【作者】 赵新生；

【导师】 梁辉；

【作者基本信息】 天津大学 ， 材料学， 2008， 硕士

【摘要】 本文采用一步沉淀和两步沉淀法制备水热反应前驱体,在不同的条件下对前驱体进行水热处理,合成了LiFePO₄粉体,并对LiFePO₄进行掺杂改性。探讨水热法制备LiFePO₄的工艺原理、水热法工艺参数和掺杂改性对LiFePO₄结构、微观形貌、电化学性能以及振实密度的影响。研究一步沉淀和两步沉淀两种工艺制备水热反应前驱体的机理,前驱体沉淀中Li与Fe元素的含量的测试表明,两种前驱体沉淀中都含有Li与Fe元素,但溶液中Li与Fe元素并未按化学计量比完全转化为沉淀,同时XRD分析表明前驱体为非晶态,因此水热反应过程是前驱体的进一步沉淀、LiFePO₄的结晶和晶体的长大过程。对一步沉淀和两步沉淀两种前驱体制备工艺制备的前驱体进行水热处理,分析前驱体制备工艺、水热时间和水热反应温度对最终产物充放电性能的影响。通过样品的充放电测试结果表明最佳水热反应条件为:一步沉淀的前驱体工艺中,反应温度150℃,反应时间1 h。两步沉淀的前驱体工艺中,反应温度为190℃,反应时间1 h。粉体的比表面积测试表明随着粉体合成温度的提高,粉体的比表面积逐渐减小,比表面积较小的平台电压较好。研究水热反应条件对振实密度的影响。通过研究发现,反应时间延长,振实密度随之降低;水热反应温度升高,相应的振实密度降低;粉体的吸潮使得粉体的流动性变差,在过80目筛的振实密度测试中发现吸潮测得振实密度很低,而过300目筛测试结果吸潮影响较小,实验中大部分粉体的振实密度在1～1.5 g·cm^-3,这与商业化产品的振实密度指标相近。对LiFePO₄正极材料进行掺杂改性研究,探讨掺杂Mg对正极材料的充放电性能和振实密度的影响。通过研究发现,掺杂5mol.%Mg使得正极材料结晶良好,形貌均一,颗粒的粒径也有所减小,并且正极材料具有明显的充放电平台,较之未掺杂的样品充放电比容量也有很大提高,这说明掺杂5mol.%Mg有效的提高了正极材料的充放电性能;掺杂5mol.%Mg样品的正极材料振实密度较之未掺杂样品有所下降。更多还原

【Abstract】 LiFePO₄ cathode materials were prepared by hydrothermal reaction method with precursors at different conditions. The precursors were synthesized by precipitation named one step and two steps separately. The study on doping modification of LiFePO₄ was carried out. The principle of hydrothermal reaction synthesizing LiFePO₄ was investigated. At the same time, influences of the technology parameters and doping modification on structure, morphology, electrochemical properties and tap density of the cathode materials were discussed.The mechanism of precursors prepared by precipitation of one step and two steps separately was investigated. The results indicate that there were Li and Fe in the precursors. However, the Li and Fe in the solution didn’t transfer to precipitation by stoichiometric ratio. X-ray diffraction analysis of the precursors suggests that the precursor is amorphous. Therefore it was a further precipitation of precursors, a crystallization and growth of LiFePO₄ during the hydrothermal reaction.The precursors were crystallized by hydrothermal reaction. Influences of the process of synthesizing the precursor, time and temperature of hydrothermal reaction on tap density, charging and discharging properties of the cathode materials were discussed. The optimized conditions of hydrothermal reaction were obtained. One is the sample with reaction temperature 150°C and reaction time 1h in precipitation of one step, the other is the sample with reaction temperature 190°C and reaction time 1h in precipitation of two steps. The special surface area of powders analysis indicates that the special surface area decrease gradually by the increasing of temperature, and the smaller one exhibit better flat voltage.The tap density of cathode powder was tested. The results suggest that, the tap density is gradually decreased by increasing reaction time and enhancing the reaction temperature. The fluidity of the cathode powder deteriorated because of the powder absorbing dampness. The tap density of the sample obtained with a 300 mesh sieve is very low. However, influence of the powder absorbing dampness on tap density of the sample obtained with a 300 mesh sieve is sensitive. Most of the cathode tap density is between 1 g·cm^-1 and 1.5 g·cm^-1. The results of tap density are close to the tap density of commercial products. The doping modification of LiFePO₄ was studied, the effects of Mg doping on the tap density, charging and discharging properties of the LiFePO₄ cathode material were discussed. The results reveal that, the doping of 5 mol. % Mg is benefit for the growth of the crystal grain by XRD and SEM, the grain size is decreased, there are charge and discharge flats of cathode material obviously, the specific capacity of the doping of 5 mol. % Mg are increased than the no doping one, however the tap density of the doping of 5 mol. % Mg is more decreased than the no doping one.更多还原