新型氟磺酰亚胺锂盐应用于锂离子电池的研究
发布时间：2018-08-31 19:06
【摘要】：非水电解液是锂离子电池的四大关键材料之一,在正负极之间主要发挥离子导电和调节电极／电解液界面的功能,与电池的循环寿命、高低温特性和安全性等关键技术性能密切相关。目前,商业化锂离子电池的电解液主要由以六氟磷酸锂(LiPF6)为导电盐、线性和环状碳酸酯的混合物为溶剂、以及必要的功能添加剂组成。这主要是因为它具有较高电导率,对铝集流体具有良好的钝化性能,且对4V正极材料具有良好的抗氧化能力。更重要的是,LiPF6作为导电锂盐的电解液,与石墨类负极表现出较好的相容性,即在电池循环的起始几周内,LiPF6电解液能在石墨化碳负极表面形成化学和电化学性能较稳定、锂离子导电而对电子绝缘的固体电解质界面(Solid electrolyte interphases, SEI)膜,阻止石墨化碳与有机电解液组分的进一步反应,从而使锂离子电池在常温(55℃)运行时具有较长的寿命和较可靠的安全性。但是,LiPF6碳酸酯电解液体系的化学稳定性差。大量研究结果证实,在高温或质子性杂质存在时,LiPF6碳酸酯电解液会发生复杂的自催化分解,并产生HF。电解液中的HF,不仅会导致正极材料金属离子的溶出,而且会毁坏石墨化碳负极表面的SEI膜,从而造成电池的容量快速衰减(特别是高温条件下),并带来安全隐患,已成为发展长寿命大型动力与储能电池的技术瓶颈之一。因此,寻找高性能的新型电解液体系(包括新型锂盐,溶剂和添加剂),以改善LiPF6碳酸酯电解液体系的缺陷,一直是国内外产业和学术界的努力目标。基于对新型氟代磺酰亚胺锂盐结构与性能关系的研究兴趣,近20年来,本课题组设计合成了多系列氟代磺酰亚胺锂盐(如Li[(FSO2)(n-CmF2m+1SO2)N], Li[(CF3CH2OSO2)(n-Cm,F2m+1SO2)N], m=0,1,2,4,6,8等)。对这些新型锂盐的物理和电化学性质的表征结果表明,(氟磺酰)(全氟丁基磺酰)亚胺锂(Li[(FSO2)(n-C4F9SO2)N], LiFNFSI)作为单一导电锂盐,在碳酸酯体系中表现出优越的综合性能,如不含HF,较高的热稳定性,较好的电化学稳定性,在3-5 V (vs. Li/Li+)能有效钝化铝箔集流体,对4 V正极材料具有较强的耐氧化能力等。初步的研究结果表明,与LiPF6相比,LiFNFSI应用于石墨/LiCoO2中间相炭微球(MCMB)/LiMn2O4钾离子电池时,表现出更好的室温和高温循环性能,初步凸显出改善锂离子电池高温循环性能的潜力,有望突破LiPF6电池高温循环时容量快速衰减的技术瓶颈。另一方面,这些前期工作并没有深入研究LiFNFSI改善锂离子电池的高温循环机制。基于以上研究背景和前期工作进展,本论文主要以提升锂离子电池的耐高温性能为研究目标,采用新型氟磺酰亚胺锂盐替代LiPF6或作为LiPF6的共导电锂盐,在室温(25℃)和高温(60和/或85℃)下,通过对比表征新型氟磺酰亚胺锂盐和传统LiPF6电解液的物理和电化学性能、石墨/LiCoO2电池的电化学性能、以及电极/电解液界面膜的阻抗和组成等,重点阐释了锂盐种类(也就是阴离子结构)对电解液的物理和电化学性能、电极/电解液界面和电池电化学性能的影响及其机制。具体研究内容和主要结果如下：(1)为了更清晰地理解传统LiPF6电解液的分解机制及其对锂离子电池电化学性能的影响,本文首先采用液态核磁共振方法系统表征了LiPF6电解液在室温(25℃)和高温(60和85℃)存储不同时间的分解产物。通过对不同温度、不同存储时间的LiPF6电解液的分解产物种类及含量的表征分析,并结合已有LiPF6电解液分解行为的研究结果,论文第二章提出了新的LiPF6电解液分解机制,即LiPF6电解液中不可避免残存的微量HF和质子性杂质(如H20, CH3OH和C2H5OH)是引发PF6-阴离子和碳酸酯分解的诱导因素;而电解液分解过程中循环产生HF和醇,导致PF6-阴离子和碳酸酯不断分解,从而持续恶化电解液。与现有文献普遍提出的由LiPF6热解引发其电解液分解的机制相比(即LiPF6→LiF+PF5),该机制能更合理地解释LiPF6电解液在室温保存时持续分解的现象。(2)采用液态核磁共振表征方法,系统研究了新型氟磺酰亚胺锂(Li[(FSO2)(n-CmF2m+1SO2)N], m=0,1,2,4,6)作为共导电盐,对LiPF6电解液高温化学分解行为的影响及其作用机制。核磁表征结果表明,高温(85℃)存储时,五种含有氟磺酰基(F-SO2-)的亚胺锂盐作为LiPF6的共导电盐,能显著抑制LiPF6电解液的高温分解。这可能主要归因于1)氟磺酰亚胺阴离子对HF的清除作用,阻断了其与碳酸酯溶剂反应持续产生质子性醇类物质；2)氟磺酰亚胺阴离子清除了电解液中活泼的O=PF3,从而阻断了HF和质子性杂质的持续产生。以上两个反应是氟磺酰亚胺锂盐抑制PF6-阴离子和碳酸酯溶剂的高温化学分解的关键。(3)系统研究了LiFNFSI与碳酸乙烯酯(EC)/碳酸甲乙酯(EMC) (3:7, v/v)组成的电解液的基础理化性能(如电导率和锂离子迁移数等),及其在铝集流体和铂电极表面的电化学行为。研究结果表明,LiFNFSI电解液具有较高的锂离子迁移数(0.50),和氧化电位(5.7V vs.Li/Li+),且在4.5 V (vs. Li/Li+)和高温(60℃)下不腐蚀铝集流体。进一步对比常用锂盐LiPF6,系统评价了LiFNFSI在石墨/LiCoO2锂离子电池中的电化学性能,主要包括高温搁置、倍率以及室温(25℃)和高温(60℃)循环性能,并结合不同循环周数的电化学交流阻抗谱(EIS)和X射线光电子能谱(XPS)的分析,重点对石墨和LiCoO2表面形成的电极／电解液界面膜进行了表征。电池测试结果表明,与LiPF6电池相比,LiFNFSI电池表现出更好的高温搁置、倍率和循环性能。这主要归因于LiFNFSI电解液所具备优异的基础性能,如热稳定性较高,且不含HF,锂离子迁移数较高,对隔膜等电极材料的浸润性较好,以及在石墨负极形成的SEI膜的化学稳定性较高等。值得注意的是,LiFNFSI电池表现出较高的阻抗值,这可能主要是由于LiFNFSI电解液在石墨负极表明形成了较厚的SEI膜。XPS结果分析表明LiFNFSI电解液在石墨负极形成的SEI膜主要由FNFSI-阴离子的还原产物组成,且具有较高的热稳定性；而LiPF6电解液在石墨负极形成的SEI膜主要为碳酸酯溶剂的还原产物,在高温下表现出明显的溶解和再生。以上结果表明,在高低温下,LiFNFSI电池具有更好的容量保持率,主要归因于较稳定的SEI膜,以及热稳定性较好且不含HF的LiFNFSI电解液；而LiPF6电池的容量衰减速度较快,主要是由于石墨负极SEI膜的溶解和再生,以及电解液中HF和质子性杂质的有害影响。(4)系统评价了LiFNFSI作为LiPF6的共导电盐,在EC/EMC (3:7, v/v)电解液中的基础理化性能和电化学行为,及其在石墨/LiCoO2电池中的高温搁置、倍率、室温(25℃)和高温(60℃)循环性能,并结合EIS和XPS研究了LiPF6-LiFNFSI混合锂盐电解液体系对电极/电解液界面膜的阻抗和组成的影响。研究表明,与LiPF6和LiFNFSI单一锂盐电解液相比,LiPF6-LiFNFSI混合锂盐电解液表现出较优异的电池循环性能。其中,LiPF6和LiFNFSI的浓度均为0.5 M时,石墨/LiCo02电池表现出较好的综合性能,如循环100周的容量保持率：92.1%(25℃)和85.0%(60℃)。这主要归因于以下三个因素的影响：1)LiFNFSI的存在显著提高了电解液的热稳定性;2)FNFSI-阴离子清除了电解液中的HF,减缓了其对石墨负极界面膜组分的化学性腐蚀,以及对正极金属离子的溶出;3) PF6-和FNFSI-阴离子在石墨负极表面共同还原,形成了较稳定的SEI膜。另一方面,EIS结果表明造成混合锂盐电解液的电池容量衰减的主要原因是不可避免存在的电极／电解液界面的反应,表现为电池阻抗值的增大,且此趋势在高温下加剧。最后,本论文从电解液的化学和电化学稳定性、界面成膜性能等多角度出发,对锂离子电池电解液的发展方向进行了展望。为了扩大锂离子电池的应用范围,特别是发展电动汽车适用的高温、长寿命锂离子电池,研究开发化学和电化学稳定性高、与电池材料相容性好的新型电解液体系是今后电解液的主要努力方向之一。其中,两种或多种锂盐混合使用,是目前提升电解液性能的重要技术策略之一,特别是在提高电池循环稳定性方面(尤其是高温性能),表现出较好的应用前景,如本文所提出的LiPF6-LiFNFSI混合锂盐电解液体系。
