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1、.Capacity Fade Mechanisms and Side Reactions inLithium-Ion Batteries Pankaj Arorat and Ralph E. WhiteCenter For Electrochemical Engineering, Department of Chemical Engineering, University of South Carolina,Columbia, South Carolina 29208, USA ABSTRACT The capacity of a lithium-ion battery decreases d
2、uring cycling. This capacity loss or fade occurs due to several different mechanisms which are due to or are associated with unwanted side reactions that occur in these batteries. These reactions occur during overcharge or overdischarge and cause electrolyte decomposition, passive film formation, ac
3、tive material dissolution, and other phenomena. These capacity loss mechanisms are not included in the present lithium-ion battery mathematical models available in the open literature. Consequently, these models cannot be used to predict cell performance during cycling and under abuse conditions. Th
4、is article presents a review of the current literature on capacity fade mechanisms and attempts to describe the information needed and the directions that may be taken to include these mechanisms in advanced lithium-ion battery models。锂离子电池容量衰减机理和界面反应研究作者：Pankaj Arorat and Ralph E. White美国，南卡罗来纳2920
5、8，哥伦比亚，南卡罗来纳州大学，化工学院化工系摘要 锂电池在循环过程中，其容量会逐渐衰减。而出现容量衰减主要归因于几个不同的机理，这些机理大多与电池内部的界面反应相关，这些反应持续性的发生在电池的充放电环节，并且引起电解液的分解、钝化膜的形成、活性材料的溶解等其它现象。关于容量衰减的机理在目前公开的锂离子电池数学模型的文献中并未加以阐述，因此在锂电池循环过程中和处于苛刻的条件下，我们无法通过模型来对锂电池的性能作出有效的预测。本篇文章将陈述容量衰减的机理，并且试着去解释其本质，为构建先进的锂电池模型指明方向。 lntroduction The typical lithium-ion cell(
6、Fig. 1) is made up of a coke or graphite negative electrode, an electrolyte which serves as an ionic path between electrodes and separates the two materials, and a metal oxide (such as LiCoO2, LiMn2O4, or LiNiO2) positive electrode. This secondary (rechargeable) lithium-ion cell has been commerciali
7、zed only recently.47 Batteries based on this concept have reached the consumer market, and lithium-ion electric vehicle batteries are under study in industry. The lithium-ion battery market has been in a period of tremendous growth ever since Sony introduced the first commercial cell in 1990.With en
8、ergy density exceeding 130 Wh/kg (e.g., Matsushita CGR 17500)and cycle life of more than 1000 cycles (e.g., Sony 18650)in many cases, the lithium-ion battery system has become increasingly popular in applications ，such as cellular phones, portable computers, and camcorders. As more lithium-ion batte
9、ry manufacturers enter the market and new materials are developed, cost reduction should spur growth in new applications. Several manufacturers such as Sony Corporation, Sanyo Electric Company, Matsushita Electric Industrial Company, Moli Energy Limited, and A&T Battery Corporation have started manu
10、facturing lithium-ion batteries for cellular phones and laptop computers. Yoda has considered this advancement and described a future battery society in which the lithium-ion battery plays a dominant role.概论 传统的锂电池由碳或石墨负极材料、作为电极间的离子传输通道的电解液、金属氧化物（例如LiCoO2、LiMn2O4、LiNiO2）正极材料三部分组成，这种二次（可充电）电池已经商业化。依照
11、这种原理制作的锂电池已经形成稳定的消费者市场，同时锂离子动力电池也在进行工业化研究。自从1990年，Sony制造出第一批商业化电池开始，锂电池市场开始进入繁荣时期。由于具有超过130wh/kg（matsushita CGR 17500）的能量密度和超过1000次循环的优势，锂电池在移动电话、手提电脑、便携式摄像机等设备领域得到更加广泛的应用。随着更多的锂电池生产商进入市场，新型材料也被陆续开发出来，同时成本控制也成为新产品增长的关键因素。像索尼电器、三洋电器公司、松下电器、莫里能源有限公司（加拿大）、日本A&T电器公司都已经在移动电话和便携式电脑等产业开始锂电池应用商业化。Yoda也已经认识到
12、锂电池的发展趋势，并且在将来的电池能源时代，锂离子电池将扮演者关键的角色。Several mathematical models of these lithium-ion cells have been published.Unfortunately, none of these models include capacity fade processes explicitly in their mathematical description of battery behavior. The objective of the present work is to review the cur
