【作者】 程琥；

【导师】 杨勇；

【作者基本信息】 厦门大学 ， 物理化学， 2007， 博士

【摘要】 聚合物锂二次电池由于具有高能量密度、可加工成任意形状以及较为安全可靠等优点而可望成为最有希望的新一代电源之一。PEO基聚合物电解质因其可能替代传统锂离子电池中的液体电解质，成为全固态聚合物锂二次电池中的电解质材料而长期受到广泛关注。但是，在其实现商品化生产之前，尚有许多实际问题需要解决。首先，PEO基聚合物电解质的室温电导率较低，不能满足实际需要，因此提高其室温电导率是当前应该解决的首要问题；其次，金属锂作为负极材料，它与聚合物电解质的界面相容性严重影响着电池的循环性能和安全性。为此，本文主要针对上述问题进行了研究，并取得以下成果：1．通过FTIR光谱技术研究了P（EO）_n-LiX（X=SCN^-，N（SO₂CF₃）₂^-，ClO₄^-，n=4～60）聚合物电解质中离子-离子、离子-聚合物基体之间的相互作用。结果表明，当锂盐加入到低介电常数的PEO中，锂盐自身存在着缔合作用，阴离子不同，缔合程度也有所不同。LiSCN较其他两种锂盐具有更严重的缔合行为，当LiSCN的加入量较大时，聚合物电解质中不仅存在大量的离子对，而且还会形成二聚体与三离子簇。此外，锂盐阴离子还会对PEO具有增塑作用，可以改变聚合物的晶相组成，增加无定形相含量。其中LiN（SO₂CF₃）₂中阴离子体积较大，增塑效果最好。从离子传导的角度来看，缔合和增塑两种作用造成的效果是相反的。离子缔合必然减少体系的有效载流子数目，不利于离子传导；而增塑作用可以改变聚合物的晶相组成，增加无定形相含量，有利于离子传导。因此，在电导率．锂盐浓度曲线上电导率会有先增大后减小的变化趋势。对于LiN（SO₂CF₃）₂、LiSCN和LiClO₄三种聚合物电解质体系，在锂盐浓度相同时，LiN（SO₂CF₃）₂以其较低的缔合行为和良好的增塑效果，从而具有最高的电导率性能；而对于增塑效果相似的LiSCN和LiClO₄，前者因其较为严重的缔合行为使电导率略低于后者。2．合成了BMPyTFSI和BMImTFSI两种离子液体，并将其引入PEO基聚合物电解质中制得新型离子液体复合聚合物电解质，通过交流阻抗、直流电位阶跃、线性电位扫描等电化学技术研究了添加离子液体前后对聚合物电解质的离子电导率、锂离子迁移数、电化学稳定窗口以及界面稳定性等性能的影响。结果发现，引入BMPyTFSI或BMImTFSI离子液体后，聚合物电解质的离子电导率明显增加，增加幅度主要出现在低温区域，其中咪唑体系的电导率稍微大于吡啶体系的电导率。当BMImTFSI的掺杂量为x=1．0时，其电导率在40℃下可以达到10^-4S／cm以上，比未添加离子液体的聚合物电解质的电导率增加两个数量级。尽管离子液体的引入导致锂离子迁移数降低，但是其锂离子电导率还是呈上升趋势。此外，离子液体的加入，大大降低了锂电极／聚合物电解质的界面电阻，改善了界面稳定性，同时拓宽了聚合物电解质的电化学稳定窗口。在两种离子液体掺入量均为x=1时，稳定窗口达到5．2V（vs．Li⁺／Li），这一结果为5V高压电池的发展提供一种可能的应用体系。DSC和FTIR研究结果表明，随着离子液体（BMImTFSI或BMPyTFSI）的加入量的提高，聚合物的玻璃态转变温度与结晶度均逐渐降低。当BMImTFSI的掺杂量为x=1．0时，聚合物的玻璃态转变温度与结晶度分别下降到-52．6℃和17．46％。因此，加入离子液体对聚合物电解质电导率的改善主要有两个影响因素：一方面较高的介电常数促进锂盐的离解，增加有效载流子数目；另一方面起到增塑作用，改变聚合物的晶相组成，增加无定形相含量，从而提高了聚合物电解质的离子电导率。3．通过XPS、FTIR技术并结合Ar⁺离子束溅射技术对含有PC或BMPyTFSI的聚合物电解质中形成在镍基体表面上的锂钝化膜的组成与结构进行了研究。结果表明，在PEO基聚合物电解质体系中，PEO相对而言比较稳定，基本没有参与钝化反应，钝化膜主要由聚合物电解质中的有机添加剂、锂盐阴离子以及少量杂质（O₂和H₂O）的还原产物组成。不同的有机添加剂对钝化膜的组成有很大的影响。若添加PC，则钝化膜主要含有ROCO₂Li和Li2CO₃等物种；若添加BMPyTFSI离子液体，则与锂金属比较稳定，钝化膜主要以LiF物种为主。其次，锂盐不同，形成的钝化膜组成也有所区别。对于LiN（SO₂CF₃）₂，LiF是钝化膜中的主要成分；而LiSCN及LiBr对钝化膜成分基本没有影响。研究还发现，在锂离子的沉积溶出过程中，钝化膜的组成没有发生太大的变化，这对于获得良好的电池充放电性能是非常重要的。4．设计并优化了一种电化学现场光谱电解池，利用显微红外光谱技术对Li／聚合物电解质固／固界面性质进行了研究。结果表明，在0-3V的首次循环伏安扫描中，由于发生了O₂和H₂O的还原反应以及锂离子欠电位沉积-溶出过程，红外光谱中各个吸收峰表现出强度先减弱后增强的趋势，并且锂盐的变化幅度较聚合物基体更加明显。这主要由两种效应造成的，一方面O₂和H₂O还原形成的钝化膜降低了红外反射率，另一方面还原反应损耗了接近工作电极表面的锂离子，导致阴离子向基体电解质移动。从同步获得的显微图象中也可以清楚观察到由于O₂和H₂O的还原过程以及锂离子欠电位沉积-溶出过程所引起的界面形貌变化。对于含有PC的聚合物电解质体系，现场红外光谱表明，经过锂离子的沉积-溶出过程，电极表面化学物种主要是PC的还原产物ROCO₂Li以及Li₂CO₃等。此外，不同选择区域的红外光谱表明，钝化膜主要形成在电极与电解质紧密接触的CE附近。更多还原

【Abstract】 The rechargeable lithium polymer batteries are considered as one of the best candidates for next generation power sources due to their high energy density, good cyclability, reliability and safety. PEO-based polymer electrolytes have received extensive attentions for their potential capability to be used as alternative candidate materials for the traditional liquid electrolytes in all solid-state rechargeable lithium polymer batteries. However, there are many problems that must be solved before these systems reach wide commercial utilization. Firstly, the battery performance is largely limited by the low ionic conductivity of PEO-based polymer electrolytes at room temperature. Therefore, it is important for PEO-based polymer electrolytes to enhance the ionic conductivity at room temperature. On the other hand, since lithium metal is used to