锂空气电池混合电解液作用机理及其电化学性能研究

发布时间：2018-09-17 19:14

【摘要】：锂空气电池,作为比能量最高的电池,有望解决电动汽车及能量存储问题,近年来备受全世界瞩目。在影响其商业化的众多问题中,正极钝化,电解液的降解,含氧量低及放电产物溶解度小等是最主要因素。但是正极的钝化和电解液的降解是现阶段限制锂空气电池发展的关键原因。稳定性差的电解液,例如有机碳酸酯等,在电池运行过程中无法可逆形成Li2O2,并且电解液会分解成诸多不可逆产物,例如甲酸锂,乙酸锂及烷基化锂等,同时许多不溶的副产物会沉积在孔空气正极孔隙,堵塞O2传输通道,加剧电池极化。电池极化增大(过电压)造成电解液的电化学不稳定,从而改变电解液体系的成分并导致电池反应机理发生改变。有机电解液又称为非质子极性溶剂,在反应体系中不能给出质子——不会释放H+腐蚀锂负极;又能使阳离子,特别是金属阳离子(Li+)溶剂化,Li+在电解液中都是以溶剂化的形式Li+(S)n迁移。同时具有电化学窗口宽,熔点高不易挥发,低粘度等优点,有望解决锂空气电池商业化问题。本文采用1 M LiTFSI作为锂盐,通过对有机溶剂环丁砜(TMS)、N,N-二甲基乙酰胺(DMA)和四乙二醇二甲醚(TEGDME)的熔点、闪点、介电常数、粘度、电导率、溶氧量、电化学窗口等理化性质研究,并运用交流阻抗测试,循环伏安测试、充放电测试表征采用三组单一电解液锂空气电池性能进行对比,提出DMA+TMS作为锂空气电池混合电解液溶剂,确认了LiTFSI-20DMA:80TMS适合作为锂空气电池电解液。最后根据DMA和TMS之间的协同作用,分别采用这两种电解液对TEGDME进行混合改性研究。(1)LiTFSI-TMS在三种电解液中粘度最高(25.26 mPa·s@28℃),离子电导率最低(1.91×10-3 S cm-1),在4℃~44℃粘度变化了43.56 mPa·s,因此温度对TMS的离子电导率影响很大;LiTFSI-TEGDME的粘度适中(9.01mPa·s@28℃),但由于其结构导致TEGDME离子电导也较低(2.06×10-3 S cm-1),其粘度随温度变化也比较明显;LiTFSI-DMA的粘度最小(2.70 mPa·s@28℃),且粘度随温度变化不大,LiFTSI-DMA的离子电导率达到了TMS和TEGDME的四倍以上(8.87×10-3 S cm-1)。CV曲线OER起始电位大小依次为DMA(3.0 V)TMS(3.2 V)TEGDME(3.3 V)。在电流密度为0.3 mA cm-2/1.77 mA gcarbon-1,比容量为1000 mAh gcarbon-1,三种溶剂循环性能大小为TMS(200圈)TEGDME(80圈)DMA(50圈),但是TEGDME和TMS循环过程中存在严重的极化。(2)本文采用DMA:TMS混合制备锂空气电池电解液。通过粘度、离子电导率、CV、气质联用技术(GCMS)对混合电解液体系分析优化后发现,TMS锂空气电池具有较高的稳定性能。添加20%DMA进入TMS电解液中制备混合电解液,在电流密度为0.3 mA cm-2,比容量为1000 mAh gcarbon-1,其充电过电压比环丁砜锂空气电池低0.2 V~0.6 V,库伦效率保持在100%,并且DMA改善了环丁砜的理化性质。(3)用扫描电子显微镜(SEM)、X射线衍射(XRD)以及核磁共振氢谱(1H NMR)对放电产物进行表征,TMS锂空气电池放电后的产物比较密集,正极表面附着了许多块状的Li2O2,20DMA:80TMS锂空气电池的产物是细小的颗粒,Li2O2本身不导电,因此会钝化正极。20DMA:80TMS锂空气电池产物较高的比表面积加快了Li2O2的充电分解。研究表明,得益于TMS稳定性能和DMA对电池极化的改善,首次提出DMA/TMS组成的双功能电解液溶剂。综合而言,DMA与TMS的协同作用使得电池的性能得到明显改善。(4)根据DMA和TMS改性研究,分别用两种溶剂对TEGDME锂空气电池进行探索。结果表明,TMS对TEGDME电池性能并无显著改善,而DMA同样使TEGDME电池充电电压有较为明显的降低。
[Abstract]:Lithium-air batteries, as the most energy-specific batteries, have attracted worldwide attention in recent years. Among the problems affecting the commercialization of lithium-air batteries, cathode passivation, electrolyte degradation, low oxygen content and low solubility of discharge products are the main factors. Bad stability electrolytes, such as organic carbonates, are unable to form Li2O2 reversibly during battery operation, and the electrolyte decomposes into irreversible products, such as lithium formate, lithium acetate and lithium alkylate, and many insoluble by-products are deposited in pore air. The increase of cell polarization (overvoltage) results in the electrochemical instability of the electrolyte, thus changing the composition of the electrolyte system and leading to changes in the reaction mechanism of the cell. Lithium anode can be corroded and the cations, especially the metal cations (Li+) can be solvated. Li + is solvated in the electrolyte form of Li + (S) n migration. At the same time, it has the advantages of wide electrochemical window, high melting point, high volatility, low viscosity, and so on, it is expected to solve the commercial problems of lithium-air batteries. The