13、rent understanding of the mechanisms of capacity fade in lithiumion batteries. Advances in modeling lithium-ion cells must result from improvements in the fundamental understanding of these processes and the collection of relevant experimental data. 关于锂离子电池的数学模型，已经有相关文献进行阐述，然而遗憾的是至今没有一篇文献能就容量衰减机理进行明
14、确解释，而本文将会在锂电池容量衰减机理进行详细阐述。先进的锂电池模型必须建立在加深对这些过程的基本理解和实验数据的整理归纳的基础之上。 Some of the processes that are known to lead to capacity fade in lithium-ion cells are lithium deposition (overcharge conditions), electrolyte decomposition, active material dissolution, phase changes in the insertion electrode mate
15、rials, and passive film formation over the electrode and current collector surfaces. Quantifying these degradation processes will improve the predictive capability of battery models ultimately leading to less expensive and higher quality batteries. Significant improvements are required in performanc
16、e standards such as energy density and cycle life, while maintaining high environmental,safety, and cost standards. Such progress will require considerable advances in our understanding of electrode and electrolyte materials, and the fundamental physical and chemical processes that lead to capacity
17、loss and resistance increase in commercial lithium-ion batteries. The process of developing mathematical models for lithiumion cells that contain these capacity fade processes not only provides a tool for battery design but also provides a means of understanding better how those processes occur. Pre
18、sent Lithium-Ion Battery Models The development of a detailed mathematical model is important to the design and optimization of lithium secondary cells and critical in their scale-up. West et al. developed a pseudo two-dimensional model of a single porous insertion electrode accounting for transport
19、 in the solution phase for a binary electrolyte with constant physical properties and diffusion of lithium ions into the cylindrical electrode particles. The insertion process was assumed to be diffusion limited, and hence charge-transfer resistance at the interface between electrolyte and active ma
20、terial was neglected. Later Mao and White developed a similar model with the addition of a separator adjacent to the porous insertion electrode.These models cover only a single porous electrode; thus, they do not have the advantages of a full-cell-sandwich model for the treatment of complex, interac
21、ting phenomena between the cell layers. These models confine themselves to treating insertion into TiS2. with the kinetics for the insertion process assumed to be infinitely fast. Spotnitz et al.accounted for electrode kinetics in their model for discharge of the TiS2 intercalation cathode.The galva
22、nostatic charge and discharge of a lithium metal/solid polymer separator insertion positive electrode cell was modeled using concentrated-solution theory by Doyle et al.The model is general enough to include a wide range of separator materials, lithium salts, and composite insertion electrodes. Conc
23、entrated-solution theory is used to describe the transport processes, as it has been concluded that ion pairing and ion association are very important in solid polymer electrolytes.This approach also provides advantages over dilute solution theory to account for volume changes. Butler-Volmertype kin
24、etic expressions were used in this model to account for the kinetics of the charge-transfer processes at each electrode. The positive electrode insertion process was described using Picks law with a constant lithium diffusion coefficient in the active material. The volume changes in the system and f
25、ilm formation at the lithium/polymer interface were neglected and a very simplistic case of constant electrode film resistances was considered. Long-term degradation of the cell due to irreversible reactions (side reactions) or loss of interfacial contact is not predictable using this model.Fuller e
