【作者】 韩鸿波；

【导师】 聂进； 周志彬；

【作者基本信息】 华中科技大学 ， 材料物理与化学， 2012， 博士

【摘要】 非水电解液是锂(离子)电池的关键材料之一,与电池的循环寿命、耐高温性、安全性等关键性能密切相关。现已商业化的二次锂(离子)电池电解液主要由有机碳酸酯(如碳酸二甲酯(DMC)、碳酸二乙酯(DEC)、碳酸甲乙酯(EMC)和碳酸乙烯酯(EC)等)、LiPF6导电盐以及各种功能添加剂等组成。有机碳酸酯电解液存在溶剂易挥发和易燃等缺点,电池在不当使用时(如过充、短路等),易造成电池热失控,发生燃烧甚至爆炸等安全事故。LiPF6本征化学活性高,对水、热敏感,热分解产生的HF和PF5是影响电池循环寿命的重要因素。因此传统的碳酸酯和LiPF6电解液体系已成为二次储能电池大型化、高功率化的技术瓶颈。本论文以“氟磺酰亚胺”为中心,围绕锂离子电池电解质的安全性和循环性展开研究。主要工作包括氟磺酰亚胺碱金属盐(尤其是锂盐)、离子液体及相关前驱体的合成,氟磺酰亚胺离子液体和氟磺酰亚胺锂-碳酸酯电解液基础物化性质的研究,以及它们作为电解液在锂(离子)电池体系中的性能评价。论文第一章回顾了锂(离子)电池的发展历史,简要介绍锂离子电池的结构组成和工作原理。然后详尽综述了锂离子电池电解液的溶剂、导电锂盐,以及离子液体电解液的研究进展,同时分析了锂离子电池的组成材料与电池的安全性能和循环寿命之间的影响关系。在第二章中,以CnF2n+1SO2NH2、SOC12和CISO3H为原料制备氯磺酰亚胺([HN(SO2C1)(SO2CnF2n+1)]),随后采用SbF3进行氟化反应得到氟磺酰亚胺([HN(SO2F)(SO2CnF2n+1)])。在CH3CN溶剂中,由氟磺酰亚胺和碳酸盐中和制备钾盐、铷盐和铯盐。通过氟磺酰亚胺钾在无水CH3CN中与LiC1O4或者NaC1O4交换制备相应的锂盐或者钠盐。各步反应产物的结构采取1H NMR(核磁共振)、19F NMR、FTIR(红外光谱)、ESI-MS(质谱)和EA(元素分析)进行了表征。碱金属盐的热性能由DSC(差示扫描量热)和TG(热重)进行分析,其中氟磺酰亚胺碱金属盐体现出较低的熔点(94-198℃)和较高的热分解温度(223-369℃)。在第三章中,碘(氯)化锍盐([SR1R2R3]+ or SR1; R1，R2, R3=alkyl or CH3OCH2CH2)与[N(SO2F)(SO2CnF2n+1)]-组合成85个新型的锍盐离子液体,并通过1H NMR、19FNMR、ESI-MS和EA对产物结构进行了表征。锍盐离子液体体现出低粘度、高电导率的优越性质,但是其还原电位较高(大约-2.50 V vs. Fc/Fc+),分解温度在236-312℃之间。引入醚键可以有效较低离子液体的粘度和玻璃化温度,提高电导率,但是同时削弱了离子液体的耐氧化还原性能。研究表明S222FSI-LiFSI (0.32 mol kg-1)电解液可以有效抑制$222+在低电位下的电化学还原反应,在Ni电极表面可以观察到可逆的锂沉积/溶出过程。在第四章中,系统研究了Li[N(SO2F)(SO2CnF2n+1)]在碳酸丙烯酯(PC)电解液中(c=1.0 M)的物化和电化学性质。研究结果表明,随着[N(SO2F)(SO2CnF2n+1)]-氟碳链的增长,电解液的粘度增大,电导率下降,氧化电位(Eox)和阴离子的HOMO值(-Ehomo)同步提高,在-150-30℃C区间内电解液仅存在玻璃化转变。Li[N(SO2F)(SO2CF3)] (LiFTFSI)在电位高于3.7 V (vs. Li+/Li)时对铝箔表现出严重的腐蚀性。恒电位直流极化测试表明Li[N(SO2F)2] (LiFSI)和Li[N(SO2F)(SO2C2F5)] (LiFPFSI)在4.5 V (vs. Li+/Li)的条件下对铝箔不能长时间保持稳定,而Li[N(SO2F)(SO2C4F9)] (LiFNFSI)、Li[N(SO2F)(SO2C6F13)] (LiFHFSI)和Li[N(SO2F)(SO2C8F17)] (LiFOFSI)对A1箔的电化学行为与LiPF6类似,表现出明显的钝化性能。LiFNFSI-PC电解液在0.01-2.00 M浓度范围内,粘度、电导率与浓度的关系分别较好地符合Jones-Dole以及Casteel-Amis关系式。此外,较高的∧imp/∧diff比值表明LiFNFSI在PC溶液中具有很好的解离性能,可以有效提高电解液的离子导电性和Li+离子迁移数。在第五章中,对比研究了LiFSI和LiPF6在EC/EMC (3:7, v/v)电解液体系中相关的性质。与LiPF6相比,LiFSI电解液具有略低的粘度、较高的电导率和锂离子迁移数,以及更优越的耐水解性能,且氧化电位高达5.6 V (vs. Li+/Li)。当电解液中C1-含量较低时,对A1箔表现出较好钝化性。基于LiFSI电解液的Li/LiCoO2半电池和graphite/LiCoO2锂离子电池比相同条件下的LiPF6电池体现出更优越的循环和倍率性能。另外,研究表明LiFSI电解液中的H20含量达到1000 ppm时,graphite/LiCoO2电池依然可以正常工作；电解液中适当的Na+离子含量对电池的循环性能影响较小；而K+则导致致命的破坏性。在第六章中,研究了一种新型的导电盐LiFNFSI,发现其在碳酸酯电解液中具有优越的高温特性。在-20-60℃温度范围内,LiFNFSI-EC/EMC (3:7, v/v)电解液的电导率与LiC1O4相当,氧化电位为5.7 V (vs. Li+/Li),对Al箔体现出良好的钝化性能。LiFNFSI的热分解温度为220℃,其电解液对水和热的稳定性明显优于LiPF6,在85℃放置两周时基本不变色,不分解。以LiFNFSI为导电盐的graphite/LiCoO2电池比LiPF6的电池具有更优越的室温或者高温(60℃)循环性能,以及高温储存性能。更多还原

【Abstract】 The commercial non-aqueous electrolyte is typically a mixture of carbonated solvents (such as dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC) and ethylene carbonate (EC)), LiPF6 and varieties of functional additives, which makes up the key material for Li-ion batteries (LIBs). Several crucial properties, such as the cycling life, resilience to elevated temperature and safety problems, are close relative to the electrolyte because of the volatile and inflammable organic solvents and the reactive LiPF6. When LIBs are under abusive conditions, the potential exists to heat a cell beyond its thermal stability limit, and then thermal runaway would be initiated, sometimes resulting in combustion and even