CAS OpenIR
反应-扩散对锂电池枝晶抑制作用研究
陈永修
Subtype博士
Thesis Advisor韩永生
2019-07-01
Degree Grantor中国科学院大学
Degree Discipline化学工程
Keyword锂电池,枝晶,浓度梯度,外加电场,外加磁场
Abstract

近年来,能源供给日益紧张的局势迫切要求人们研发兼具高能量密度和长使用寿命的金属二次电池(如锂、钠、锌、镁金属电池等),以满足电动汽车、便携式电子设备、航天电源等领域的能源需求。在上述二次电池中,锂金属作为负极时,因具有最低的还原电势(-3.04 V vs标准氢电极)和最高的理论容量(3860 mAh g-1),引起了科研工作者强烈的兴趣。但目前,锂金属电池因锂枝晶问题使其应用前景大打折扣。这主要是因为:(1)锂枝晶的生长与蔓延极易诱发电池内部短路导致电池安全问题;(2)锂枝晶在快速放电过程中,极易脱落于电解质液中形成“死锂”,降低电池容量和缩短电池的使用寿命。(3)锂枝晶刺破固体电解质保护层,与电解液发生副反应,消耗电解液,缩短电池的使用寿命。由此可见,锂枝晶的抑制是开发高功率密度和长使用寿命储能电池的首要前提。在过去几十年中,科研工作者通过对电解液改性、固体电解质界面保护、固态电解质、结构化或功能化负极、隔膜改性、充电方式等技术手段来抑制锂枝晶。上述有关材料创新的研究工作已取得重要进展,但锂枝晶难题依然悬而未决。从枝晶结构的生成机制角度抑制锂枝晶的研究工作鲜有报道,洞悉枝晶结构生长的驱动力将有助于提出新的抑制锂枝晶思路。结合本课题组已有研究工作基础:电沉积中,金属离子在传递受限的情况下,倾向于生成树枝状结构,而强化金属离子传质后,金属枝晶结构逐渐消失。因此,本研究在探讨锂枝晶生成机制的基础场,分别采用外加电场和外加磁场强化锂离子传输,削弱电沉积界面处离子浓度梯度,达到抑制锂枝晶生成的目的。首先,本研究对诱发锂枝晶生成的影响因素和锂枝晶对电池循环性能的影响进行了探讨。影响枝晶生成因素的研究主要涉及反应速率(电流密度)、反应时间、循环次数。具体地:(1)随着反应速率的提升(0.2 ~10 mA cm-2),锂枝晶结构会变得越来越纤长:棒状枝晶结构(长度低于5 μm)过渡至线状锂枝晶(长度可达50 μm)。这主要是因为较快反应条件下,锂离子供给(传递)速率低于消耗(反应)速率,在晶体生长前端形成稳定的浓度梯度,诱发锂枝晶结构的持续性生长。(2)当反应速率为定值时,随着反应时间的推移,锂枝晶的结构在反应前、中和后期分别呈现出线状(不具有分叉结构)、苔藓丛(扭曲线状团簇)、灌木丛状(具有分叉结构)。这主要是因为:反应前期,锂离子在电沉积界面处传递受限,倾向于在枝晶尖端富集、成核和生长,展现出一维线状枝晶结构;反应中期,沉积界面处锂离子进一步被消耗,枝晶尖端处的浓度场发生变化,导致枝晶生长方向发生变化,故生成由扭曲线状组成的团簇状形貌(苔藓状);反应后期,伴随着锂离子极度消耗,锂离子逐渐在枝晶缺陷处成核,故演变成扭曲状分叉枝状团簇(灌木丛状)。(3)高电流密度充放电实验中,随着循环次数的增加,锂枝晶的长度逐渐延长,且锂枝晶在延伸过程中,锂枝晶的直径变得粗细不一,该形貌结构极易断落于电解液中形成“死锂”。另外,伴随着充放电循环的增加,循环至放电终点时,负极表面残余的惰性锂逐渐增多,这也是导致电池库伦效率降低的诱因。(4)采用COMSOL对负极枝晶形貌进行了模拟,发现:负极表面锂离子流分布极为不均匀和枝晶尖端存在尖锐的浓度梯度,从而验证了浓度梯度是锂枝晶生成驱动力的论断。其次,本研究采用了外加电场强化锂离子传质,削弱沉积界面处浓度梯度,高电流密度充放电下实现了锂枝晶的抑制:(1)在电池负极处施加外加交流电场(电场方向平行于负极表面,即与锂离子运动方向垂直),使得锂离子负极处均匀分布,发现适中的交流频率(30 Hz)在均布锂离子方面展现出优异的性能。在外加交流电场(5 V cm-1和30 Hz)和高电流密度充放电条件下(2 mA cm-2),电池使用寿命为同电流密度对照实验的3倍,且沉积锂形貌为小颗粒;(2)在电池外部施加外加直流电场以强化锂离子传递,所述的外加直流电场方向与电池内部正极指向负极的方向一致。故在外电场力(与锂离子运动方向同向)的作用下,可使得锂离子传质增强。研究结果表明:伴随着外加直流电场场强的增加,电池内部锂离子转移数和扩散系数均提高。且沉积界面处的浓度梯度得以削弱,这主要得益于外加电场促使锂离子向负极表面迁移,增加界面处锂离子浓度而导致的结果,该结论也被COMSOL模拟研究工作证实。接下来,外加直流电场(5 V cm-1)和高电流密度充放电测试表明电池使用寿命为同电流密度下无外场对照实验的2倍,电池循环后沉积锂的形貌为小颗粒(如珍珠状等),证明了强化传质削弱沉积界面处浓度梯度抑制锂枝晶生成的论断;(3)考察了同时采用外加交流电场(5 V cm-1和30 Hz)和外加直流电场(5 V cm-1)对电池多循环性能的影响及其对锂枝晶的抑制情况,研究发现:即便是高电流密度充放电条件下,电池的使用寿命分别是同电流密度下对照实验的5倍,且负极表面沉积锂的形貌为小颗粒,表明两场同用在保护负极和延长电池使用寿命方面表现出优异的性能。最后,本研究考察了采用外加磁场强化传质对电池循环性能的影响及其对锂枝晶的抑制情况。所述的外加磁场布置于电池负极,磁场方向与负极表面平行,即与锂离子运动方向垂直。在电池充电过程中,快速切割磁感线运动的顺磁性锂离子流受洛伦兹力的影响,在负极表面附近产生微流动,强化了锂离子传质的同时,也避免了锂离子在负极表面的局部富集,因而能够有效抑制锂枝晶的生成。研究中发现:(1)随着磁场强度的增加,锂离子扩散系数随之增大,而沉积界面处界面浓度梯度降低,证实了外加磁场强化传质的有效性。(2)适中的磁场强度(0.8 T)对延长电池使用寿命、负极保护和锂枝晶抑制等电池长使用寿命指标十分重要。磁场强度为0.8 T,电池使用寿命为同电流密度下对照实验的5.5倍,电池的电化学性能最为优异。另外,伴随着磁场强度的增加,沉积锂形貌依次表现为线状枝晶、针状枝晶、大颗粒和小颗粒沉积形貌,有效证明了强化锂离子传质调控沉积锂形貌的推断。(3)采用COMSOL模拟对负极处沉积锂形貌进行了追踪,并对负极表面至体相电解液方向的浓度分布进行了量化,模拟结果表明:无外场条件下,电池负极表面存在明显尖锐的、稳定的浓度梯度,导致锂枝晶结构的蔓延;而引入外加磁场后,电池中锂离子传输增强,削弱了负极界面处浓度梯度,锂枝晶生长驱动力得以削弱,故倾向于生成均匀沉积的形貌结构。;The ever-increasing needs of energy has stimulated the research on second metal batteries (i.e., Li, Na, Zn, etc.), which hold high energy density and show widespread uses in emerging applications represented by electrical vehicles, portable electronics, aerospace application, and etc. Among these potential candidates, lithium metal battery (LMB) stirs intense research because of its highest theoretical capacity (3860 mAh g-1) and lowest electrochemical potential (-3.04V versus standard hydrogen electrode). However, the uncontrolled dendrite formation during charging process leads to safety concern and shortened life for lithium battery. In detail, the formidable lithium dendrites during repeated cycling tend to connect the anode and cathode, resulting in internal short circuits and safety issues. Also, after piercing solid electrolyte interface (SEI layer) by dendrites, the undesired reaction between deposited lithium and electrolytes as well as the dead lithium during discharge process