新型氟磺酰亞胺鋰鹽應(yīng)用于鋰離子電池的研究
發(fā)布時間：2018-08-31 19:06
【摘要】：非水電解液是鋰離子電池的四大關(guān)鍵材料之一,在正負(fù)極之間主要發(fā)揮離子導(dǎo)電和調(diào)節(jié)電極／電解液界面的功能,與電池的循環(huán)壽命、高低溫特性和安全性等關(guān)鍵技術(shù)性能密切相關(guān)。目前,商業(yè)化鋰離子電池的電解液主要由以六氟磷酸鋰(LiPF6)為導(dǎo)電鹽、線性和環(huán)狀碳酸酯的混合物為溶劑、以及必要的功能添加劑組成。這主要是因?yàn)樗哂休^高電導(dǎo)率,對鋁集流體具有良好的鈍化性能,且對4V正極材料具有良好的抗氧化能力。更重要的是,LiPF6作為導(dǎo)電鋰鹽的電解液,與石墨類負(fù)極表現(xiàn)出較好的相容性,即在電池循環(huán)的起始幾周內(nèi),LiPF6電解液能在石墨化碳負(fù)極表面形成化學(xué)和電化學(xué)性能較穩(wěn)定、鋰離子導(dǎo)電而對電子絕緣的固體電解質(zhì)界面(Solid electrolyte interphases, SEI)膜,阻止石墨化碳與有機(jī)電解液組分的進(jìn)一步反應(yīng),從而使鋰離子電池在常溫(55℃)運(yùn)行時具有較長的壽命和較可靠的安全性。但是,LiPF6碳酸酯電解液體系的化學(xué)穩(wěn)定性差。大量研究結(jié)果證實(shí),在高溫或質(zhì)子性雜質(zhì)存在時,LiPF6碳酸酯電解液會發(fā)生復(fù)雜的自催化分解,并產(chǎn)生HF。電解液中的HF,不僅會導(dǎo)致正極材料金屬離子的溶出,而且會毀壞石墨化碳負(fù)極表面的SEI膜,從而造成電池的容量快速衰減(特別是高溫條件下),并帶來安全隱患,已成為發(fā)展長壽命大型動力與儲能電池的技術(shù)瓶頸之一。因此,尋找高性能的新型電解液體系(包括新型鋰鹽,溶劑和添加劑),以改善LiPF6碳酸酯電解液體系的缺陷,一直是國內(nèi)外產(chǎn)業(yè)和學(xué)術(shù)界的努力目標(biāo)�；趯π滦头酋啺蜂圎}結(jié)構(gòu)與性能關(guān)系的研究興趣,近20年來,本課題組設(shè)計合成了多系列氟代磺酰亞胺鋰鹽(如Li[(FSO2)(n-CmF2m+1SO2)N], Li[(CF3CH2OSO2)(n-Cm,F2m+1SO2)N], m=0,1,2,4,6,8等)。對這些新型鋰鹽的物理和電化學(xué)性質(zhì)的表征結(jié)果表明,(氟磺酰)(全氟丁基磺酰)亞胺鋰(Li[(FSO2)(n-C4F9SO2)N], LiFNFSI)作為單一導(dǎo)電鋰鹽,在碳酸酯體系中表現(xiàn)出優(yōu)越的綜合性能,如不含HF,較高的熱穩(wěn)定性,較好的電化學(xué)穩(wěn)定性,在3-5 V (vs. Li/Li+)能有效鈍化鋁箔集流體,對4 V正極材料具有較強(qiáng)的耐氧化能力等。初步的研究結(jié)果表明,與LiPF6相比,LiFNFSI應(yīng)用于石墨/LiCoO2中間相炭微球(MCMB)/LiMn2O4鉀離子電池時,表現(xiàn)出更好的室溫和高溫循環(huán)性能,初步凸顯出改善鋰離子電池高溫循環(huán)性能的潛力,有望突破LiPF6電池高溫循環(huán)時容量快速衰減的技術(shù)瓶頸。另一方面,這些前期工作并沒有深入研究LiFNFSI改善鋰離子電池的高溫循環(huán)機(jī)制。基于以上研究背景和前期工作進(jìn)展,本論文主要以提升鋰離子電池的耐高溫性能為研究目標(biāo),采用新型氟磺酰亞胺鋰鹽替代LiPF6或作為LiPF6的共導(dǎo)電鋰鹽,在室溫(25℃)和高溫(60和/或85℃)下,通過對比表征新型氟磺酰亞胺鋰鹽和傳統(tǒng)LiPF6電解液的物理和電化學(xué)性能、石墨/LiCoO2電池的電化學(xué)性能、以及電極/電解液界面膜的阻抗和組成等,重點(diǎn)闡釋了鋰鹽種類(也就是陰離子結(jié)構(gòu))對電解液的物理和電化學(xué)性能、電極/電解液界面和電池電化學(xué)性能的影響及其機(jī)制。