explosion incidents. Meanwhile, HF and PF5 would be generated, which have been certified as the main factors to break down the cycling performance of LIBs, once the LiPF6-based electrolyte is thermal operated or contaminated by moisture or alcohol impurities. Therefore, the conventional non-aqueous electrolytes based on carbonates and LiPF6 have become the bottle-neck of rechargeable energy-storage devices with large energy density and high power source. A serial lithium salts and ionic liquids (ILs) based on the weakly-coordinating fluorosulfonylimide anions (n=0,1,2,4,6,8) were designed and prepared in this thesis. Their physicochemical, electrochemical properties and application in the field of LIBs electrolytes were also investigated extensively.In chapter 1, the development and status corresponding to LIBs, as well as their constituents and operation mechanism, were introduced. Afterward the electrolyte reviews, including the organic solvents, conductive salts and ionic liquids, were detained. And the influencing factors of the safty issues and capacity fading for LIBs were also analyzed.In chapter 2, the intermediates of HN(SO2C1)(SO2CnF2n+1) would be obtained by reaction of CnF2n+1SO2NH2, SOC12 and CISO3H in one pot, subsequently to derive HN(SO2F)(SO2CnF2n+1) by fluorination with SbF3. The alkali salts of potassium, rubidium and cesium were prepared according to the neutralization of HN(SO2F)(SO2CnF2n+1) with alkali carbonates. Meanwhile, the lithium and sodium salts should be received by exchanging KN(SO2F)(SO2CnF2n+1) with LiC104 or NaC104 in CH3CN solvent for the purpose of excluding the H2O contamination. All the products were selectively conformed by means of 1H NMR,19F NMR, ESI-MS, FT-IR and EA. And the thermal properties of the alkali salts were characterized using DSC and TG methods, with the results that Tm ranging form 94 to 198℃, and Td from 223 to 369℃, respectively.Eighty-five novel ILs comprised of the sulfonium cations ([SR1R2R3]+ or SR1; R1, R2, R3 =alkyl or CH3OCH2CH2) and [N(SO2F)(SO2CnF2n+1)]- anions were prepared in chapter 3, and conformed employing 1H NMR,19F NMR, ESI-MS and EA analytical methods. The sulfonium serial ILs generally showed low viscosity and high conductivity, and would decompose from 236 to 312℃. However, they were more weakly resistant toward reduction (ca.-2.50 V vs. Fc+/Fc) as compared with the tetraalkyl ammonium based salts. As introducing ether function to the sulfonium cations, the results indicated that it would tend to bring down the viscosity and glass transition temperature and increase the ionic conductivity, while their resistance to anodic oxidation and cathodic reduction was weakened simultaneously. A reverse Li+ deposit-dissolve process on the Ni electrode was detected for S222FSI-LiFSI (0.32 mol kg-1) electrolyte, indicating an effective SEI film would be formed on the Ni electrode to restrain the S222+ cation decomposing, which was absolutely different from S222TFSI-LiTFSI electrolyte.Solutions of Li[N(SO2F)(SO2CnF2n+1)] dissolved in PC solvent were studied intensively in chapter 4. Several regulations, that the viscosity of the electrolyte increasing, the conductivity decreasing, and the oxidation potential improving