[Abstract]:Non-aqueous electrolyte is one of the four key materials for lithium-ion batteries. It plays an important role in conducting ions between positive and negative electrodes and adjusting the interface between electrode and electrolyte. It is closely related to the cycle life, high and low temperature characteristics and safety of batteries. Lithium (LiPF6) is a conductive salt, a mixture of linear and cyclic carbonates as a solvent, and a necessary functional additive. This is mainly due to its high conductivity, good passivation of aluminum collectors, and good oxidation resistance to 4V cathode materials. Graphite anodes exhibit good compatibility, i.e. LiPF6 electrolyte can form a solid electrolyte interphase (SEI) film on the surface of graphitized carbon anode with stable chemical and electrochemical properties during the first few weeks of the battery cycle, and lithium ion conducts on the surface of graphitized carbon anode to prevent graphitized carbon from forming organic electrolyte. However, the chemical stability of LiPF6 carbonate electrolyte system is poor. A large number of studies have confirmed that complex Autocatalytic Decomposition of LiPF6 carbonate electrolyte occurs in the presence of high temperature or protonic impurities. HF is produced in the electrolyte, which not only leads to the dissolution of metal ions from cathode material, but also destroys SEI film on graphitized carbon anode surface, resulting in rapid capacity decay (especially at high temperature) and potential safety hazards, and has become one of the technical bottlenecks in developing long-life large-scale power and energy storage batteries. Finding new electrolyte systems with high performance (including new lithium salts, solvents and additives) to improve the defects of LiPF6 carbonate electrolyte systems has always been the goal of industry and academia at home and abroad. The physical and electrochemical properties of these new lithium salts (e.g. Li [(FSO2) (n-CmF2m + 1SO2) N], Li [(CF3CH2OSO2) (n-Cm, F2m + 1SO2) N], M = 0, 1, 2, 4, 6, 8, etc.) were characterized. The results show that, (fluorosulfonyl) (perfluorobutyl sulfonyl) lithium imide (Li [(FSO2) (n-C4F9SO2) N], LiNFSI) is a single conducting lithium salt in carbonate system. It shows excellent comprehensive properties, such as no HF, high thermal stability, good electrochemical stability, effective passivation of aluminum foil in 3-5 V (vs. Li / Li +) and strong oxidation resistance to 4V cathode materials. The preliminary results show that LiFNFSI can be applied to graphite / LiCoO2 mesocarbon microspheres (MCMB) / LiMn2 as compared with LiPF6. O4 potassium ion batteries exhibit better performance at room temperature and high temperature cycling, which initially highlights the potential to improve the high temperature cycling performance of lithium ion batteries and is expected to break through the technical bottleneck of rapid capacity degradation during high temperature cycling of LiPF6 batteries. Mechanisms. Based on the above research background and the progress of the previous work, this paper mainly aims at improving the high-temperature performance of lithium-ion batteries. New fluorosulfonyl imide lithium salts are used to replace LiPF6 or as co-conducting lithium salts of LiPF6. The new fluorosulfonyl imide lithium salts and their transfer are characterized by comparing at room temperature (25 C) and high