具體研究內(nèi)容和主要結(jié)果如下：(1)為了更清晰地理解傳統(tǒng)LiPF6電解液的分解機(jī)制及其對鋰離子電池電化學(xué)性能的影響,本文首先采用液態(tài)核磁共振方法系統(tǒng)表征了LiPF6電解液在室溫(25℃)和高溫(60和85℃)存儲不同時間的分解產(chǎn)物。通過對不同溫度、不同存儲時間的LiPF6電解液的分解產(chǎn)物種類及含量的表征分析,并結(jié)合已有LiPF6電解液分解行為的研究結(jié)果,論文第二章提出了新的LiPF6電解液分解機(jī)制,即LiPF6電解液中不可避免殘存的微量HF和質(zhì)子性雜質(zhì)(如H20, CH3OH和C2H5OH)是引發(fā)PF6-陰離子和碳酸酯分解的誘導(dǎo)因素;而電解液分解過程中循環(huán)產(chǎn)生HF和醇,導(dǎo)致PF6-陰離子和碳酸酯不斷分解,從而持續(xù)惡化電解液。與現(xiàn)有文獻(xiàn)普遍提出的由LiPF6熱解引發(fā)其電解液分解的機(jī)制相比(即LiPF6→LiF+PF5),該機(jī)制能更合理地解釋LiPF6電解液在室溫保存時持續(xù)分解的現(xiàn)象。(2)采用液態(tài)核磁共振表征方法,系統(tǒng)研究了新型氟磺酰亞胺鋰(Li[(FSO2)(n-CmF2m+1SO2)N], m=0,1,2,4,6)作為共導(dǎo)電鹽,對LiPF6電解液高溫化學(xué)分解行為的影響及其作用機(jī)制。核磁表征結(jié)果表明,高溫(85℃)存儲時,五種含有氟磺酰基(F-SO2-)的亞胺鋰鹽作為LiPF6的共導(dǎo)電鹽,能顯著抑制LiPF6電解液的高溫分解。這可能主要?dú)w因于1)氟磺酰亞胺陰離子對HF的清除作用,阻斷了其與碳酸酯溶劑反應(yīng)持續(xù)產(chǎn)生質(zhì)子性醇類物質(zhì)；2)氟磺酰亞胺陰離子清除了電解液中活潑的O=PF3,從而阻斷了HF和質(zhì)子性雜質(zhì)的持續(xù)產(chǎn)生。以上兩個反應(yīng)是氟磺酰亞胺鋰鹽抑制PF6-陰離子和碳酸酯溶劑的高溫化學(xué)分解的關(guān)鍵。(3)系統(tǒng)研究了LiFNFSI與碳酸乙烯酯(EC)/碳酸甲乙酯(EMC) (3:7, v/v)組成的電解液的基礎(chǔ)理化性能(如電導(dǎo)率和鋰離子遷移數(shù)等),及其在鋁集流體和鉑電極表面的電化學(xué)行為。研究結(jié)果表明,LiFNFSI電解液具有較高的鋰離子遷移數(shù)(0.50),和氧化電位(5.7V vs.Li/Li+),且在4.5 V (vs. Li/Li+)和高溫(60℃)下不腐蝕鋁集流體。進(jìn)一步對比常用鋰鹽LiPF6,系統(tǒng)評價了LiFNFSI在石墨/LiCoO2鋰離子電池中的電化學(xué)性能,主要包括高溫擱置、倍率以及室溫(25℃)和高溫(60℃)循環(huán)性能,并結(jié)合不同循環(huán)周數(shù)的電化學(xué)交流阻抗譜(EIS)和X射線光電子能譜(XPS)的分析,重點(diǎn)對石墨和LiCoO2表面形成的電極／電解液界面膜進(jìn)行了表征。電池測試結(jié)果表明,與LiPF6電池相比,LiFNFSI電池表現(xiàn)出更好的高溫擱置、倍率和循環(huán)性能。這主要?dú)w因于LiFNFSI電解液所具備優(yōu)異的基礎(chǔ)性能,如熱穩(wěn)定性較高,且不含HF,鋰離子遷移數(shù)較高,對隔膜等電極材料的浸潤性較好,以及在石墨負(fù)極形成的SEI膜的化學(xué)穩(wěn)定性較高等。值得注意的是,LiFNFSI電池表現(xiàn)出較高的阻抗值,這可能主要是由于LiFNFSI電解液在石墨負(fù)極表明形成了較厚的SEI膜。