【作者】 薄红志；

【导师】 韩绍昌；

【作者基本信息】 湖南大学 ， 材料学， 2006， 硕士

【摘要】 采用固相混合法制备了新型正极材料LiFePO₄。以不同工艺条件试样的充、放电性能及其微观结构为考察指标,采用正交法对最佳的合成工艺进行了优化。考察了试样与九种不同电解液的相容性。采用正交法考察了试样的颗粒粒度、电极膜中导电剂乙炔黑和粘结剂聚四氟乙烯（PTFE）的含量以及充、放电电流密度等因素对其充、放电性能的影响。采用不同碳源对试样进行了包覆,考察了碳包覆对试样晶体结构和电化学性能的影响。实验结果表明:1.各预分解工艺因素对试样D₃（第三循环放电容量）影响的大小顺序为:预分解温度>保护气体流量>预分解时间>球磨时间。其最佳工艺组合为:球磨时间2h、预分解温度350℃、预分解时间5h、保护气体流量1.5L/min,此时试样的D₃=121.78mAh/g,第三循环充、放电效率η3=98.56%。2.各合成工艺因素对试样D₃影响的大小顺序为:合成温度>锂/铁/磷配比>合成时间>成型压强,其最佳工艺条件为:锂/铁/磷原子比1.05:1:1、合成温度650℃、合成时间为18h、成型压强60MPa,此时试样的D₃=127.49mAh/g,η3=99.46%;各合成工艺因素对晶胞体积V和晶粒尺寸D₁₃₁的影响大小顺序为:合成温度>锂/铁/磷配比>合成时间>成型压强,其最佳组合为锂/铁/磷原子比1.05:1:1、合成温度650℃、合成时间为18h、成型压强60MPa,此时试样的晶胞体积V=0.29172nm3,与理论值相差最小,且晶粒尺寸适中(D₁₃₁=34.383nm)。由此可见,以充、放电性能为考察指标与以微观结构为考察指标所得到的最佳工艺组合是完全一致的,两者必然同步优化。3.试样在九种不同电解液中进行恒电流充、放电时都能形成良好的充、放电平台,在分别以EC+DEC（1:1,v:v）、EC+DMC（1:1,v:v）和PC+DME（1:1,v:v）为溶剂、以LiClO4为溶质的电解液中都可以获得良好的电化学性能。其中,试样在1mol/L LiClO4/EC+DMC（1:1）电解液中的电化学性能最佳,在以LiPF6和LiBF4为溶质的电解液中进行充、放电时,其循环性能较差,容量衰减较大。4.试样粒度、电极膜中的导电剂乙炔黑含量和粘结剂PTFE的含量以及充、放电时的电流密度四个因素对试样D₃的影响大小顺序为:乙炔黑用量>充、放电电流密度>试样颗粒粒度>PTFE用量,各因素的最佳组合为:试样颗粒粒度-400目、PTFE用量5%、乙炔黑用量20%、充、放电电流密度10mA/g,此时试样的D₃=131.25mAh/g,η3=99.37%。5.采用乙炔黑为碳源对试样进行包覆时,不能得到结晶良好的试样,试样的电化学性能也没有得到有效地改善;采用葡萄糖为碳源对试样进行包覆能够得到结晶良好的试样,试样的电化学性能得到了较大的改善,平台性能和倍率性能较好;采用酚醛树脂为碳源对试样进行包覆时,获得了良好的碳包覆结构,试样的颗粒细小均匀,甚至达到了纳米级,其结晶度高、晶粒尺寸小,电化学性能得到很大改善,具有优异的平台性能和倍率性能,当包覆碳含量为10%时,试样的电化学性能最佳,其第三循环放电容量为D3为164.89mAh/g,充、放电效率η₃为99.20%,该试样在1C倍率时的第三循环放电容量D3=149.12mAh/g。更多还原