temperature (60 and / or 85 C). The physical and electrochemical properties of LiPF6 electrolyte, the electrochemical properties of graphite/LiCoO2 batteries, and the impedance and composition of electrode/electrolyte interfacial film are described. The effects of lithium salts (i.e. anionic structure) on the physical and electrochemical properties of the electrolyte, the electrode/electrolyte interface and the electrochemical properties of the batteries and their mechanisms are emphatically explained. The main results are as follows: (1) In order to understand the decomposition mechanism of traditional LiPF6 electrolyte and its effect on the electrochemical performance of lithium-ion batteries more clearly, the decomposition products of LiPF6 electrolyte stored at room temperature (25 C) and high temperature (60 and 85 C) for different time were systematically characterized by liquid nuclear magnetic resonance. After characterization and analysis of the types and contents of decomposition products of LiPF6 electrolyte at different temperatures and storage times, and combined with the results of previous studies on Decomposition Behavior of LiPF6 electrolyte, a new decomposition mechanism of LiPF6 electrolyte, i.e. the inevitable residual trace HF and protonic impurities (such as H20, CH3OH and C) in LiPF6 electrolyte, is proposed in the second chapter of this paper. 2H5OH is the inducing factor for the decomposition of PF6-anion and carbonate, while HF and alcohol are produced in the decomposition process of electrolyte, which leads to the decomposition of PF6-anion and carbonate and consequently deteriorates the electrolyte continuously. (2) Using liquid nuclear magnetic resonance (NMR) characterization method, the effects of novel lithium fluorosulfonymide (Li [(FSO2) (n-CmF2m + 1SO2) N], M = 0, 1, 2, 4, 6) as co-conducting salts on the high temperature chemical decomposition behavior of LiPF6 electrolyte and its mechanism were studied. The results showed that five imide lithium salts containing fluorosulfonyl group (F-SO2-) as co-conductive salts of LiPF6 could significantly inhibit the high temperature decomposition of LiPF6 electrolyte at high temperature (85 C). This may be attributed to 1) the scavenging effect of fluorosulfonyl imide anions on HF, which prevented the reaction with carbonate solvents from continuously producing protonic alcohols; The above two reactions are the key to the inhibition of high temperature chemical decomposition of PF6-anions and carbonate solvents by lithium fluorosulfonymide. (3) LiFNFSI and vinyl carbonate (EC) / ethyl carbonate (EMC) (3:7, V / v) were systematically studied. The results show that LiFNFSI electrolyte has high lithium ion mobility (0.50) and oxidation potential (5.7 V vs. Li / Li +) and does not corrode aluminum at 4.5 V (vs. Li / Li +) and high temperature (60 C). The electrochemical performance of LiFNFSI in graphite/LiCoO2 lithium-ion batteries was systematically evaluated by comparing the common lithium salts LiPF6. The electrochemical performance of LiFNFSI in graphite/LiCoO2 lithium-ion batteries included high temperature shelving, multiple, cycling performance at room temperature (25 C) and high temperature (60 C). The electrochemical impedance spectroscopy (EIS) and X-ray photoelectron spectroscopy (XPS) with different cycling cycles were also analyzed. The electrode/electrolyte interfacial films formed on graphite and LiCoO2 surfaces were characterized. The results of battery tests showed that LiFNFSI batteries exhibited better