XPS結(jié)果分析表明LiFNFSI電解液在石墨負(fù)極形成的SEI膜主要由FNFSI-陰離子的還原產(chǎn)物組成,且具有較高的熱穩(wěn)定性；而LiPF6電解液在石墨負(fù)極形成的SEI膜主要為碳酸酯溶劑的還原產(chǎn)物,在高溫下表現(xiàn)出明顯的溶解和再生。以上結(jié)果表明,在高低溫下,LiFNFSI電池具有更好的容量保持率,主要?dú)w因于較穩(wěn)定的SEI膜,以及熱穩(wěn)定性較好且不含HF的LiFNFSI電解液；而LiPF6電池的容量衰減速度較快,主要是由于石墨負(fù)極SEI膜的溶解和再生,以及電解液中HF和質(zhì)子性雜質(zhì)的有害影響。(4)系統(tǒng)評價了LiFNFSI作為LiPF6的共導(dǎo)電鹽,在EC/EMC (3:7, v/v)電解液中的基礎(chǔ)理化性能和電化學(xué)行為,及其在石墨/LiCoO2電池中的高溫擱置、倍率、室溫(25℃)和高溫(60℃)循環(huán)性能,并結(jié)合EIS和XPS研究了LiPF6-LiFNFSI混合鋰鹽電解液體系對電極/電解液界面膜的阻抗和組成的影響。研究表明,與LiPF6和LiFNFSI單一鋰鹽電解液相比,LiPF6-LiFNFSI混合鋰鹽電解液表現(xiàn)出較優(yōu)異的電池循環(huán)性能。其中,LiPF6和LiFNFSI的濃度均為0.5 M時,石墨/LiCo02電池表現(xiàn)出較好的綜合性能,如循環(huán)100周的容量保持率：92.1%(25℃)和85.0%(60℃)。這主要?dú)w因于以下三個因素的影響：1)LiFNFSI的存在顯著提高了電解液的熱穩(wěn)定性;2)FNFSI-陰離子清除了電解液中的HF,減緩了其對石墨負(fù)極界面膜組分的化學(xué)性腐蝕,以及對正極金屬離子的溶出;3) PF6-和FNFSI-陰離子在石墨負(fù)極表面共同還原,形成了較穩(wěn)定的SEI膜。另一方面,EIS結(jié)果表明造成混合鋰鹽電解液的電池容量衰減的主要原因是不可避免存在的電極／電解液界面的反應(yīng),表現(xiàn)為電池阻抗值的增大,且此趨勢在高溫下加劇。最后,本論文從電解液的化學(xué)和電化學(xué)穩(wěn)定性、界面成膜性能等多角度出發(fā),對鋰離子電池電解液的發(fā)展方向進(jìn)行了展望。為了擴(kuò)大鋰離子電池的應(yīng)用范圍,特別是發(fā)展電動汽車適用的高溫、長壽命鋰離子電池,研究開發(fā)化學(xué)和電化學(xué)穩(wěn)定性高、與電池材料相容性好的新型電解液體系是今后電解液的主要努力方向之一。其中,兩種或多種鋰鹽混合使用,是目前提升電解液性能的重要技術(shù)策略之一,特別是在提高電池循環(huán)穩(wěn)定性方面(尤其是高溫性能),表現(xiàn)出較好的應(yīng)用前景,如本文所提出的LiPF6-LiFNFSI混合鋰鹽電解液體系。
[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.
【學(xué)位授予單位】：華中科技大學(xué)
【學(xué)位級別】：博士
【學(xué)位授予年份】：2016
【分類號】：TQ131.11;TM912
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本文編號：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.
【學(xué)位授予單位】：華中科技大學(xué)
【學(xué)位級別】：博士
【學(xué)位授予年份】：2016
【分類號】：TQ131.11;TM912
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本文編號：2215892
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