【作者】 黄承焕；

【导师】 黄可龙；

【作者基本信息】 中南大学 ， 应用化学， 2011， 博士

【摘要】 大力开发新能源、可再生能源以及发展新能源汽车,是解决我国能源可持续利用以及环境污染问题的有效途径。而大规模利用这些新能源,需要与之配套的高性能电池。在众多现行开发的二次电池中,锂离子电池因具有工作电压高、功率密度和能量密度高和无记忆效应等优点,成为人们的首先。但是,锂离子电池在首次充电、存储和充放电循环等过程中发生性能衰减甚至安全问题,减少了电池的使用寿命,提高其使用成本,并限制了其在交通领域的应用。因此,充分认识这些过程中锂离子电芯的容量衰减、性能失效等机制以及电极界面的特征,对于管理、使用和设计锂离子电芯具有重要的指导意义。本论文首先研究了LiNi1/3Mn1/3Co1/3O2/LiPF6-EC:DEC:PC/人造石墨电芯在不同条件下的存储行为,采用交流阻抗技术(EIS)研究电芯及电极阻抗变化；辅助XRD、ICP、FTIR.SEM、EDS和DSC等分析技术,考察存储条件对电极材料以及隔离膜的化学物理性质的影响。发现28周内电芯容量衰减的活化能为28.8-35.8kJ/mol,容量损失率与存储时间的平方根呈线性关系。低温阻碍电极界面副反应发生,减少了容量衰减和阻抗增加。0℃下,28周后,电芯的容量损失率仅为4.1%,比60℃下少39.4%。通过胀气气体成分分析及EIS测试表明负极/电解液界面的副反应占主导。主要气体(CO、CH4、C2H6)是由电解液在负极表面上的还原产生的。高温存储未改变石墨的晶体结构,但导致了电解液在正极表面的氧化,并造成了隔离膜孔隙率的下降；同时改变石墨表面形貌和及其固体电解质界面(SEI)膜的成分,促使SEI膜分解温度提高。此外,还考察了充电状态和石墨颗粒大小对该电芯体系高温存储性能的影响。制备参比电极,原位监测三电极电芯在存储、使用过程中电极的电势和阻抗变化。发现满充状态下的LiN11/3Mn1/3Co1/3O2/人造石墨电芯、LiCoO2/混合石墨电芯,在高温存储下,石墨负极的界面阻抗随存储时间的延长先减小而后增大；这说明了SEI膜的溶解与成长存在相互竞争过程。然而,正极界面阻抗则逐渐地增大。存储过程中,负极的电势因LiC6发生脱锂反应而逐渐地上升,正极的电势则因LixNi1/3或LixCoO2材料发生嵌锂反应而逐渐地降低。还发现在电芯循环过程中,正、负极的阻抗变化趋势跟高温存储过程类似。分析了锂离子电芯的可逆和不可逆容量损失机理。发现几乎所有可逆损失的容量在电芯存储后的首次充电过程重新获得。可逆损失部分主要有两种来源：从嵌锂石墨中损失掉的Li+,一部分转化为亚稳态的烷基锂,还有一部分Li+因SEI膜的溶解迁移到正极并重新嵌入L1xNi1/3Mn1/3Co1/3或LixCoO2晶格中；这些损失掉的Li+在以后的充放电中可重新利用。此外,还讨论了电芯在循环过程中不可逆容量损失的机理。探讨了首次充电条件对石墨SEI膜和电芯电化学性能的影响。提高化成温度可以促进SEI膜中一些亚稳态的锂配合物转化为更稳定的成分,从而改善SEI膜的化学稳定性。当化成温度从25℃提高到45℃时,电芯在60℃存储10周或者在常温下循环300次(-1C)后,不可逆容量损失率减少了约8%。但在同一温度下,化成电流密度在0.044-1.077mA/cm2范围内,对电芯的电化学性能衰减影响小。对于LiCoO2/LiPF6-EC:DEC:EMC/混合石墨电芯,不同种类的石墨颗粒间因晶体结构、比表面积的差异,造成存储中活性Li+损失量的不同,从而导致了混合石墨电极中浓差电池(Li1-xC6)(Li1-x-yC6(x≠y)的形成,并诱发了该电极在低频区电感的出现。对于满充电芯,Li+在石墨电极界面的转移活化能为64kJ/mol,比其在LiCoO2正极界面多24kJ/mol。由于石墨界面主导了离子转移电阻及活化能,导致低温下电芯充电时,负极表面容易析锂,造成不可逆容量损失。为此,减小石墨界面电荷转移电阻及其活化能,有利于提高锂离子电池的低温循环和倍率性能。更多还原