be the anode material, the interface compatibility of polymer electrolyte with lithium metal severely affects the safety and cycle life of battery.In this dissertation, many efforts have been invested in the study of the problems above, and the results as follows:1. FTIR spectra of PEO-LiX （X=SCN, N（SO₂CF₃）₂, ClO₄） polymer electrolytes have been obtained for EO/Li ratios from 60:1 to 4:1 in order to investigate the interactions of ion-ion and ion-polymer. When a lithium salt is dissolved in the PEO with low dielectric constant, the ion association is commonly present and the degree of ion association varies with the type of anion from lithium salt. The results show that the ion association in the PEO-LiSCN system is more serious and at high concentration of LiSCN contact ion pairs, triple ions and dimers are main ion species. Furthermore, the crystalline phase of PEO was progressively transformed into amorphous phase since the large-size anions from lithium salt can play a role of plasticizer. LiN（SO₂CF₃）₂ with the largest size anion in this work possess the best plasticizing effect.From the ion transport of view, the results induced by the ion association and plasticizing role are opposites. The ion association necessarily diminishes the number of effective charge carrier and then leads to low ionic conductivity, whereas plasticizing role can change the phase composition of PEO and increase the content of the amorphous phase in which the highest ionic conductivity occurs. As a result, a maximum of conductivity is observed when the salt concentration is higher than a certain value. Based on the FTIR analysis, we commendably interpret the fact that ionic conductivities of PEO-LiX （X=SCN, N（SO₂CF₃）₂, ClO₄） polymer electrolytes follow the order LiN（SO₂CF₃）₂ > LiClO₄ > LiSCN at the same salt concentration.2. BMPyTFSI and BMImTFSI ionic liquids were synthesized by anion exchange reaction, then new gel polymer electrolytes containing ionic liquid were prepared by solution casting method. The electrochemical properties such as ionic conductivity, lithium transference number, electrochemical stability windows, and the compatibility with Li electrode were investigated with ac impedance, dc polarization, and linear sweep voltammetry techniques. The results show that the incorporation of BMPyTFSI or BMImTFSI to the P（EO）₂₀LiTFSI electrolyte improves the ionic conductivity over the entire temperature range investigated, but the greatest enhancement is at lower temperatures. The ionic conductivity of the P（EO）₂₀-LiTFSI electrolyte at 40℃with a BMIm⁺/Li⁺ mole fraction of 1.0 showed an increase of about two orders of magnitude reaching 4×10^-4 S/cm. In despite of decreasing for the lithium ion transference number with the increase of the amount of BMPyTFSI or BMImTFSI, the lithium ionic conductivity increased. The electrochemical stability and interfacial stability for these gel polymer electrolytes were significantly improved due to the incorporation of BMPyTFSI or BMImTFSI. At high concentration of ionic liquid, the electrochemical stability window reaches 5.2 V. The results suggest that the polymer electrolyte containing ionic liquid can be applied safely in 5 V lithium secondary