high temperature shelving, multiplication and cycling performance than LiPF6 batteries. This was mainly attributed to the excellent basic properties of LiFNFSI electrolytes, such as high thermal stability, free of HF and lithium ions. It is noteworthy that LiFNFSI batteries exhibit high impedance values, which may be mainly due to the formation of thicker SEI films on graphite negative electrolyte in LiFNFSI electrolyte. The SEI film formed by electrolyte on graphite anode is mainly composed of FNFSI-anion reduction products with high thermal stability, while the SEI film formed by LiPF6 electrolyte on graphite anode is mainly the reduction products of carbonate solvent, which shows obvious dissolution and regeneration at high temperature. The better capacity retention was attributed to the more stable SEI film and the better thermal stability and HF-free LiFNFSI electrolyte, while the faster capacity decay rate of LiPF6 battery was attributed to the dissolution and regeneration of graphite negative SEI film and the harmful effects of HF and protonic impurities in the electrolyte. (4) The performance of LiFNFSI was systematically evaluated. The basic physical and chemical properties and electrochemical behavior of LiPF6 as a co-conductive salt in EC/EMC (3:7, v/v) electrolyte, as well as its high temperature shelving, rate, room temperature (25 C) and high temperature (60 C) cycling performance in graphite/LiCoO2 batteries were investigated. The impedance and electrochemical behavior of LiPF6-LiFNFSI mixed lithium salt electrolyte system to electrode/electrolyte interfacial film were investigated by EIS and XPS. The results show that the LiPF6-LiFNFSI mixed lithium salt electrolyte exhibits better cycling performance than LiPF6 and LiFNFSI single lithium salt electrolyte. When the concentration of LiPF6 and LiFNFSI is 0.5 M, the graphite/LiCo02 battery exhibits better comprehensive performance, such as capacity retention rate of 92.1% (25 C) and 85. This is mainly attributed to the following three factors: 1) the presence of LiFNFSI significantly improves the thermal stability of the electrolyte; 2) FNFSI-anion eliminates HF in the electrolyte, slows down its chemical corrosion to the components of graphite anode interfacial film, and dissolves metal ions from the anode; 3) the presence of PF6-and FNFSI-anions in the graphite anode. On the other hand, EIS results show that the main reason for the capacity decay of mixed lithium salt batteries is the inevitable electrode/electrolyte interface reaction, which shows that the impedance value of batteries increases and this trend increases at high temperature. Finally, this paper focuses on the electrolyte chemistry. In order to expand the application range of lithium-ion batteries, especially to develop high-temperature and long-life lithium-ion batteries suitable for electric vehicles, high chemical and electrochemical stability and good compatibility with battery materials were studied and developed. The new electrolyte system is one of the main efforts in the future. Among them, the mixed use of two or more lithium salts is one of the important technical strategies to improve the performance of the electrolyte at present, especially in improving the cyclic stability of the battery (especially at high temperature), showing a good application prospect, such as LiPF6-LiF proposed in this paper. NFSI mixed lithium salt electrolyte system.
【学位授予单位】：华中科技大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：TQ131.11;TM912
，
本文编号：2215892
