高电压锂离子电池关键材料的开发及界面膜的优化研究

发布时间：2018-03-14 17:48

本文选题：高电压锂离子电池　切入点：界面膜　出处：《北京科技大学》2017年博士论文　论文类型：学位论文

【摘要】：随着石油资源的逐渐枯竭,以及汽车尾气排放带来的环境污染等问题,使得新能源汽车成为未来汽车的主要发展方向,成为行业研究热点。锂离子电池作为新能源汽车的核心零部件得到越来越多的关注。为了更好的满足用户需求,解决里程焦虑的问题,开发更高比能量的电池来延长汽车续航里程成为必然。提高锂离子电池的比能量需要提高活性材料的克容量发挥,扩宽电池的电压窗口,开发高电压锂离子电池。本文以开发高电压锂离子电池化学体系为目标,将关键原材料的研发和正负极界面膜优化等过程关键参数的识别有机结合起来,首先研究了决定高电压锂离子电池能量的正极富锂材料和决定电压窗口的电解液体系;然后为了高电压锂离子电池拥有更加优异的性能,识别了适用于高电压锂离子电池负极表面界面膜精修的低电位恒压化成参数和正极表面可控生长界面膜的高电位下高压化成参数,并研究了加工过程中不可避免带入的痕量水分对整个电池体系稳定性的影响;最后将优化的正极富锂材料、耐高压分解的电解液和界面膜优化有机结合起来,开发了性能优异的高电压锂离子电池化学体系,并对体系的寿命衰减机理进行了分析,进一步优化了高电压锂离子电池化学体系。富锂材料具备较高的克容量发挥,是开发高电压高能量锂离子电池优异的正极材料。但富锂材料在充放电过程中,由于Mn元素的变价,会发生类似尖晶石结构的相变,导致性能的快速劣化。本文采用Cu~(2+)元素掺杂的方法,取代一部分Mn~(4+),增强材料的结构稳定性和导电性,从而提高材料的电化学性能。通过液相共沉淀和球磨法,合成了含有不同比例Cu~(2+)的富锂材料0.5Li2MnO3·0.5Li(Ni0.5Mn0.5-xCux)O2,采用XRD,SEM,粉末电子电导和电化学充放电等测试方法对掺杂Cu~(2+)的富锂材料进行了表征。研究表明:掺杂Cu~(2+)之后,材料形貌由颗粒状变为类棒状,材料的层间距增大,克容量发挥降低,随着掺杂Cu~(2+)比例的提高,材料电子电导率逐渐增大,克容量发挥随之提高,寿命改善也越明显。掺杂能明显增加富锂材料的晶胞体积和层间距,有利于锂离子在循环过程中的脱嵌,增强材料的结构稳定性,进而改善循环性能。为了拓宽电池的工作电压窗口,本文研究了基于氟代溶剂D2的耐氧化分解的电解液体系,研究各溶剂组成、锂盐浓度对有机电解液的电导率、饱和蒸汽压、热稳定性、电化学窗口、正极耐高压稳定性和负极成膜稳定性的影响。结果表明:随着电解液中锂盐浓度的增大,电导率先增大再降低,饱和蒸汽压降低,热稳定性变差;随着电解液中D2比例的增大,电导率逐渐降低,饱和蒸汽压逐渐升高;D2溶剂明显提升了电解液的耐氧化分解特性,并参与了碳酸酯类溶剂在负极上的还原反应。识别化成参数可以优化电池正负极在首次充放电过程中所形成的界面膜,提升高电压锂离子电池的循环性能。对于负极,本文研究低电位下恒压精修SEI膜的化成方法,识别了关键控制参数,发现在3.3V下恒压60min,能够在负极表面上形成致密、低阻抗、良好均一性的界面SEI膜。针对正极在高电位下持续氧化分解电解液造成电池性能劣化的问题,本文寻找合适的恒压电位,控制电解液中VC的含量,从化成参数优化的角度,研究VC在电解液中的最佳比例,促使VC添加剂在正极表面充分、有序、可控分解,形成稳定的界面SEI膜,阻止电解液进一步的氧化分解,从而提高高电压锂离子电池高压下的循环寿命。研究发现当电解液中VC含量为0.8%,首次充电截止电压为4.63V,恒压40min,电池的循环性能最佳,300次循环后容量保持率达到70%。痕量水分对电池体系的稳定性有较大影响。本文研究了痕量水分对电池体系稳定性的影响,研究发现:水分对于正极,主要影响了电解液在正极表面发生氧化分解反应的电位,水分含量越高,电解液的氧化分解电位越低,氧化分解的产物在正极表面沉积,导致正极界面膜阻抗增大,而对正极材料结构没有影响;水分在石墨负极表面参与SEI的形成反应,改变了负极界面膜的组成和形貌,水分含量高的负极形成较厚的、疏松多孔的界面膜;不同水份含量全电池的寿命测试显示,负极水分含量的增大对电池循环寿命的影响要比正极水分含量增大显著的多;通过三电极分析发现,负极界面膜Rf的增大是电池阻抗增大的主要原因,负极水分含量越高,Rf随循环增长就越快,最终导致电池寿命衰减加剧。最后,本文采用掺Cu~(2+)改性后的富锂材料为正极,普通人造石墨为负极,常规PP/PE/PP三层复合聚烯烃隔膜,匹配D2氟化溶剂为主体成分的耐高压分解电解液,并采用负极界面SEI膜精修的低电位恒压化成参数和高电位下正极表面可控生长界面膜的高压化成参数,严格控制过程水分在合理的区间范围,制备了高电压锂离子电池,并对寿命衰减机理进行分析,进一步优化了高电压锂离子电池的化学体系。
[Abstract]:With the gradual depletion of oil resources, and vehicle emissions caused by environmental pollution and other issues, making the new energy vehicles have become the main direction of future car development, become a hot research area. Lithium ion batteries as the core components of new energy vehicles to get more and more attention. In order to better meet the needs of users, solve the mileage anxiety problems. The development of more high energy battery to prolong the vehicle mileage become inevitable. Improve the lithium ion battery than the energy needed to improve the active material capacity per gram of play, widening the battery voltage window, the development of high voltage lithium ion battery. In this paper, the development of high voltage lithium ion battery chemistry system as the goal, the key raw materials research and development and the positive and negative polar circle mask key parameters optimization process identification organically, firstly studies the positive decision of high voltage lithium ion battery energy The electrolyte lithium rich materials and then to determine the voltage window; high voltage lithium ion battery have more excellent performance and recognition for high voltage lithium ion battery cathode surface film refinement of low potential parameters and constant pressure into the anode surface controlled growth bounded in the high potential and high voltage forming parameters, and the effects of the trace the water in the process of the inevitable into the whole cell system stability; cathode lithium rich materials will finally be optimized, high pressure decomposition of the electrolyte and the film optimization combine the development of excellent high voltage lithium ion battery chemical system, and the system lifetime decay mechanism is analyzed, further optimization of the high voltage lithium ion battery chemistry system. Lithium rich materials have relatively high capacity, is the development of high voltage high energy lithium ion battery is excellent The electrode materials for lithium rich materials. But in the process of charge and discharge, because the Mn element valence, phase transition occurs similar to spinel structure, leading to rapid deterioration of the performance. This paper uses the method of Cu~ (2+) doped, replacing a part of Mn~ (4+), enhanced structural stability and conductivity of the material, so as to improve the the electrochemical properties of materials through liquid phase coprecipitation and ball milling method, with different proportion of synthesis of Cu~ (2+) of the lithium rich materials (Ni0.5Mn0.5-xCux), 0.5Li2MnO3 0.5Li O2, by XRD, SEM, test method of powder electronic conductivity and electrochemical charge discharge of