【作者】 胡拥军；

【导师】 陈白珍；

【作者基本信息】 中南大学 ， 电化学工程， 2007， 博士

【摘要】 聚合物锂离子电池是一种新型高能化学电源,具有质量比能量高、安全、无电解液泄漏、可使用软包装、外形设计灵活等优点,符合化学电源的发展趋势。本论文的目标是制备性能优良的锂离子电池用聚合物电解质,研究聚合物电池的交流阻抗,讨论电池容量衰减机理和离子传输机理。研究工作主要从以下几个方面展开:研究了相转移法制备PVDF-HFP基微孔型聚合物电解质的工艺条件和造孔机理,制备了改性P（VDF-HFP/MMA）聚合物电解质和无纺布增强型PVDF-HFP聚合物电解质。前者通过引入MMA结构单元改善了电解质的表面性质,后者通过以无纺布为支撑介质提高了电解质的机械强度,获得的电解质孔穴丰富,吸液率高,室温离子电导率均超过1mS·cm^-1,电化学稳定窗口在5.0V以上。其中无纺布增强型聚合物电解质制备的电池界面性质很稳定,50次循环后容量保持率为97%,1C放电容量是0.1C放电容量94%,表现出较好的电化学性能。提出了制备聚合物微孔膜的另一种方法——直接挥发溶剂法,研究了挥发溶剂法造孔的机理,并用该法制备了PVDF-HFP/有机蒙脱石复合聚合物电解质,避免了相转移法制备有机/无机复合聚合物电解质时出现的无机颗粒“团聚”现象。当有机蒙脱石添加量为5%时,获得的复合聚合物电解质孔穴丰富,吸液率达到290%,室温离子电导率为1.51mS·cm^-1,机械性能得到提高,制备的聚合物电池界面阻抗降低,循环50次后容量保持率达到95.9%,倍率放电性能也有所改善。研究了聚合物电池交流阻抗图谱各部分表示的意义,提出了聚合物电池的等效电路并以此对制备的电池进行了模拟,拟合结果与实验测试结果基本一致。讨论了聚合物电池充放电循环时的容量衰减机理,认为循环过程中电解质与电极的界面性质变差使电池界面阻抗增加以及正极晶格粉化使电化学反应阻抗增加是电池容量衰减的主要原因,由此提出了改善聚合物电池循环性能的方法。用季戊四醇和甲基丙烯酸合成出一种含较多双键和羰基的多臂星形有机物单体,并用热聚合方法制备了凝胶型聚合物电解质,通过在聚合物结构中引入乙二醇甲基丙烯酸酯,改善了电解质的性质。该方法聚合速度快,形成的聚合物网络对电解液的包容能力强,单体用量少,当电解液含量为92%时,获得的是粘性好,较柔软的半透明凝胶,室温离子电导率达到1.89mS·cm^-1,制备的聚合物电池电化学性能与使用液态电解质电池的性能接近。另外,还用不同分子量的聚乙二醇甲基丙烯酸酯通过热聚合制备了凝胶电解质,讨论了分子链长度对电解质性能的影响,其中以二缩乙二醇甲基丙烯酸酯与电解液按1:9的配比制备的电解质性能最佳。该方法成本低,操作简单,是制备凝胶型聚合物电解质的一种简单实用的方法。研究了凝胶电解质电导率随温度变化的关系,测量了锂离子传输,表观活化能和离子迁移数随聚合物含量的变化,发现制备的几种凝胶电解质电导率与温度的关系均偏离Arrhenius方程,而活化能及离子迁移数测试结果则表明,凝胶电解质中聚合物含量越高,化学交联点越多,锂离子迁移所需活化能就增加,离子迁移数也减少,据此提出了改善凝胶聚合物电解质性能的方法:选择分子支链长度适当的具有网状结构的高分子;在保持电解液能被聚合物网络包容的条件下,尽量降低聚合物含量,从而增加所包容的电解液及分子链本身的活动能力。更多还原