deteriorate the lifespan of batteries. Thus, inhibiting lithium dendrites is the primary prerequisite for developing storage batteries with high power density and long lifespan.During the past decades, numerous investigations have been focused on suppressing dendritic lithium via electrolyte modification and additive, protecting SEI layer or in-situ formation robust SEI layer, all solid-state electrolyte, 3D and structure anodes, charging style, separator modification,and etc. Although significant progress has been achieved, this problem is still unsolved. However, there are few efforts to inhibit lithium dendrites based on the formation mechanism of dendritic structures. A good understanding on the growth mechanism of dendrites may lead to a breakthrough solution to mitigate or eliminate these structures in recharging batteries. In our previous studies, the dendritic structures are prone to form at diffusion limitation of chemicals. However, the dendritic structures can be inhibited by enhancing the transport of metal ions. Thus, in this dissertation, diffusion enhancements by external fields (i.e., electric fields, magnetic fields) are employed to suppress the formation of dendrites in lithium battery at a quick charging process.Firstly, the factors (i.e., reaction rate (current density), reaction time, and cycles) were examined to probe their effects on dendrite proliferation. Also, the battery is also tested to explore the lifespan of battery without external fields. The conclusions in this part are presented as follows: (1) higher reaction rate gives rise to the slender and elongated dendrites. With the increasing current density from 0.2 to 10 mA cm-2, the dendritic morphologies are evoluted from needle-like dendrites with length less than 5 μm to wire-like dendritic structure with a length of approximate 50 μm. When the reaction rate is faster than the diffusion rate of lithium ions, a concentration gradient is formed at the growth front of crystals. Once the gradient is stable, lithium ions tend to accumulate at the tip of dendrites to deposit, lengthening the dendrites. (2) With the repeated charging process at a high current density, three dendritic morphologies appear on the anode surface at the sequence of wire-like dendrites, moss-like dendrites, and shrub-like dendrites. At the initial stage, the transport of lithium ions near the anode surface is slower than the consumption rate, which results in the established of stable gradients at the growth front of crystals. Lithium ions tend to accumulate, nucleate, and grow at the tip of dendrites, lengthening the dendritic structures. During the middle stage, lithium ions are further consumed and the concentration gradients have changed, inducing the unstable growth direction and forming moss-like structures. At the last stage, the lithium ions are extremely consumed, which generates defects in the craystal and shapes twisted shrub-like dendrites. (3) The lithium dendrites are elongated with the repeated cycling. These structures tend to break into electrolyte, forming dead lithium and lowering battery capacity. In addition, more and more inactive dendrites still remain on the anode surface at the end of discharge in each cycle, deteriorating battery life. (4) The COMSOL simulation shows that lithium ions fluxs are uneven distribution at the anode surface. Also, there appears steep concentration gradient at the tip of dendrites, confirming the interface concentration gradient is the driving force for dendrite growth.Secondly, external electric field was employed to enhance the diffusion of lithium ions, lower the concentration gradient at the deposition interface, and suppress lithium dendrites even at a high charge current density: (1) An external