【作者】 蔡志军；

【导师】 李磊；

【作者基本信息】 上海交通大学 ， 高分子物理和化学， 2011， 硕士

【摘要】 随着经济的发展和文明的进步,人类对于能源的依赖越来越偏向于可持续、环境友好的新能源体系。锂离子二次电池由于具备高能量密度、无记忆效应、无污染等突出优势,成为最受青睐的二次电池。经过多年的发展,锂离子电池已在移动电话、笔记本电脑、数码相机等便携式电子设备上得到广泛应用。但迄今为止,锂离子电池在性能上还不能完全满足动力锂离子电池的要求,尤其是其高温循环稳定性,例如:目前电池的循环寿命仅为5年左右,低于电动汽车或混合动力汽车所要求的10-15年的使用年限;在较高的环境温度下(>50oC)电池容量快速下降等。电解质作为锂离子电池的关键材料之一,对电池的性能和成本有着重要的影响。本文拟通过新型添加剂改善锂离子电池的电解液和开发新型的聚合物电解质两种途径,旨在解决锂离子电池目前存在的高温循环稳定性问题。本文首先选取3-三甲基-硅烷硼酸酯(TMSB)作为锂离子电池的添加剂,以改善电解液的高温稳定性,进而提高电池的高温循环稳定性,同时通过相关的交流阻抗、电镜扫描、循环伏安、红外、表面元素分析等测试验证了TMSB对电解液稳定性的作用,并研究其作用机理。研究发现, 55℃下,电解液中含有1wt% TMSB的LiFePO4/Li电池经过80次循环之后的容量衰减仅为6%,而不含添加剂的电池经过同样的循环之后的容量衰减达到25%。交流阻抗测试表明,在55℃时,含有1wt%的TMSB添加剂的电池经过循环之后比不含添加剂的电池的界面阻抗更低。SEM以及XPS表明添加剂的使用对电极的界面形貌以及表面组分都有较大的影响。研究表明,TMSB的缺电子结构,使其易与PF6-相互作用,降低LiPF6的热分解能力,提高电解液的热稳定性,从而提高了锂离子电池的循环性能,特别是高温下的循环性能。另外,本文制备了全氟磺酸锂离子交换膜聚合物电解质,并首次验证其在锂离子电池中的应用可行性。通过溶液浇铸成膜、锂化、脱水、溶胀等过程制得碳酸丙烯酯溶胀的全氟磺酸锂聚合物电解质(PC-PFSA-Li膜)。交流阻抗测试显示,所制得PC-PFSA-Li膜的室温电导率为4.63×10-4 S/cm,而其在60℃时的电导率达到1.03×10-3 S/cm;同时,通过阻抗分析表明PC-PFSA-Li膜和Li金属具有良好的界面稳定性。循环伏安测试表明,PC-PFSA-Li膜具备良好的电化学稳定性,在2.5-4.35V(vs. Li/Li+)内并没有发生氧化分解反应。LiFePO4/PC-PFSA-Li膜/Li电池在80℃时具有良好的循环性能,其经过100次循环之后的容量保存率在90%以上,而使用传统电解液的LiFePO4/LiPF6-EC-DMC/Li电池,在同样条件下经过45次循环之后,电池的容量迅速衰减,在第58次循环时,其容量的保存率仅有75%。相比于传统的液态电解质,新型的全氟磺酸锂聚合物电解质由于避免锂盐的使用,提高了电池的高温循环性能的同时,也降低了电池的成本,因而是一种具备良好应用前景的聚合物电解质。更多还原

【Abstract】 With the development of the economy and society, much more attention has been paid to the utilization of the new energy for the shortage of traditional energy. Due to its high energy density, high voltage and long cycle life, lithium-ion battery becomes the most popular secondary battery since it was firstly introduced in 1990. After many years’development, lithium-ion batteries have been widely used as power source of cell phones, laptops and digital cameras, as well as other potable electric devices. However, lithium-ion battery still cannot be used as the power source for electric vehicles (EV) or plug-in hybrid electric vehicles (PHEV) because of safety concerns, short cycle life and high cost. For example, the battery can only work less than 5 years at room temperature, which is much shorter for the requirements of electric vehicles, 10 to 15 years; the battery will cause safety problems such as explosion under abused conditions; and the battery decays quickly at elevated temperatures (≥50oC). These problems are closely related to the electrolyte used in the battery. In this paper, we try to use electrolyte additives and develop a new polymer electrolyte to solve the existing high temperature cycle stability problems of the lithium ion battery.Tris(trimethylsilyl) borate (TMSB) was used as a new electrolyte additive to improve high temperature performance of LiFePO4 based lithium-ion battery. The effects of the TMSB on the LiFePO4 electrode are investigated via a combination of electrochemical impedance spectroscopy (EIS), cyclability, scanning electron microscope (SEM) and X-ray photoelectron spectroscopy (XPS). It is found that the LiFePO4 battery with a composite LiPF6-based electrolyte containing 1 wt% TMSB additive exhibits higher discharge retention and better cycling performance than the battery without TMSB additive at both 30°C and 55°C. SEM and XPS measurements show the changes of surface morphology and formation of solid electrolyte interface (SEI). EIS results indicate that the interfacial impedances of the batteries after cycled at 55°C with the electrolyte containing TMSB additive are significantly smaller than the batteries without additive. The improved performances are ascribed to the enhancement of the thermal stability of the electrolyte and the modification of SEI component on the LiFePO4 electrode. New polymer electrolyte based on lithiated perfluorinated sulfonic ionomer with high ion exchange capacity for lithium-ion battery is investigated.Through polymerization, solution-casting, salification and dehydration/swelling processes, lithiated perfluorinated sulfonic polymer electrolyte swelled with PC (PC-PFSA-Li) is prepared. The PC-PFSA-Li polymer electrolyte shows ionic conductivity of 4.63×10-4 S/cm at room temperature and 1.03×10-3 S/cm at 60 oC, respectively. Cyclic voltammetry indicates that the PC-PFSA-Li polymer electrolyte possessed good electrochemical stability in the range 2.5-4.5 V (vs. Li/Li+). EIS shows that PC-PFSA-Li polymer electrolyte is compatible with lithium foil. The LiFePO4/PC-PFSA-Li/Li batteries show higher capacity retention at 80 oC after 100 cycles than that of the battery using the LiPF6-EC-DMC conventional liquid electrolyte. The replacement of conventional liquid electrolytes and the use of polymer electrolytes with the ion-exchange membranes not only gives an extra contribution in improving the performance and safety of the lithium-ion battery, but also reduces the cost of the battery system. Therefore, this novel lithium-ion polymer battery would be used as new, advanced types of power sources for hybrid and electric vehicles.更多还原