and phosphorus, Li2C2O4as the source of lithium, citric acid as reductant, sucrose as the source of carbon. Composite metals dopped LiFePO4/C was synthesized via ultrasonic aided precipitation combined carbon thermal reduction process. Ni and Nb dopped LiFePO4/C was prepared at650℃for12h. The particles were in uniform distribution and the sizes were200nm. It showed excellent discharge capacity of158.8mAh/g at0.1C for the first time, and the cell delivered157.4mAh/g after30cycles, with the capacity retention rate of99.1%. And Ni-Nb dopped LiFePO4/C delivered150.2mAh/g as first discharge capacity at1C. And the cell retained97.8%after100cycles. EIS result showed that the dopped composite metals reduced the charge transfer impedance of LiFePO4/C by a large margin. Afterwards, excharge current density was increased, and the diffusion coefficient of lithium ion improved one order of magnitudes. It illustrated dopped composite metals could improve the diffusion rate of lithium ion effectively as the efficient path to ameliorating the electrochemical performance of LiFePO4.Nanosized spherical FePO4·2H2O was as the source of iron and phosphorus, Li2C2O4as the source of lithium, ascorbic acid as reductant, sucrose as the source of carbon. Composite metals dopped LiFePO4/C was synthesized via spray-drying combined carbon thermal reduction process. The Ti-Nb-Co dopped product synthesized with ball-milling time of2h, liquid solid ratio of6mL/g, spray inlet air temperature of200℃, spray speed of1000mL/h, roasting temperature of690℃and the roasting time of18h had the best performance. And the spherical sample had the fine and uniform particle sizes close to200nm. The surface of material was coated flocculent carbon. The particles had good crystallinity and excellent electrochemical performance. It exhibited a capacity of160.0mAh/g at0.1C for the first discharge process with the charging and discharging efficiency of95.5%. It delivered discharge capacity of151.6mAh/g,128.9mAh/g and115.6mAh/g at1C,5C and10C respectively when the charge rate was0.1C, with the discharge voltage platform of3.4V,3.2V and3.1V respectively, showing a good rate performance。It had a good cycle performance with a retention rate of99.9%after100cycles at1C. Electrochemical impedance was36.6Ω. Cycle volt-ampere curve was in centric symmetric and the potential difference of oxidation peak and reduction peak was only0.16V, with good charging and discharging reversibility.EDX results showed that the sample had the elements of Fe, P, O, Ti, Nb and Co, indicating that Ti, Nb and Co had entered into the crystal lattice of LiFePO4/C. TEM result showed that the surface of sample distributed a layer of amorphous carbon about40nm. Carbon film coated LiFePO4/C resulted in a good electric network, contributing to improving the electron transport. FTIR result showed that the LiFePO4/C was pure and had no characteristic peak of other impurity. Raman result showed that the surface of Ni-Nb dopped spherical LiFePO4/C scattered a graphited carbon layer, increasing the conductivity。FeVO4prepared by ultrasonic aided precipitation combined heat treatment was as the source of iron and vanadium, and LiH2PO4was as the source of lithium and phosphorus, and oxalic acid was choosen to be reductant, and then glucose was as the source of carbon. The2LiFePO4·Li3V2(PO4)3/C composite materials were synthesized via mechanical activation combined solid sintering process. The particles prepared at700℃for16h were in uniform distribution. It showed excellent discharge capacity of144.5mAh/g at0.1C for the first time, and the cell delivered140.7mAh/g,133.2mAh/g and125.6mAh/g at0.1C,0.5C and1C respectively after50cycles, with the capacity retention rate of97.4%,96.2%and95.1%. Cycle volt-ampere curve was in centric symmetric and the potential difference of oxidation peak and reduction peak was0.27V for LiFePO4and less than0.15V for Li3V2(PO4)3with good charging and discharging reversibility. EIS result showed that the Electrochemical impedance of2LiFePO4·Li3V2(PO4)3/C composite materials was130Ω.The2LiFe1-xCoxPO4·Li3V2(PO4)3/C composite materials were synthesized via mechanical activation combined solid sintering process. The best cobalt dopping amount was0.04and the2LiFe0.96Co0.04PO4·Li3V2(PO4)3/C synthesized at700℃for16h were in uniform distribution. It delivered discharge capacity of144.1mAh/g,142.2mAh/g,133.4mAh/g and124.8mAh/g at0.1C,1C,3C and5C respectively when the charge rate was0.1C. It had a good cycle performance with a retention rate of99.8%after50cycles at0.1C, retaining a capacity of143.8mAh/g. Cycle volt-ampere curve was in centric symmetric and the potential difference of oxidation peak and reduction peak was0.17V for LiFePO4and less than0.1V for Li3V2(PO4)3with good charging and discharging reversibility. Therefore, Co dopping decreased the polarization degree. EIS result showed that the electrochemical impedance of2LiFe0.96Co0.04PO4·Li3V2(PO4)3/C composite materials was100Ω. Fe4(VO4)4/C precursor was prepared by ultrasonic aided precipitation combined heat treatment. And2LiFePO4·Li3V2(PO4)3/C composite materials for lithium ion batteries were synthesized via spray-drying combined carbon thermal reduction process. The2LiFePO4·Li3V2(PO4)3/C synthesized with ball-milling time of2h, liquid solid ratio of20mL/g, spray inlet air temperature of260℃, spray speed of1000mL/h, the carbon source ratio for glucose and oxalic acid of6:4, roasting temperature of750℃and the roasting time of18h had the best performance. And the spherical sample had the fine and uniform particles. It showed excellent discharge capacity of147.6mAh/g at0.1C for the first time, and the cell delivered145.8mAh/g after30cycles, with the capacity retention rate of98.78%. And2LiFePO4·Li3V2(PO4)3/C delivered145.0mAh/g and123.0mAh/g as first discharge capacity at1C and10C respectively, and the discharge median voltage was3.5V and3.2V. EIS result showed that the Electrochemical impedance of2LiFePO4-Li3V2(PO4)3/C composite materials was23.4Ω. Cycle volt-ampere curve was in centric symmetric and the potential difference of oxidation peak and reduction peak was0.12V for LiFePO4and0.04V for Li3V2(PO4)3with good charging and discharging reversibility.EDS results showed that the sample had the elements of Fe, P, O and V, indicating that every element had entered into the crystal lattice of2LiFePO4·Li3V2(PO4)3/C evenly. TEM result showed that the surface of sample distributed a layer of amorphous carbon about5nm. Carbon film coated2LiFePO4-Li3V2(PO4)3/C resulted in a good electric network, contributing to improving the electron transport. FTIR result showed that the2LiFePO4·Li3V2(PO4)3/C was pure and had no characteristic peak of other impurity. Raman result showed that the surface of2LiFePO4·Li3V2(PO4)3/C scattered a graphited carbon layer, increasing the conductivity。 LiFePO4/C composite materials dopped with Ni/Nb metals were prepared via pilot production. Six samples with different particle size distribution were obtained by screening and made into18650-type power batteries. The result showed that the sample in normal distribution with mean grain size of3.503μm had the best processability, with solid content of slurry of47.5%and compacted density of2.35g/cm3. At low rate, size distribution had the less effect on discharge capacity of LiFePO4/C. However, average particle size played an important role on cycle performance of LiFePO4/C. The result showed that the sample in normal distribution with mean grain size of3.503μm had the best cycle performance. LiFePO4/C with small particle size had the better low-temperature discharging performance than the particles with large size at-20℃. |