【Abstract】 A new kind of lithium ion batteries cathode material, lithium iron phosphate, was prepared by solid-state method. The charging-discharging performance and the microstructure of different samples prepared by this method were taken as the investigation targets of optimization, and the technological conditions were optimized. The compatibility of the sample with nine different kinds of electrolytes was investigated. The influences of some other factors: the granularity of sample, the content of conductive additive—acetylene black and the content of binder—polytetrafluoroethylene （ PTFE ） in the electrode membrane and the charging-discharging current density etc, on the charging-discharging performances of the sample were investigated by the orthogonal method. The sample was coated by different carbon sources, and the influence of carbon coating on the structure and electrochemical properties of the sample was investigated. The experimental results are as follows:1. The influence degree order of the decomposition conditions on D₃ （ the discharging capacity in the third cycle ） is: decomposition temperature > flow rate of protection gas > decomposition time > milling time. The best combination of the technological conditions is: the ball milling time is 2h, the decomposition temperature is 350℃, the decomposition time is 5h, and the flow rate of protection gas is 1.5L/min, while D₃ is 121.78mAh/g andη3 （ the Coulombic efficiency of the third cycle ） is 98.56%.2. The influence degree order of the synthesis conditions on D₃ is synthesis temperature > Li/Fe/P molar ratio > synthesis time > briquetting pressure. In the best combination of the technological conditions, the Li/Fe/P molar ratio is 1.05:1:1, the synthesis temperature is 650℃, the synthesis time is 18h, and the briquetting pressure is 60MPa, while D₃ is 127.49mAh/g andη3 is 99.46%. The influence degree order of the synthesis conditions on crystalline cell volume （ V ） and average size of crystallites ( D₁₃₁ ) is: synthesis temperature > Li/Fe/P molar ratio > synthesis time > briquetting pressure. The best combination of the technological conditions is: the Li/Fe/P molar ratio is 1.05:1:1, the synthesis temperature is 650℃, the synthesis time is 18h, and the briquetting pressure is 60MPa, while V is 0.29172nm3, which is most close to the crystalline cell volume （ V ） of the theoretical value, and the D₁₃₁ is 34.383nm, which is suitable for the lithium ions to diffuse into the central part of the crystallites. It is obvious that the best combination of the technological conditions determined by microstructure is the same with those determined by charging-discharging performances. They must be optimized simultaneously.3. The sample shows good charging-discharging potential platforms during charging-discharging in nine different electrolytes and good electrochemical performance in the electrolytes using LiClO₄ as solute, EC+DEC （1:1,v:v）, EC+DMC （1:1,v:v） and PC+DME （1:1,v:v） as solvent. The best electrochemical performance of the sample can be obtained in 1mol/L LiClO₄/EC+DMC （1:1）, the cyclic performances are worse and the fading capacities are larger in the electrolytes using LiPF₆ and LiBF4 as solute.4. The factors such as the granularity of sample, the content of acetylene black and the content of PTFE in the electrode membrane and the charging-discharging current density also make influences on D₃. The influence degree order of above four factors on D₃ is: content of acetylene black in the electrode membrane > charging-discharging current density > granularity of sample > content of PTFE in the electrode membrane. In the best combination of the four factors, the granularity of sample is -400 mesh, the content of PTFE in the electrode membrane is 5%, the content of acetylene black in the electrode membrane is 20%, and the charging-discharging current density is 10mA/g, while D₃ is 131.25mAh/g andη₃ is 99.37%.5. The crystal structure is not perfect and the electrochemical properties are not significantly enhanced when the samples are coated by acetylene black, but both of them can be great improved after coated by using glucose as the carbon source, and then the samples demonstrate good charging-discharging potential platform properties and rate capabilities. After coating with phenol formaldehyde resin as the carbon source, samples of high purity and small average size of crystallites are prepared, the pyrolytic carbon forms good coating structure, the particles’size of samples is very small and uniform, even nano-meter particles are prepared, the electrochemical properties of the samples are greatly improved and the samples show excellent charging-discharging potential platforms and rate capabilities. While the coating carbon content is 10%, the sample possesses the best electrochemical properties, then D₃ is 164.89mAh/g andη₃ is 99.20%, and D₃ could achieve 149.12mAh/g at 1C rate.更多还原