凝聚态物理
化学电源化学电源
    
    本报告主要开展两方面的工作，一是开发了磷酸铁锂材料新的合成工艺并对其中试化进行初步探索；二是建立了差分电化学质谱系统并将其应用于锂离子电池体系方面研究。
    在磷酸铁锂合成方面，我们开发了新的合成路线，通过两步法合成了LiFePO4/C复合正极材料。通过XRD粉末衍射、元素分析、SEM等手段对合成的材料进行了表征，对合成工艺中的烧结温度、烧结时间、锂源、碳含量等参数进行了优化，确定了实用化合成LiFePO4/C正极复合材料的最优条件。电化学性能的测试结果表明，优化的工艺参数为：碳酸锂为锂源，原料混合物以500 rpm球磨4h，高温烧结温度为750oC，烧结时间为1h，产物中碳含量为3.19%。通过优化工艺合成的实验室样品以0.1C电流充放电，首次充放电比容量达到160 mAh-1以上，1C最高放电比容量达到140 mAh-1以上。材料的具有优异的循环性能，室温下经过300次循环，容量几乎没有衰减，高温50oC下经过300次循环，容量保持在75%以上。
    在实验室合成的基础上，我们对该合成路线的中试化工艺进行初步探索。与实验室材料相比，中试工艺下合成的材料性能有所下降。以0.1C电流充放电，首次充放电比容量约130 mAh-1，1C最高放电比容量在120 mAh-1以上。材料性能的下降与铁原料的纯度以及烧结的均匀性有关。成本分析结果表明，用该工艺合成的LiFePO4/C材料，原料成本大约在3.19万元/吨。
    在差分电化学质谱方面，我们成功建立了该系统并应用于锂离子电池体系研究。AlF3 包覆Li[Li0.2Ni0.13Mn0.54Co0.13]O2正极材料前后的DEMS研究表明，包覆层的存在为材料与电解液提供了一个缓冲，抑制了高电位下正极材料脱氧造成的电解液氧化。在乙烯基亚硫酸乙烯酯（VES）添加剂的研究中，通过DEMS技术直接检测到了还原产物丁二烯，为VES还原机理的提出提供了直接证据。在商业电池的首次充电过程研究中，DEMS结果证实，电池首次充电过程产生的气体是分阶段进行的，不同的阶段产生的气体并不相同。 [英文摘要]：     In this report, two parts of work were introduced. First, a new synthesis route of LiFePO4 material was studied and its pilot-technology was also explored. Second, differential electrochemical mass spectrometry (DEMS) technique was developed and applied to the research of lithium ion batteries.
    In the part of LiFePO4 synthesis, a new two-step route was proposed and a LiFePO4/C composite material was synthesized successfully. The samples were characterized by XRD, elemental analysis and SEM techniques and the electrochemical performances were also measured. The effects of sintering temperature, sintering time, lithium sources and carbon content on the electrochemical performance of LiFePO4/C composite material were investigated. The optimal conditions of preparing LiFePO4/C material are as follows: Li2CO3 as lithium source, the raw materials are firstly ball-milled 4h at a rate of 500rpm, and then calcined 1h at 750oC, and the carbon content is 3.19%. The first discharge special capacity of lab-synthesis LiFePO4/C sample is above 160 mAhg-1 at 0.1C and the highest discharge special capacity is above 140 mAhg-1 at 1C. In addition, the sample also exhibits excellent cyclic performance. After 300 cycles, the capacity retention is almost 100% in room temperature and above 75% in high temperature (50oC).
    Based on lab-synthesis results, the pilot-technology of this synthesis route was explored. Compared with lab sample, the electrochemical performance of the pilot sample was not good as the lab-synthesis results. The first discharge special capacity decreases approximately to 130 mAhg-1 at 0.1C and the highest discharge special capacity decreases approximately to 120 mAhg-1 at 1C. The reason may relate to the purity of iron source and the uniformity of calcined process. The analysis results shows that the cost of raw materials is about ￥31,900/ton by this synthesis route.In the second part, the DEMS devices were set-up and applied into the field of lithium ion batteries. In AlF3-coated Li[Li0.2Mn0.54Ni0.13Co0.13]O2 system, the DEMS results indicate that AlF3 coating layer can provide a buffer layer between cathode surface and electrolytes, which causes the activity of initial extracted oxygen species is greatly reduced and the decomposition of the electrolyte is significantly suppressed. In the research of Vinyl ethylene sulfite (VES) additive, the reduced production butadiene C4H6 is detected by DEMS technique, which provides a directly evidence for the reductive process research of VES. In addition, the first charge process of commercial lithium ion batteries is also researched by DEMS. The results show that these gases do not generate at the same time during the charge process and these gases will generate in different charge stage. [参考文献]：     1. Winter, M.; Besenhard, J.O.; Spahr, M.E.; et al. Insertion electrode materials for rechargeable lithium batteries. Adv. Mater,1998,10(10):725-763.
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