together with the HOMO values, were found as the fluoroalkyl chain was enlarged. And only glass transitions were observed in the temperature range of-150 to 30℃. Li[N(SO2F)(SO2CF3)] (LiFTFSI) exhibited severe corrosion to Al foil with an initial potential of 3.7 V (vs. Li+/Li) by cyclic voltammetry (CV) measurement. Li[N(SO2F)2] (LiFSI) and Li[N(SO2F)(SO2C2F5)] (LiFPFSI) were testified unstable when polarized at 4.5 V (vs. Li+/Li) for long time, whereas anions with longer fluoroalkyl chains, such as Li[N(SO2F)(SO2C4F9)] (LiFNFSI)、Li[N(SO2F)(SO2C6F13)] (LiFHFSI) and Li[N(SO2F)(SO2C8F17)] (LiFOFSI), showed similar electrochemical response toward Al electrode to LiPF6. A serial of LiFNFSI-PC electrolytes with concentrations from 0.01 to 2.00 M were also investigated. Correlations of the concentration with viscosity (or conductivity) was perfectively fitted by Jones-Dole (or Casteel-Amis) function. And the large values of∧imp/∧diff implied a high dissociation degree for LiFNFSI in PC solution, which was beneficial for high ionic conductivity and Li+ transference number.In chapter 5, lithium bis(fluorosulfonyl)imide (LiFSI) has been studied in comparison with LiPF6 in EC/EMC (3:7, v/v) electrolyte, in terms of the physicochemical and electrochemical properties. It exhibited far superior stability towards hydrolysis than LiPF6. Solution comprised of LiFSI was less viscous, more conductive and provided with larger Li+ transference number than that containing LiPF6. The stability of LiFSI-based electrolyte on the Pt electrode was detected highly up to 5.6 V (vs. Li+/Li). And a non-corrosion performance toward Al in the high potential region (3.0～5.0 V vs. Li+/Li) had been confirmed for high purity LiFSI electrolytes using CV, SEM and chronoamperometry, whereas Al corrosion indeed occurred in the LiFSI-based electrolytes tainted with trace amounts of LiCl (50 ppm). With high purity, LiFSI outperformed LiPF6 in both Li/LiCoO2 and graphite/LiCoO2 cells. Furthermore, in contrary to LiPF6, graphite/LiCoO2 cell constituted of LiFSI-based electrolyte operated normally, even though 1000 ppm H2O was contained. Appropriate Na+ content in the LiFSI electrolyte hardly affected the cycling performance of the graphite/LiCoO2 cell, while K+ added with 0.2 M content almost destroyed the cell.A novel lithium salt, lithium (fluorosulfonyl)(nonafluorobutanesulfonyl)imde (LiFNFSI), was investigated as conducting salt for lithium-ion cells in chapter 6. The neat salt (Td:220℃) and the corresponding electrolyte showed better thermal stability than LiPF6. Contrary to the absolutely decomposition of LiPF6 in EC/EMC (3:7, v/v) solvent after aging at 85℃for 2 w, LiFNFSI electrolyte stayed colorless and transparent at the same condition. The electrolyte comprised of 1.0 M LiFNFSI in EC/EMC (3:7, v/v) showed high conductivity comparable to LiC1O4, good electrochemical stability, and would not corrode Al collector. At both room temperature (25℃) and elevated temperature (60℃), the graphite/LiCoO2 cells with LiFNFSI exhibited better cycling performances than those with LiPF6. These outstanding properties of LiFNFSI made it an attractive candidate to overcome the rapid capacity fading of LIBs at elevated temperatures.更多还原