非水锂空气电池电解液稳定性及氧还原电极过程的研究

发布时间：2018-07-08 11:18

本文选题：锂空气电池 + 非水电解液体系　；参考：《国防科学技术大学》2014年博士论文

【摘要】：随着人们对高能量密度储能系统的需求日益迫切,具有极高理论能量密度的锂空气电池(11300 Wh kg-1)自问世起便得到了研究人员的广泛关注。非水锂空气电池因具有结构简单、电极反应可逆的特点,已成为目前锂空气二次电池领域的研究热点。虽然具有诸多优势,但非水锂空气电池在实际应用过程中依然面临着电解液体系不稳定、电池循环性能较差,空气电极结构和表面性质对电极放电性能影响机制不明确等问题,导致目前非水锂空气电池实用化进程面临着严峻挑战。针对上述问题,本文首先利用循环伏安、恒流充放电测试及离线光谱测试对三种有机电解液体系—碳酸酯类(碳酸乙烯酯,EC和碳酸二乙酯,DEC)、醚类(乙二醇二甲醚,DME)和酰胺类(N-甲基-2-吡咯烷酮,NMP)溶剂在锂空气电池中的稳定性进行系统研究。研究结果表明,碳酸酯类分子易于被O2-亲核进攻发生分解生成Li2CO3和烷基碳酸锂;醚类分子的自氧化反应则会导致醚类电解液在长时间放电过程中发生分解,副反应产物为Li2CO3。由于存在不稳定分解,两种溶剂分子均不适用于非水锂空气电池。在研究的三种溶剂分子中,酰胺类溶剂具有最好的稳定性,其首次放电产物为Li2O2,首次循环过程的库伦效率达到97%。NMP在放电过程中对O2-良好的稳定性为后续O2还原电极过程研究提供了稳定的电解液体系。但对采用NMP电解液锂空气电池的循环性能进一步研究表明,充电过程中,NMP在空气电极表面会发生电化学氧化分解,生成Li2CO3和LiNOx;同时在锂负极表面发生电化学还原,生成亚胺基醚锂、氨基酸锂和LiOH。充电过程中NMP在正负极两侧的分解均会导致电池循环性能的衰退。以两种标准平板电极金(Au)电极和玻碳(GC)电极为研究对象,综合考虑O2还原电极过程中,O2异相电荷转移过程、O2-均相转化过程和Li2O2沉积三个步骤间的相互影响。利用稳态极化曲线,阴极极化条件下的交流阻抗谱和整体电解法系统研究了两种电极表面O2还原过程的特征。研究结果表明,O2在不同电极材料表面上的还原过程存在显著差异。相对于GC电极,Au电极表面O2具有更快的异相电荷转移速度和较慢的O2-均相转化速率,Li2O2在其表面的沉积倾向于垂直表面生长。电化学还原产物O2-在Li2O2表面的富集导致O2还原过程中,沉积的Li2O2电阻率水平显著低于其本体电阻率。O2的电荷转移过程越快,O2-的消耗速率越慢,Li2O2电阻率水平越低,因此Au电极表面沉积的Li2O2具有较小的电阻率数值。为进一步研究多孔空气电极中材料表面性质同电极放电性能间的关系,本文利用溶液浸渍方法制备了一种全碳纳米管(CNT)空气电极。在不改变电极宏观孔道结构的前提下,通过改变热处理温度,控制CNT表面性质,系统分析了CNT电极表面性质对O2还原电极过程的影响。结果表明,CNT表面具有吸电子能力的含氧基团会抑制O2的异相电荷转移过程,同时含氧基团的引入会提高O2-的均相转化过程速率。在含氧官能团含量较高,表面孔道孔径较小的CNT电极表面,Li2O2表现出倾向平行电极表面生长的特征。降低CNT表面含氧官能团含量,扩大表面孔道孔径,Li2O2在CNT表面垂直生长的趋势增强。根据CNT表面Li2O2电阻率水平和不同热处理条件下CNT电极表面形貌随放电深度的变化规律,本文确定了放电过程中电极电子传导能力并不是导致放电终止的主要因素,电极放电性能主要决定于O2在Li2O2表面的后续还原过程。在含氧官能团含量低,表面孔道孔径较大的CNT电极表面,较高的相对生长指数使得Li2O2具有椭球状形貌结构,该形貌结构有利于O2在Li2O2颗粒表面的后续电荷转移过程,这使得上述CNT电极体现了良好的放电性能。结合上一部分的研究结果,电极材料表面性质直接决定了电极表面O2还原过程特征。因此,电极材料表面性质是影响电极放电性能的根本性关键因素之一。本文还在现有单一电解液体系无法同时满足充电过程中锂空气电池正负极稳定性需求的研究结果基础上,探索了一种新型双电解液体系在非水锂空气电池中的应用。以CNT电极为空气电极,金属锂为负极,引入固体电解质LAGP,正负极两侧分别采用NMP和碳酸酯类电解液,构成具有双电解液结构的锂空气电池(Li|1 mol L-1 LiPF6-EC\DEC|LAGP|0.1 mol L-1 LiClO4-NMP|CNT)。测试结果表明,通过利用固体电解质的隔离作用避免非水电解液在正负极的副反应历程,该电池体系在限制3000 mAh g-1比容量的条件下,40次后的循环性能无明显衰减。该结构体系的提出,为现有锂空气电池的应用探索了一条可行途径。
[Abstract]:With the increasing demand for high energy density energy storage systems, lithium air batteries with high theoretical energy density (11300 Wh kg-1) have been widely concerned by researchers since they were asked. The non lithium ion battery has become the research of the two battery field of lithium air because of its simple structure and reversible electrode reaction. Although it has many advantages, the non water lithium air battery still faces the problems of the instability of the electrolyte system, the poor performance of the battery cycle, the unclear influence mechanism of the air electrode structure and the surface properties on the discharge performance of the electrode, which leads to the severe choice of the practical process of the current non water lithium air battery. In order to solve the above problems, the stability of three organic electrolyte systems - carbonate (ethylene carbonate, EC and two ethyl carbonate, DEC), ethers (ethylene glycol two methyl ether, DME) and amides (N- methyl -2- pyrrolidone, NMP) solvents in lithium air batteries The results show that the carbonic acid esters are easily decomposed by O2- nucleophilic attack to produce Li2CO3 and alkyl lithium carbonate, and the self oxidation of ether molecules will result in the decomposition of the ether electrolyte during the long discharge process. The side reaction product is Li2CO3. because of the existence of unstable decomposition, the two solvents are discomfort. For the non water lithium air battery. Among the three solvent molecules studied, amides have the best stability, the first discharge product is Li2O2, the Kulun efficiency of the first cycle process reaches 97%.NMP and the good stability of O2- during the discharge process provides a stable electrolyte system for the subsequent O2 reduction electrode process. Further study on the cyclic performance of NMP electrolyte lithium air battery shows that in the charging process, the electrochemical oxidation of NMP will occur on the surface of the air electrode to produce Li2CO3 and LiNOx, and the electrochemical reduction on the surface of the lithium anode produces imino ether lithium, and the decomposition of NMP on both sides of the positive and negative poles during the charging process of amino acid lithium and LiOH. will be all Two standard flat electrode gold (Au) electrode and glassy carbon (GC) electrode are used as the research object. The interaction between the O2 heterphasic charge transfer process, the O2- homogeneous transformation process and the Li2O2 deposition in the O2 reduction electrode