【Abstract】 Polymer lithium battery is a high energy-density power source. Because it has many advantages such as high specific capacity, little possibility of electrolyte leakage, flexible packaging and shapes, polymer battery has broad prospects and has received great interest in recent years. This paper aims to prepare a lithium battery-used polymer electrolyte which has excellent properties, and study the A.C impedance and mechanism of energy decline and ionic transmission. The main contents of this dissertation are as follows:The technology of preparing PVDF-HFP-based polymer electrolyte using phase inversion method and its mechanism of producing micro-pores were researched, the modified poly（vinylidene fluoride-hexafluoro-propylene/methyl methacrylate） polymer electrolyte and non-woven fabric reinforced poly(vinylidene fluoride-hexafluoropropylene （PVDF-HFP） polymer electrolyte were prepared. By introducing methyl methacrylate, the surface properties of polymer electrolyte are improved, and by using non-woven fabric as supporting medium, the mechanical intensity of PVDF-HFP based polymer electrolyte is enhanced. Both the polymer electrolyte membranes have rich micro-pores. After absorbing electrolyte, their ionic conductivity at room temperature exceeds 1mS·cm^-1, and their electrochemical window reaches 5.0V. The interfacial property of polymer battery prepared by non-woven fabric reinforced PVDF-HFP polymer electrolyte is stable. After 50 cycles, its capacity maintenance ratio is up to 97%, when discharging at 1.0 current rate, it can retain 94% of the discharge capacity of 0.1 current rate. So the electrochemical properties of this polymer battery are very good.By directly volatilizing solvent, a new method to prepare micro-porous polymer membrane was put forward, and its mechanism to produce micro-pores was investigated. When using this method to prepare PVDF-HFP/organo-Montmorillonite（OMMT） composite polymer membrane, the reunition of inorganic particles can be restrained. The polymer electrolyte with 5% OMMT has rich micro-pores, its uptake of liquid electrolyte is 290%, and its ionic conductivity at room temperature is up to 1.51mS·cm^-1. By using PVDFHFP/OMMT composite polymer electrolyte, the interfacial impedance of polymer battery is reduced, the capacity maintenance ratio after 50 cycles reaches 95.5%, and its rate performance is also improved.The meaning represented by A.C impedance plot of polymer lithium battery was studied, an equivalent circuit of electrode system was put forward, and the polymer lithium battery is simulated by using this equivalent circuit. The result of simulation is consistent to that of the experiment. The capacity declining mechanism of polymer battery was then discussed. It is suggested that during cycling, the increase of interfacial impedance resulting from deterioration of interfacial properties and the increase of electrochemical impedance bring by crystal lattice pulverization of cathode material are the main reason of battery’s capacity decline. Based on this investigation, methods to improve the electrochemical properties of polymer battery are educed.A monomer with three arms synthesized by pentaerythritol and methyl acrylic acid was used to prepared gel polymer electrolyte, and by introducing glycol methyl methacrylate to the polymer framework, pliability and viscidity of gel polymer electrolyte were improved. This method has many advantages such as quick polymerizing speed, better capability for polymer to contain liquid electrolyte. The gel with 92% of liquid electrolyte has an ionic conductivity up to 1.89mS·cm^-1 at room temperature, and the electrochemical performance of gel polymer battery is as good as that of liquid electrolyte battery. Beside this polymer electrolyte, another gel polymer electrolyte synthesized by polydiethyleneglycol methacrylate with different molecular weight was prepared, and the effect of molecular longness on its electrochemical performance was also discussed. The results show that when the ratio of diglycol to electrolyte is 1:9, the gel polymer electrolyte has optimal electrochemical properties. Compared with other gel electrolytes, polydiethyleneglycol methacrylate based gel polymer electrolyte has many characters such as low cost, simple operation and good electrochemical properties. Relationship of gel polymer electrolyte’s ionic conductivity to temperature and the effect of polymer content on the apparent activation energy of lithium ion transmission as well as its transfer number were investigated. It turns out that the relationship of ionic conductivity to temperature is not consistent with Arrhenius equation. Additionally, the results also show that with the increase of polymer content, the apparent activation energy of lithium ion transmission enhances, and its transfer number declines. Based on this research, some methods were put forward to improve the properties of gel polymer electrolyte. If a polymer with network structure and right longness is selected to prepare gel polymer electrolyte, and the content of polymer is apropos, the movement ability of both the liquid electrolyte and the polymer itself can be enhanced, so the electrochemical performance of gel polymer electrolyte can be improved.更多还原