[Abstract]:Non-aqueous electrolyte is one of the four key materials for lithium-ion batteries. It plays an important role in conducting ions between positive and negative electrodes and adjusting the interface between electrode and electrolyte. It is closely related to the cycle life, high and low temperature characteristics and safety of batteries. Lithium (LiPF6) is a conductive salt, a mixture of linear and cyclic carbonates as a solvent, and a necessary functional additive. This is mainly due to its high conductivity, good passivation of aluminum collectors, and good oxidation resistance to 4V cathode materials. Graphite anodes exhibit good compatibility, i.e. LiPF6 electrolyte can form a solid electrolyte interphase (SEI) film on the surface of graphitized carbon anode with stable chemical and electrochemical properties during the first few weeks of the battery cycle, and lithium ion conducts on the surface of graphitized carbon anode to prevent graphitized carbon from forming organic electrolyte. However, the chemical stability of LiPF6 carbonate electrolyte system is poor. A large number of studies have confirmed that complex Autocatalytic Decomposition of LiPF6 carbonate electrolyte occurs in the presence of high temperature or protonic impurities. HF is produced in the electrolyte, which not only leads to the dissolution of metal ions from cathode material, but also destroys SEI film on graphitized carbon anode surface, resulting in rapid capacity decay (especially at high temperature) and potential safety hazards, and has become one of the technical bottlenecks in developing long-life large-scale power and energy storage batteries. Finding new electrolyte systems with high performance (including new lithium salts, solvents and additives) to improve the defects of LiPF6 carbonate electrolyte systems has always been the goal of industry and academia at home and abroad. The physical and electrochemical properties of these new lithium salts (e.g. Li [(FSO2) (n-CmF2m + 1SO2) N], Li [(CF3CH2OSO2) (n-Cm, F2m + 1SO2) N], M = 0, 1, 2, 4, 6, 8, etc.) were characterized. The results show that, (fluorosulfonyl) (perfluorobutyl sulfonyl) lithium imide (Li [(FSO2) (n-C4F9SO2) N], LiNFSI) is a single conducting lithium salt in carbonate system. It shows excellent comprehensive properties, such as no HF, high thermal stability, good electrochemical stability, effective passivation of aluminum foil in 3-5 V (vs. Li / Li +) and strong oxidation resistance to 4V cathode materials. The preliminary results show that LiFNFSI can be applied to graphite / LiCoO2 mesocarbon microspheres (MCMB) / LiMn2 as compared with LiPF6. O4 potassium ion batteries exhibit better performance at room temperature and high temperature cycling, which initially highlights the potential to improve the high temperature cycling performance of lithium ion batteries and is expected to break through the technical bottleneck of rapid capacity degradation during high temperature cycling of LiPF6 batteries. Mechanisms. Based on the above research background and the progress of the previous work, this paper mainly aims at improving the high-temperature performance of lithium-ion batteries. New fluorosulfonyl imide lithium salts are used to replace LiPF6 or as co-conducting lithium salts of LiPF6. The new fluorosulfonyl imide lithium salts and their transfer are characterized by comparing at room temperature (25 C) and high temperature (60 and / or 85 C). The physical and electrochemical properties of LiPF6 electrolyte, the