doped Cu~ (2+) of the lithium rich materials were characterized. Research shows that the doping of Cu~ (2+), the material morphology from granular into class bar, materials increased the interlayer spacing, G capacity decreased with the doping of Cu~ (2+) the increase in the proportion of materials, electronic conductivity increases grams capacity play increase, improve service life Is more obvious. Doping can obviously increase the cell volume and the layer spacing of lithium rich materials, for lithium ions in the circulation process disembedded, enhance the structural stability of materials, and improve the cycle performance. In order to broaden the battery voltage window, in this paper the fluorinated solvent resistance of D2 electrolyte decomposition oxidation based on the study of the solvent composition, the conductivity of lithium salt concentration on the organic electrolyte, saturated vapor pressure, thermal stability, electrochemical window, positive and negative effects of high pressure resistant stability of membrane stability. The results show that: with the increase of salt concentration in the electrolyte, the conductivity increased first and then decreased, reducing the saturated vapor pressure and thermal stability becomes worse; with the increase of the proportion of D2 in the electrolyte, the conductivity decreased gradually, the saturated vapor pressure gradually increased; D2 solvent significantly enhance the oxidation decomposition of the electrolyte, and part of the carbonate dissolution The reduction reaction in the cathode of the battery can be optimized. Formed in the first charge discharge process to identify parameters of interfacial film, improved cyclic performance of high voltage anode for lithium ion batteries. In this paper, the formation process of low potential under constant refinement of the SEI film, identify the key control parameters, found in 3.3V constant pressure 60min, can form a dense, in the anode on the surface of low impedance, good uniformity of interface of SEI films. For the positive continuous oxidative decomposition of electrolyte caused by the deterioration of the battery performance in high potential, the potential for constant pressure fit, to control the content of VC in electrolyte, a parameter optimization point of Conghua, the best ratio of VC in the electrolyte, the VC additive in the cathode surface is sufficient, orderly, controllable decomposition, interface form stable SEI film, prevent further oxidation electrolyte decomposition, so as to improve high voltage lithium Ion battery cycle life under high pressure. The study found that when the content of VC in the electrolyte is 0.8%, the first charging voltage is 4.63V, voltage 40min, cell cycle performance, after 300 cycles the capacity retention rate has great influence on the stability of 70%. trace water battery system. This paper investigates the influence of trace water on the battery system the study found that: the stability of water on the cathode, the main impact of the electrolyte on the cathode surface potential oxidation decomposition reaction, the higher water content, electrolyte oxidation potential is low, product decomposition oxidation deposition on the cathode surface, resulting in the increase of cathode interface impedance, but no effect on the structure of cathode material; water formation reaction in the surface of graphite anode in SEI, changed the composition and morphology of the cathode film anode, high moisture content to form thick, porous film; not Life test with moisture content of whole cell showed negative effect on the moisture content increase the battery cycle life than the cathode water content increased significantly more; through the three electrode analysis showed that the increase of Rf cathode film is the main reason for the battery impedance increase, anode water content is higher, with the cycle of growth is more Rf fast, resulting in increased battery life attenuation. Finally, this paper uses Cu~ (2+) doped modified lithium rich materials of the cathode, the common artificial graphite as anode, conventional PP/PE/PP three layer composite polyolefin diaphragm, high pressure resistance, solvent based electrolyte decomposition D2 fluoride, body composition, and surface controlled cathode high potential growth sector mask of the high voltage forming low potential and formation parameters of constant parameters using SEI membrane anode surface refinement, strict control of water in a reasonable range, high voltage lithium ion battery was prepared, and the life The mechanism of life attenuation is analyzed, and the chemical system of high voltage lithium ion battery is further optimized.

【学位授予单位】：北京科技大学
【学位级别】：博士
【学位授予年份】：2017
【分类号】：TM912

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