Studies On Synthesis And Modification Of High Conductive Microspherical LiFePO₄Cathode Materials

LiFePO₄has been extensively recognized as one of the most promising cathodematerials in lithium-ion batteries, owing to its nontoxicity, superior cycle stability,good thermal stability and safety, remarkable tolerance to over charge and overdischarge and low cost, and so on. However, the low conductivity of LiFePO₄resultsin poor rate capability, especially the high rate capability, which has hampered itsextensive application in high-rate lithium ion batteries. In addition, the tap density ofLiFePO₄is low. In generally, the tap density of industrialized LiFePO₄is less than1.0g/cm3, leading to relatively low specific energy.In this dissertation, the aim is enhancement of the conductivity and tap density ofLiFePO₄, thus attaining the purpose of improving the electrochemical property ofcathode materials, especially the rate capability and specific volume energy. Themonodisperse microspherical LiFePO₄precursor were synthesized via ammoniaassisted hydrothermal route, then the microspherical LiFePO₄/C were obtained bysolid state reaction. The influence of preparation technology on the performance ofLiFePO₄cathode materials has been discussed. The physical and electrochemicalproperties of microspherical LiFePO₄hybrid coated with La_0.56Li_0.33TiO₃and carbonhave been investigated. The effect of Ni and F ion co-doping on the performance ofmicrospherical LiFePO₄/C has also been observed. Moreover, the thermodynamicsand kinetics behaviors of lithium ion extraction/insertion in microspherical LiFePO₄/Chave been investigated. The main works are as follows:（1） The microspherical LiFePO₄precursor were synthesized by ammonia assistedhydrothermal route. The influences of hydrothermal preparation technology on theperformance of microspherical LiFePO₄precursor were probed. It was found that theLiFePO₄precursor with good spherical degree, uniform and narrow particle sizedistribution could be prepared from the conditions as follows: the concentration ofammonia was0.4mol/L, hydrthermal reaction at180oC for6h.（2） The effect of calcination conditions on the physical and electrochemicalperformance of the microspherical LiFePO₄/C was discussed. The results showed thatthe performances of microspherical LiFePO₄/C were markedly affected by thetemperature and time of calcination. The optimal calcination conditions ofmicrospherical LiFePO₄/C which exhibited high degree of crystallinity, good spherical degree, uniform and narrow particle size distribution and fineelectrochemical performance were as follows: the temperature of calcination was700oC, the time of calcination was10h. The initial discharge capacity of themicrospherical LiFePO₄/C was155.9mAh/g without capacity decreasing after100cycles at rate of1C. When the discharging current was increased from0.1C to5C, itdelivered a discharge capacity of168.3（0.1C）,165.0（0.2C）,161.0（0.5C）,154.9（1C）,143.1（2C） and120.4（5C） mAh/g, respectively.（3） In order to improve the electrochemical performance of microsphericalLiFePO₄/C, La_0.56Li_0.33TiO₃, as Li fast ion conductor, was coated on the surface of themicrospherical LiFePO₄/C. The microspherical LiFePO₄/(C+La_0.56Li_0.33TiO₃)composites were synthesized via situ coating method. The results showed that thesurface of microspherical LiFePO₄/C was coated with a hybrid conductor layercomposing of La_0.56Li_0.33TiO₃nanoparticles and amorphous carbon, which can notonly enhance electronic and Li ion transport, but also avoid the erosion of electrolyteon active material and restrain the dissolution of iron from the cathode material, andthus can evidently enhance the electrochemical performance, especially high ratecapability. The microspherical LiFePO₄/(C+2wt.%La_0.56Li_0.33TiO₃) exhibited thehighest rate performance among all the samples. The tap density of the materialsreached1.30g/cm3. When the discharging current was increased from0.1C to20C,it exhibited167.6（0.1C）,155.5（1C）,142.1（2C）,126.1（5C）,105.8（10C） and86.0（20C） mAh/g, respectively.（4） In order to further improve the electrochemical performance ofmicrospherical LiFePO₄/C, modification with Ni and F ions co-doping was performed.The microspherical LiFe_1-xNi_x（PO₄）_1-xF_3x/C （x=0.00,0.01,0.02,0.03） were preparedvia ammonia assisted hydrothermal method combining with solid state reaction. Theresults indicated that the Ni and F co-doping did not destroy the olivine structure ofLiFePO₄, but it could stabilize the crystal structure, lengthen the Li-O bond, decreasecharge transfer resistance, enhance Li ion diffusion velocity, and thus improve itscycle and rate capability of the LiFePO₄/C, especially at a high-rate capability. It hasbeen found that microspherical LiFe0.99Ni0.01（PO4）0.99F0.03/C showed a best ratecapability and excellent cycle stability among all the samples. The tap density of thematerials reached1.23g/cm3. The initial discharge capacity of microsphericalLiFe0.99Ni0.01（PO4）0.99F0.03/C was156.6and130.1mAh/g as well as stable cycleperformance （the capacity retention ratio was99.6and96.7%till100cycles） at1C （170mA/g） and5C （850mA/g）, respectively. When the discharging current wasincreased from1C to20C, it delivered a discharge capacity of167.9（0.1C）,154.9（1C）,143.1（2C）,133.3（5C）,113.7（10C） and91.6（20C） mAh/g, respectively.（5） The thermodynamics behavior of lithium ion extraction/insertion inmicrospherical LiFePO₄was preliminarily studied. It was found that the insertion freeenergy of lithium ion （G） increased as a linear way with the increase of theextraction/insertion amount of lithium ion （x）. It suggested that the highercharge/discharge capacity can be obtained with the larger Gvalues. Moreover,the chemical diffusion coefficients （DLi） of lithium ion extraction/insertion inmicrospherical LiFePO₄were calculated by galvanostatic intermittent titrationtechnique （GITT）, electrochemical impedance spectroscopy （EIS） and cyclicvoltammogram （CV）, respectively. The results showed that the DLiobtained from EIS,GITT and CV were generally consistent within the same order of magnitude（10-20¹0-14cm2/s）.