alternating current field (ACF) is delicately built up perpendicular to the anode to perturb lithium ion distribution around the anode. The ACF with 30 Hz shows excellent performance in lengthening battery life and suppressing lithium dendrites due to its superior uniform distribution of Li+ along the anode surface. In detail, the battery life with ACF (5 V cm-1 and 30 Hz) are three times of the control case. The deposited morphologies at high current density are small particles, confirming the efficacy of dendrite inhibition. (2) An external direct current field (DCF) was applied to the battery to speed up the diffusion of Li+ in electrolytes. The DCF was parallel to the internal electric field, which was expected to accelerate the transport of lithium ions in electrolytes. The results show both the transfer number and diffusion coefficient of lithium ions increase with the intensity of DCF. Also, the concentration gradient decreases with the intensity of DCF, indicating that the DCF speeds up the lithium ions transport from cathode to anode. The COMSOL simulation shows that the deposited morphologies are uniform deposition layer after enhancing the transport of lithium ions. The life of the battery increases two times after the introduction of the DCF (5 V cm-1) even at high current density of 2 mA cm-2. There are no dendritic structures after cycling at the anode, confirming the effectiveness of DCF in suppressing lithium dendrites. (3) A simultaneous adoption of ACF (5 V cm-1 and 30 Hz) and DCF (5 V cm-1) was employed to investigate their effects on the electrochemical performance of the lithium battery. The lifespan of the lithium battery is almost five times of the control case at the high current density of 2 mA cm-2, showing excellent performance of DCF and ACF in protecting the anode surface, prolonging battery life, and suppressing lithium dendrites.Finally, the external magnetic field (EMF) was employed to examine its effect on battery electrochemical performance and dendrite inhibition. The EMF was parallel to the anode surface, namely vertical to the lithium ions movement. The magnetic field force (Lorentz force) could be exerted on the motional lithium ions, inducing micro fluids at the anode surface, which avoids the accumulation of lithium ions at the tip, resulting in uniform distribution of lithium ions and dendrite-free deposited morphology. The conclusions in this part are as follows: (1) Li+ diffusion coefficient increases with the intensity of EMF. Also, the increased intensity of EMF leads to the decrease of concentration gradients at the anode surface. This validates the effectiveness of EMF in the enhancement of mass transport. (2) A moderate EFM with 0.8T during battery tests is evaluated for well inhibiting lithium dendrites and prolonging battery life. The life of battery with EMF of 0.8 T is 5.5 times of the control case at the high current density of 2 mA cm-2. With the increased strength of magnetic field, the deposited morphologies are needle-like dendrites, wire-like dendrites, large particles, and tiny particles. This confirms that the enhancement of mass transport via EMF could effectively shape the deposited lithium morphologies. (3) The COMSOL simulations were conducted to track the morphologies evolution. It shows that the sharp concentration gradient exists on the tip of dendrites and drives the proliferation of dendrites. However, the enhancement of transport via EMF lowers the concentration gradient, forming dendrite-free deposited morphologies.

Language中文
Document Type学位论文
Identifierhttp://ir.ipe.ac.cn/handle/122111/40639
Collection中国科学院过程工程研究所
Recommended Citation
GB/T 7714
陈永修. 反应-扩散对锂电池枝晶抑制作用研究[D]. 中国科学院大学,2019.
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