26、t al developed a general model for lithiumion insertion cells that can be applied to any pair of lithium-ion insertion electrodes and any binary electrolyte system given the requisite physical property data. Fuller et als work demonstrated the importance of knowing the dependence of the open-circuit
27、 potential on the state of charge for the insertion materials used in lithium-ion cells. The slopes of these curves control the current distribution inside the porous electrodes, with more sloped open-circuit potential functions leading to more uniform current distributions and hence better utilizat
28、ion of active material. Optimization studies were carried out for the Beilcore plastic lithium-ion system.The model was also used to predict the effects of relaxation time on multiple charge-discharge cycles and on peak power.Doyle et al.modified the dual lithium-ion model to include film resistance
29、s on both electrodes and made direct comparisons with experimental cell data for the LixC6/LiPF6, ethylene carbonate/dimethyl carbonate (EC/DMC), Kynar FLEX/LiyMn2O4 system. Comparisons between data and the numerical simulations suggested that there is additional resistance present in the system not
30、 predicted by present models. The discharge performance of the cells was described satisfactorily by including either a film resistance on the electrode particles or by contact resistances between the cell layers or current-collector interfaces. One emphasis of this work was in the use of the batter
31、y model for the design and optimization of the cell for particular applications using simulated Ragone plots.Thermal modeling is very important for lithium batteries because heat produced during discharge may cause either irreversible side reactions or melting of metallic lithium, Chen and Evans car
32、ried out a thermal analysts of lithiumion batteries during charge-discharge and thermal runaway using an energy balance and a simplified description of the electrochemical behavior of the system.Their analysis of heat transport and the existence of highly localized heat sources due to battery abuse
33、indicated that localized heating may raise the battery temperature very quickly to the thermal runaway onset temperature, above which it may keep increasing rapidly due to exothermic side reactions triggered at high temperature. Pals and Newman developed a model to predict the thermal behavior of li
34、thium metal-solid polymer electrolyte cells and cell stacks. This model coupled an integrated energy balance to a fullcell-sandwich model of the electrochemical behavior of the cells. Both of these models emphasized the importance of considerations of heat removal and thermal control in lithium-poly
35、mer battery systems.一些常见的引起锂电池容量衰减的因素包括1、锂枝晶的生成（过充电压条件下）2、电解液分解3、活性材料的溶解4、电极材料嵌锂过程中发生相变5、电极材料和集流体表面钝化膜的形成。对以上这些降解过程进行量化，将能够提升电池模型的电池容量，并最终制造出成本低、质量好的电池，能量密度和循环寿命是作为提升电池性能的重要指标，同时电池的安全性能、环境友好程度、成本标准也是衡量电池的指标。在此过程中，我们需要对电解液和电极材料有更深层次的理解，并且对商业化锂电池体系中引起容量衰减、阻抗增加的基本物理原理和化学过程做出进一步探究。构建包含这些容量衰减因素的锂电池数字模型不仅
36、能为锂电池的设计提供帮助，更为探究这些因素如何发生提供方法。如今的锂离子电池模型 一种详细的数字模型的构建对于锂离子二次电池的结构设计和性能优化显得极其重要，并且在后续的电池比例扩大过程中起到决定性的作用。西方学者首先构建出一种虚拟的二维模型，在该模型中，存在着单一的多孔可嵌入的电极，它能够保证具有常数物理性质的二元电解液在液态环境中传输，并且能让锂离子在电极的球形颗粒中扩散，锂离子的嵌入过程被认为是扩散能力有限的，因此在电解液和电极材料界面形成的电荷转移电阻通常是被忽略不计的。随后Later Mao和White构建出另外一种相似的模型，此模型中，在多孔的嵌入电极相邻处加入隔膜。这些模型中都只
37、包含一个多孔电极，因此它们不像“三明治”模型那样具备层间相互作用合成处理的优势。在这些模型中，它们将自己定位为TiS2的嵌入处理，并且认为该嵌入过程中的动力学是无限快的，其中Spotnitz学者对TiS2嵌入式正极在放电过程中的电极动力学进行过相关研究。Doyle学者通过溶液浓度理论构建出由锂金属/固态聚合物隔膜/可嵌入的活性材料三部分组成的恒流充放电体系。该模型一般包括广泛的隔膜材料、锂盐和复合式插入电极。溶液浓度理论则可以解释粒子传输过程，并且认为在固态聚合物电解质环境中，离子的配对和结合是相当重要的，相对于稀溶液理论，这种模型在体积变化上具备优势。？这个模型要运用Butler-Volme