process is taken into consideration. The stable polarization curve and the AC impedance under the cathodic polarization condition are used. The characteristics of the O2 reduction process on the surface of the two electrodes are studied by the spectrum and the whole electrolysis system. The results show that there is a significant difference in the reduction process of O2 on the surface of different electrode materials. Relative to the GC electrode, O2 has a faster rate of heterogeneous charge transfer and slower O2- phase transformation rate on the surface of Au electrode, and the deposition of Li2O2 on the surface of the electrode has been deposited on the surface of the electrode. The growth of the electrochemical reduction product O2- on the Li2O2 surface leads to the O2 reduction process, and the resistivity of the deposited Li2O2 is significantly lower than that of its bulk resistivity.O2, the faster the charge transfer process, the slower the O2- consumption rate, the lower the Li2O2 resistivity, so that the Li2O2 deposited on the Au electrode surface has a smaller number of resistivity. In order to further study the relationship between the surface properties of porous materials and the performance of electrode discharge in the porous air electrode, a kind of full carbon nanotube (CNT) air electrode was prepared by solution impregnation. Under the premise of changing the macro channel structure of the electrode, the surface properties of CNT were controlled by changing the temperature of heat treatment, and the CNT electrode table was systematically analyzed. The effect of surface properties on the process of O2 reduction electrode shows that the oxygen - containing groups with the ability of absorbing electrons on the surface of CNT will inhibit the heterogeneous charge transfer process of O2, and the introduction of oxygen containing groups will increase the rate of the homogeneous transformation process of O2-. In the CNT electrode surface with higher oxygen functional group content and smaller pore diameter of the surface, the Li2O2 shows the dip. The characteristics of the growth of the parallel electrode surface. Reducing the content of oxygen functional groups on the surface of CNT, expanding the pore diameter of the surface and increasing the vertical growth of Li2O2 on the CNT surface. According to the Li2O2 resistivity of the CNT surface and the change law of the surface morphology of the CNT electrode with the discharge depth under different heat treatment conditions, the electrode electricity in the discharge process is determined. The conductivity is not the main factor that leads to the termination of the discharge. The discharge performance of the electrode is mainly determined by the subsequent reduction process of O2 on the surface of Li2O2. In the CNT electrode surface with a low oxygen functional group content and a larger surface pore diameter, the higher relative growth index makes the Li2O2 elliptically shaped structure, which is beneficial to O2 in Li2O. The following charge transfer process on the surface of 2 particles makes the CNT electrode exhibit good discharge performance. Combining the results of the previous part, the surface properties of the electrode directly determine the characteristics of the O2 reduction process on the electrode surface. Therefore, the surface properties of the electrode material are one of the key factors affecting the discharge performance of the electrode. On the basis of the research results that the existing single electrolyte system can not meet the positive and negative stability requirements of the lithium air battery in the charging process, a new dual electrolyte system is used in the non water lithium air battery. The CNT electrode is used as the air electrode, the metal lithium is negative, the solid electrolyte LAGP is introduced and the positive and negative poles are divided. The lithium air battery (Li|1 mol L-1 LiPF6-ECDEC|LAGP|0.1 mol L-1 LiClO4-NMP|CNT) with double electrolyte structure is not used in the electrolyte of NMP and carbonate. The test results show that the negative reaction process of non water electrolyte in positive and negative electrode is avoided by the isolation effect of solid electrolyte, and the battery system is limited to 3000 mAh g-1 specific capacitance. Under the condition of volume, the cycle performance of the 40 cycles is not obviously attenuated. The proposed structure system has explored a feasible way for the application of the existing lithium air battery.
【学位授予单位】：国防科学技术大学
【学位级别】：博士
【学位授予年份】：2014
【分类号】：TM911.41

【共引文献】