electrochemical properties of graphite/LiCoO2 batteries, and the impedance and composition of electrode/electrolyte interfacial film are described. The effects of lithium salts (i.e. anionic structure) on the physical and electrochemical properties of the electrolyte, the electrode/electrolyte interface and the electrochemical properties of the batteries and their mechanisms are emphatically explained. The main results are as follows: (1) In order to understand the decomposition mechanism of traditional LiPF6 electrolyte and its effect on the electrochemical performance of lithium-ion batteries more clearly, the decomposition products of LiPF6 electrolyte stored at room temperature (25 C) and high temperature (60 and 85 C) for different time were systematically characterized by liquid nuclear magnetic resonance. After characterization and analysis of the types and contents of decomposition products of LiPF6 electrolyte at different temperatures and storage times, and combined with the results of previous studies on Decomposition Behavior of LiPF6 electrolyte, a new decomposition mechanism of LiPF6 electrolyte, i.e. the inevitable residual trace HF and protonic impurities (such as H20, CH3OH and C) in LiPF6 electrolyte, is proposed in the second chapter of this paper. 2H5OH is the inducing factor for the decomposition of PF6-anion and carbonate, while HF and alcohol are produced in the decomposition process of electrolyte, which leads to the decomposition of PF6-anion and carbonate and consequently deteriorates the electrolyte continuously. (2) Using liquid nuclear magnetic resonance (NMR) characterization method, the effects of novel lithium fluorosulfonymide (Li [(FSO2) (n-CmF2m + 1SO2) N], M = 0, 1, 2, 4, 6) as co-conducting salts on the high temperature chemical decomposition behavior of LiPF6 electrolyte and its mechanism were studied. The results showed that five imide lithium salts containing fluorosulfonyl group (F-SO2-) as co-conductive salts of LiPF6 could significantly inhibit the high temperature decomposition of LiPF6 electrolyte at high temperature (85 C). This may be attributed to 1) the scavenging effect of fluorosulfonyl imide anions on HF, which prevented the reaction with carbonate solvents from continuously producing protonic alcohols; The above two reactions are the key to the inhibition of high temperature chemical decomposition of PF6-anions and carbonate solvents by lithium fluorosulfonymide. (3) LiFNFSI and vinyl carbonate (EC) / ethyl carbonate (EMC) (3:7, V / v) were systematically studied. The results show that LiFNFSI electrolyte has high lithium ion mobility (0.50) and oxidation potential (5.7 V vs. Li / Li +) and does not corrode aluminum at 4.5 V (vs. Li / Li +) and high temperature (60 C). The electrochemical performance of LiFNFSI in graphite/LiCoO2 lithium-ion batteries was systematically evaluated by comparing the common lithium salts LiPF6. The electrochemical performance of LiFNFSI in graphite/LiCoO2 lithium-ion batteries included high temperature shelving, multiple, cycling performance at room temperature (25 C) and high temperature (60 C). The electrochemical impedance spectroscopy (EIS) and X-ray photoelectron spectroscopy (XPS) with different cycling cycles were also analyzed. The electrode/electrolyte interfacial films formed on graphite and LiCoO2 surfaces were characterized. The results of battery tests showed that LiFNFSI batteries exhibited better high temperature shelving, multiplication and cycling performance than LiPF6 batteries. This was