melting point, flash point, dielectric constant, viscosity, conductivity, dissolved oxygen content, electrochemical window and other physical and chemical properties of solvent sulfolane (TMS), N, N-dimethylacetamide (DMA) and tetraethylene glycol dimethyl ether (TEGDME) were studied. The AC impedance test, cyclic voltammetry test and charge-discharge test were used to characterize the performance of three groups of single electrolyte lithium air batteries. Finally, according to the synergistic effect between DMA and TMS, two kinds of electrolytes were used to modify TEGDME. (1) LiTFSI-TMS had the highest viscosity in the three electrolytes (25.26 mPa @ s @ 28 C). The ionic conductivity is the lowest (1.91 *10-3 S cm-1), and the viscosity changes 43.56 mPa s at 4 44 C, so the temperature has a great influence on the ionic conductivity of TMS; the viscosity of LiTFSI-TEGDME is moderate (9.01 mPa s @ 28 C), but the ionic conductivity of TEGDME is also lower (2.06 10-3 S cm-1) because of its structure, and its viscosity changes obviously with temperature. The ionic conductivity of LiFTSI-DMA is more than four times that of TMS and TEGDME (8.87 *10-3 S cm-1). The starting potential of OER in CV curve is DMA (3.0 V) TMS (3.2 V) TEGDME (3.3 V). The current density is 0.3 mA cm-2/1.77 mA gcarbon-1, and the specific capacity is 1000 mAh carbon-1. 1. The performance of three solvents is TMS (200 cycles) TEGDME (80 cycles) DMA (50 cycles), but there is serious polarization in the process of TEGDME and TMS cycling. (2) In this paper, the electrolyte of lithium-air batteries was prepared by mixing DMA: TMS. Adding 20% DMA into the TMS electrolyte to prepare the mixed electrolyte, the current density is 0.3 mA cm-2, the specific capacity is 1000 mAh gcarbon-1, the charge overvoltage is 0.2 V~0.6 V lower than that of the sulfolane lithium air battery, the coulomb efficiency is maintained at 100%, and the physical and chemical properties of sulfolane are improved by DMA. Microscope (SEM), X-ray diffraction (XRD) and nuclear magnetic resonance hydrogen spectroscopy (1H NMR) were used to characterize the discharge products. After discharging, the products of the lithium-air battery were dense. The positive surface of the lithium-air battery was coated with many pieces of Li2O2,20DMA:80TMS particles. Li2O2 itself was not conductive, so the cathode would be passivated.20DMA:80TMS lithium-air battery. The high specific surface area of the gas cell products accelerated the charge decomposition of Li2O2. The results showed that the stability of TMS and the improvement of cell polarization by DMA were beneficial to the improvement of the performance of the battery. The results show that the performance of TEGDME battery is not improved significantly by using TMS, and the charging voltage of TEGDME battery is also decreased significantly by DMA.
【学位授予单位】：深圳大学
【学位级别】：硕士
【学位授予年份】：2016
【分类号】：TM911.41

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