【Abstract】 Development of new and renewable energy, as well as new energy vehicles is an efficient solution to the problems of energy sustainable utilization and environment pollution. In order to store and utilize these new energies in large-scale, batteries with high performance are required. Among several types of secondary batteries, Li-ion battery is concerned firstly owe to its favorable properties of high working voltage, high power and energy density and no memory effect. However, Li-ion battery suffers performance decay and even safety problem during the period of the initial charge, storage and cycle, which will shorten battery life. Consequently, it would lead to increase battery cost and impede its wide application, especially in the transportation field.In this thesis, storage behaviors of one type of Li-ion cells based on LiNi1/3Mn1/3Co1/3O2/LiPF6-EC:DEC:PC/artificial graphite under various storage condition were investigated using impedance spectroscopy (EIS), XRD, ICP, FTIR, SEM, EDS, DSC, etc. With aims of these analysis tools, effects of storage conditions on chemical and physical properties of electrodes and separators were studied. It was found that activation energy of cell capacity loss was in the range of28.8-35.8kJ/mol and that there was linear relationship between capacity loss and square root of storage time. Low temperature impedes the interfacial side reactions, capacity loss and growth of impedance of a cell. The capacity loss of an aged cell after storage for28weeks was only4.1%at0℃,39.4%lower than60℃. Gas and EIS analysis results showed that the interfacial reaction on graphite dominated the side reactions of the aged cells. The main gases including CO、CH4、C2H6detected in a swelling aged cell are caused by the reduction of electrolyte at graphite interface. High temperature does not alter structure of graphite, but surface morphology of graphite and composition of solid electrolyte interphase (SEI) film, which leads to increase the decompose temperature of SEI film. The effects of state of charge (SOC) and particle size of graphite on storage performance of Li-ion cells at elevated temperature were also studied.Changes of potentials and impedance of electrodes were in-situ monitored with the aid of reference electrode in3-electrode cells during storage. It was observed that the interfacial impedance of graphite electrode decreased firstly and then increased with the prolonged storage for both of LiNi1/3Mn1/3Co1/3O2/artificial graphite and LiCoO2/mixed graphite cells, indicating that the competition between the damage and growth of SEI film occurs during storage. Yet, the interfacial impedance of positive electrode increased with the storage. During storage, the potentials of negative electrode gradually increased due to the de-lithiation of LiC6, whereas the negative electrode gradually dropped due to Li+re-intercalation into LixNi1/3Mn1/3Co1/3or LixCoO2. It was also found that the impedance changes of both electrodes during cycle were similar with high-temperature storage.Mechanisms of reversible and irreversible capacity loss of Li-ion cells have been discussed. Experiment results showed that most of reversible loss of capacity for an aged cell after storage could be regained after recharging. There are two main sources to regain reversibly lost capacity or lithium ions during storage. Some reversibly lost lithium ions react with electrolyte solvent to become metastable lithium alkyl complex (as a part of SEI film), which can be cyclable again after recharging. On the other hand, some lithium ions of SEI film release in electrolyte due to the dissolution of SEI film and re-intercalation into LixNi1/3Mn1/3Co1/3or LixCoO2crystal driven by Li+concentration gradient. The irreversible capacity loss of Li-ion cells during cycle was also discussed.The effects of initial charge conditions on SEI film properties and electrochemical performance of Li-ion cells have been discussed. Some metastable lithium complex can be transformed to the stable one by increasing formation temperature, which can enhance the chemical stability of SEI film. The irreversible capacity loss of an aged cell after storage at60℃for10weeks or after300cycles (at～1C) at room temperature reduced ca.8%after increase the formation temperature from25℃to60℃. However, under the same temperature, the formation current density within the range of0.044-1.077mA/cm2hardly impacted on the performance of Li-ion cell.As for LiCoO2/LiPF6-EC:DEC:EMC/Mixed graphite Li-ion cell, the amount of lost active Li+during cell storage differs among various types of graphite particles due to their difference of crystal structure and specific surface area, resulting in formation of a concentration cell of (Li1-xC6)/(Li1-x-yC6)(x≠y) in the mixed graphite electrode. The concentration cell can induce inductive phenomena in impedance spectra of the mixed graphite electrode at lower frequency range. It was shown that in the fully charged celll, the activation of lithium ion transfer at graphite/electrolyte interface was64kJ/mol,24kJ/mol more than that of lithium ion transfer at LiCoO2/electrolyte interface. Because the graphite electrode dominates interfacial charge transfer resistance and its activation energy, lithium trends to be plating on the graphite, causing irreversible capacity loss for a cell during a charge process at low temperatures. By reducing both of them, it will be beneficial in improving the low-temperature and the high rate cycle performance of Li-ion cell.更多还原