mainly attributed to the excellent basic properties of LiFNFSI electrolytes, such as high thermal stability, free of HF and lithium ions. It is noteworthy that LiFNFSI batteries exhibit high impedance values, which may be mainly due to the formation of thicker SEI films on graphite negative electrolyte in LiFNFSI electrolyte. The SEI film formed by electrolyte on graphite anode is mainly composed of FNFSI-anion reduction products with high thermal stability, while the SEI film formed by LiPF6 electrolyte on graphite anode is mainly the reduction products of carbonate solvent, which shows obvious dissolution and regeneration at high temperature. The better capacity retention was attributed to the more stable SEI film and the better thermal stability and HF-free LiFNFSI electrolyte, while the faster capacity decay rate of LiPF6 battery was attributed to the dissolution and regeneration of graphite negative SEI film and the harmful effects of HF and protonic impurities in the electrolyte. (4) The performance of LiFNFSI was systematically evaluated. The basic physical and chemical properties and electrochemical behavior of LiPF6 as a co-conductive salt in EC/EMC (3:7, v/v) electrolyte, as well as its high temperature shelving, rate, room temperature (25 C) and high temperature (60 C) cycling performance in graphite/LiCoO2 batteries were investigated. The impedance and electrochemical behavior of LiPF6-LiFNFSI mixed lithium salt electrolyte system to electrode/electrolyte interfacial film were investigated by EIS and XPS. The results show that the LiPF6-LiFNFSI mixed lithium salt electrolyte exhibits better cycling performance than LiPF6 and LiFNFSI single lithium salt electrolyte. When the concentration of LiPF6 and LiFNFSI is 0.5 M, the graphite/LiCo02 battery exhibits better comprehensive performance, such as capacity retention rate of 92.1% (25 C) and 85. This is mainly attributed to the following three factors: 1) the presence of LiFNFSI significantly improves the thermal stability of the electrolyte; 2) FNFSI-anion eliminates HF in the electrolyte, slows down its chemical corrosion to the components of graphite anode interfacial film, and dissolves metal ions from the anode; 3) the presence of PF6-and FNFSI-anions in the graphite anode. On the other hand, EIS results show that the main reason for the capacity decay of mixed lithium salt batteries is the inevitable electrode/electrolyte interface reaction, which shows that the impedance value of batteries increases and this trend increases at high temperature. Finally, this paper focuses on the electrolyte chemistry. In order to expand the application range of lithium-ion batteries, especially to develop high-temperature and long-life lithium-ion batteries suitable for electric vehicles, high chemical and electrochemical stability and good compatibility with battery materials were studied and developed. The new electrolyte system is one of the main efforts in the future. Among them, the mixed use of two or more lithium salts is one of the important technical strategies to improve the performance of the electrolyte at present, especially in improving the cyclic stability of the battery (especially at high temperature), showing a good application prospect, such as LiPF6-LiF proposed in this paper. NFSI mixed lithium salt electrolyte system.
【学位授予单位】：华中科技大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：TQ131.11;TM912
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本文编号：2215892
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