【作者】 余红明；

【导师】 赵新兵；

【作者基本信息】 浙江大学 ， 材料学， 2011， 博士

【摘要】 橄榄石型磷酸亚铁锂作为锂离子电池正极材料具有较高的比容量,良好的循环稳定性,可靠的安全性以及低廉的价格等特征成为目前学术界和产业界共同关注的焦点。本文从实用性的角度出发,以改善材料的碳包覆性能和电化学性能为目的,从固相合成和液相制备两个方向讨论了优化碳包覆及其对材料电化学性能的影响。同时,进行了磷酸亚铁锂的10公斤级中试项目研究和100公斤级的小批量生产试验,摸索了各工艺条件对产物物理特性和电化学性能的影响。首先研究了二茂铁在一步固相法制备磷酸亚铁锂的过程中对碳包覆的优化效果。实验发现,二茂铁可以在碳包覆的过程中起催化剂的作用,使包覆碳的石墨化程度提高,从而改善材料的电化学性能。二茂铁对热解温度更高的碳源聚丙烯有着更好的催化效果,产物的电导率和电化学性能都得到了很大的提高。基于二茂铁高温分解出纳米Fe对碳的催化效果,考虑直接使用前驱体中本身的Fe2O3作为催化剂,分析其在高温下对碳包覆的催化作用。以乙炔气体作为碳源和还原性气氛,采用化学气相沉积的方式来对LiFePO4进行包碳。实验发现,不同的原料体系在特定的条件下,都能得到LiFePO4与碳纤维的复合材料。碳纤维的生长与磷酸铁锂的生长是竞争关系。如果前驱体各组分的活性较高,则倾向于生成LiFePO4,Fe2O3没有机会催化碳纤维的生成。因此需要针对不同的前驱体设置不同的气氛通入机制和加热机制。而碳纤维的生长会消耗部分Fe2O3,因此需控制碳纤维的含量。同时,我们发现,高温下沉积的碳除了碳纤维外,还有大量无定形态的碳,这些无定形态碳容易在颗粒表面形成连续致密的碳膜,影响锂离子在颗粒表面的迁移,造成充放电过程中的极化。因此有必要合理控制CVD的条件,使碳纤维和无定形碳的比例达到最佳,在提高材料电导率的同时,尽量降低碳含量,提高材料的比容量和能量密度。通过共沉淀法制备磷酸铁锂与石墨烯的复合材料。实验发现,复合材料的电化学性能受制于磷酸亚铁锂本身的结晶完整性和包覆碳的电导率。前者可以通过延长共沉淀的时间来改善,而后者可以通过升高热处理的温度来改善。提高材料本身的结晶完整性可以提高小倍率充放电时的比容量,而改善包覆碳的电导率则可以改善材料在大倍率下的比容量。通过对中试过程由小到大整个过程的摸索,掌握了一步法制备磷酸亚铁锂的工艺中各参数对产物的物相、形貌和电化学性能的影响。湿法球磨可以获得分散均匀的前驱体；合适的喷雾干燥温度可以避免二次颗粒的过分团聚,控制粉末的粒径分布；焙烧过程中合理的升温制度需要考虑前驱体的热分解特性,及时除水可以有效的降低炉膛的氧化性气氛,保证产物的纯度。还可以在原料中添加适量的聚合物碳源,双重碳源既能增加炉膛的还原性气氛,又能改善材料碳包覆的均匀性。对产物的适当球磨或者糅合处理可以有效地改善产品的加工性能。最终,我们在10公斤级中试中和100公斤级小规模试生产中都获得了性能良好的实用的磷酸亚铁锂正极材料,圆满完成了中试试验。更多还原

【Abstract】 Olive type LiFePO4 as cathode material of Li-ion batteries has attracted the eyes from both academy and industry world because of its high specific capacity, long cycle life and calendar life, high safety and low cost. For the purpose of application, we try to optimize carbon coating during the synthesis of LiFePO4 using either solid-state or aqueous precipitation method, and discuss the influence on the electrochemical performance of LiFePO4 Besides, we conducted the pilot plant test of the LiFePO4/C composites on a 10 kg scale and mass production on a 100 kg scale, during which we discuss the mechanism of process conditions that influence the physical and electrochemical performance of LiFePO4 cathode material.Ferrocene has been chosen as the catalyst of graphitizing material during one step solid state synthesis of LiFePO4. Carbon coating was improved by lowering D/G(disordered/graphene) ratio and increasing sp2/sp3 ratio. As a result, the conductivity and the electrochemical performance was improved. Moreover, the catalization ability of ferrocene seems to be different for different kinds of carbon sources. As for polypropylene, the pyrolyze temperature was high and the pyrolyzed carbon was more easily catalyzed into sp2-coordinated structure, thus the conductivity was improved more significantly.It is common to relate ferrocene with Fe2O3 because they can both be reduced into nano Fe particles under reduction atmosphere. And hence we chose the precusor with Fe2O3 to catalyze carbon during carbon coating. Acetylene gas was used as carbon source and a CVD method was proposed to coating LiFePO4 with carbon. After some experiments, we discoverd that each system with Fe2O3 can catalyze acetylene into carbon nano fibers under certain conditions. And the process that carbon fiber grows is competing with that of LiFePO4. As for those precusor system composed of more active components, the growth temperature of carbon nano fibers should be lower. Thus for different kind of precusor, different CVD conditions shoule be adopted. Besides, the growth of carbon fibers comsumed some Fe2O3, which may reduce the content of active material thus the process should be well controled. In addition, we find that amorphous carbon is depositing along with carbon fibers, and forms a continous carbon film outside LiFePO4 particles, while its thickness increases with carbon content. This kind of film has a negative impact on the electrochemical performance by preventing Li ions transfering between electrolyte and cathode particles. Thus proper conditions should be controled to get balance between amouphous carbon and carbon nano-fibers, and increase the electronic conductivity without increasing carbon content, improve the specific capacity and energy density.In order to get a thin carbon film coating and good electronic conductivity, we prepared LiFePO4/graphene composites with co-precipitation method. The electrochemical performance of the composites was proved to be influenced by the intrinsic crystallinity and electronic conductivity of coating carbon. The intrinsic crystallinity can be improved by extending the reaction time of co-precipitation, and the electronic conductivity of coating carbon actually depends on the removing of oxygen containing groups, which can be improved by high temperature heat treatment. The specific capacity and rate performance of the composites can be improved by increasing the crystallinity of LiFePO4 and purity of graphene respectively.Also, we conducted the pilot plant test of LiFePO4 massive producing, during which we found the parameters that had influences on the morphology and electrochemical performance of LiFePO4. For example, wet ball milling can improve the homogeneity of the precusor, suitable temperature of spray dry can prevent the size of the secondary particles of the precusor from growing up. The pyrolyzing properties of the precusors should also be taken into accout when setting the temperature of the furnace. Expeling of moisture from the furnace in time made a significant improve of the atmosphere in the furnace and promise the purity of LiFePO4. Adding polymer as a second carbon source to improve the reduction atmosphere and a homogeneous carbon coating is also important. Ball mill of the product can improve the processing property of the cathode material. At last, we obtained products with good electrochemical performance on both 10 kg scale and 100 scale mass prodution.更多还原