batteries.The DSC and FTIR results show that the glass transition temperature （T_g） and the crystallinity obviously decrease with increase of the content of ionic liquid. These revealed ionic liquid can weaken the interaction among the polymer chains and accelerate the segmental motion of the PEO-based polymer electrolyte, thus the ionic conductivity of the gel polymer electrolytes increases.3. The passive layers formed on lithium at a nickel substrate in polymer electrolytes containing PC and BMPyTFSI were characterized by using XPS, FTIR as well as Ar ion sputtering technique. PEO seems to be rather inert to lithium and has no effect on the composition of the passive layer. The passive layers are mainly composed of the reduction products of organic plasticizer, anion from lithium salt, and impurities existing in electrolyte such as traces of O₂ and H₂O. The results show that PC was reduced on Li to ROCO₂Li and Li₂CO₃ species and the main reduction product of LiN（SO₂CF₃）₂ was LiF. Polymer electrolytes containing BMPyTFSI ionic liquid seemed to be stable with lithium and only formed a fewer passive layers mainly including LiF. LiSCN and LiBr salts have no effect on the composition of the passive layer. During the lithium deposition-dissolution process, there is no change for the composition and structure of the passive layer. It is very important to obtain good performance of battery.4. A spectroelectrochemical cell was designed and optimized, then the solid-solid interface between lithium electrode and polymer electrolytes was explored to characterize by using in situ micro-FTIR spectroscopy. The cyclic voltammetric results indicated that the reducing reactions of oxygen and water as well as the under-potential deposition （UPD） of lithium occur in the electrode/electrolyte interface in the different potential region. The infrared spectral changes observed during the CV process revealed that there is a direct correlation between the CV peaks and the magnitude of the infrared peaks. This change is most likely due to an increase or decrease in the infrared reflectivity induced by the formation of a thin layer at the Au/polymer electrolyte interface. It is shown that the infrared reflectivity from the solid-solid interface is very sensitive to the formation of the passive layer on the lithium electrodes. On the other hand, in situ FTIR results show that there is a sharply decrease in the amount of Li salt probed by the beam during the reducing process of water. The reason is that the reducing process leads to the loss of lithium ion from the surface of the working electrode and accompanies by the migration of anion into the bulk electrolyte. Optical micrographs obtained simultaneously also displayed directly the formation of the passive layer along with lithium deposition and dissolution process. It is correlated well with in-situ FTIR and electrochemical experiments.In situ FTIR results obtained from the polymer electrolyte containing PC after lithium deposition-dissolution process show that the surface chemistry of Li is dominated, as expected, by PC reduction to ROCO₂Li and Li₂CO₃ species. These are in agreement with the results obtained from ex situ experiments.更多还原