38、rtype运动学公式去计算每个电极中电荷转移过程中的动力学。在正极材料的嵌入过程中，利用菲克定理来计算活性材料中锂离子扩散系数，整个体系中体积变化和锂与聚合物界面形成的钝化膜均忽略不计，但是会将电极界面电阻作为恒量纳入考虑范围。通过这种模型，我们无法预测由不可逆反应（副反应）或界面接触损失引起的持续性衰减。富勒等人构建出锂离子嵌入式电池的综合性模型，这种模型能兼容各种类型的锂离子嵌入式电极和二元电解液形成的体系，这能测定出我们想要的物理属性数据。富勒等人的工作阐述了理解充电状态的开路电压对于锂离子电池嵌入材料应用的重要性。通过这些曲线的斜率可以控制多孔电极内部的电流分布，利用开路电压曲线函数来
39、更好的统一电流分布，因此使活性材料得到更好的使用，而关于贝尔塑料锂离子电池系统的最优化设计已经完成，这个模型可以预测由弛豫时间给电池多次充放电循环和峰值功率带来的影响。Doyle学者修改了双电极锂离子电池模型，他考虑到两电极表面的钝化膜阻抗，并且制备出LiC6/LiPF6 EC/DMC(阿柯玛股份有限公司)/LiyMn2O4电池体系测试出的相关比对数据，对比实验数据和数字模型可以得出在该系统中出现的附加阻抗，而这个阻抗无法通过现有的模型进行预测。在将活性材料表面的钝化膜阻抗和电极间的接触阻抗或集电器的界面阻抗纳入考虑范围后，该电池的放电性能令人满意。此项工作的重点是利用模拟Ragone图来进行
40、电池设计和最优化应用。热反应建模也是锂电池的重要组成部分，通常认为在放电过程中产生的热量将会导致不可逆的副反应和金属锂的溶解。Chen和Evans两人制备出一套关于锂电池热分解系统，当电池处于充放电状态或热失控状态，通过能量平衡和简单描述该系统的电化学行为来构建模型，他们关于由电池滥用引起的热量传输分析和局部温度过高的理论表明：局部升温可能会很快地引起电池温度升高以致电池热失控，超出设定温度后，高温将会引发放热性界面反应从而使整个电池的温度急剧上升。Pals和Newman也构建出一种模型，利用该模型可以预测金属固态聚合物电解质电池和电池推的热反应，这个模型将综合能量平衡系统与“三明治”式的全电
41、池模型相联接，从而测定整个电池的电化学行为，以上所有模型均强调锂离子聚合物电池系统的热散失和热控制的重要性。Verbrugge and Koch developed a mathematical model for lithium intercalation processes associated with a cylindrical carbon microfiber. They characterized and modeled the lithium intercalation process in single-fiber carbon microelectrodes includi
42、ng transport processes in both phases and the kinetics of charge transfer at the interface. The primary purpose of the model was to predict the potential as a function of fractional occupancy of intercalated lithium. The overcharge protection for a Li/TiS2, cell using redox additives has been theore
43、tically analyzed in terms of a finite linear diffusion model by Narayanan et al。Darling and Newman modeled a porous intercalation cathode with two characteristic particle sizes.They reported that electrodes with a particle size distribution show modestly inferior capacity-rate behavior and relaxatio
44、n on open circuit is substantially faster when the particles are uniformly sized. Nagarajan et al modeled the effect of particle size distribution on the intercalation electrode behavior during discharge based on packing theory.They observed that during pulse discharge, an electrode consisting of a
45、binary mixture displays higher discharge capacity than an electrode consisting of singlesized particles. The current from the smaller particles reverses direction during the rest period which cannot be observed in the case of an electrode comprised of the same-sized particles. Recently Darling and N
46、ewman made a first attempt to model side reactions in lithium batteries by incorporating a solvent oxidation side reaction into a lithium-ion battery model, Even though a simplified treatment of the oxidation reaction was used, their model was able to make several interesting conclusions about self-
47、discharge processes in these cells and their impact on positive electrode state-of-charge. Verbrugge和Koch构建成一个关于锂离子嵌入圆柱形碳纤维的数字化模型，该模型可以表征并且可以模拟锂离子在单个碳纤维电极中的嵌入过程。包括锂离子在两相中的传输和界面传输动力学，该模型主要意图是为了预测嵌入的锂离子数量和电动势函数关系。在Nareyamal等学者构建的有限线性扩散模型中，理论分析了可以通过氧化还原添加剂来对Li/TiS2电池进行过充保护的结论。Darling和Newman构建成两种不同粒径尺寸的
48、多孔嵌入式正极模型，他们表明当材料的颗粒尺寸分布不不均匀时，将会导致较低的容量保持率和较快的自放电现象。？Nagarajan等学者研究了在充放电过程中颗粒尺寸分布对嵌入式电极的影响？，他们发现在放电过程中，二种粒度混合电极要比单一尺寸颗粒电极具有更高的放电容量。在后期放电过程中，小颗粒里面形成的电流将会改变方向，而这种现象在粒径分布均匀的电极里面不会存在。最近Darling和Newman开始尝试构建锂电池副反应模型，他们的想法是在锂电池模型中引入溶液氧化副反应系统，尽管对氧化反应进行了简单处理，但他们的模型仍然在电池自放电过程以及它对正极电极充电状态的影响方面能得出一些有价值的结论。A num
49、ber of models having varying degrees of sophistication have been developed for lithium rechargeable batteries. For the most part, these models consider the ideal behavior of the systems, neglecting the phenomena that lead to losses in capacity and rate capability during repeated charge-discharge cycles. Fundamental models of these latter phenomena a