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1、Capacity Fade Mechanisms and Side Reactions inLithium-Ion Batteries锂离子电池容量衰减机机理和副反应Pankaj Arorat and Ralph E. White*作者：Pankaj Arorat and Ralph E. White*Center For Electrochemical Engineering, Department of Chemical Engineering, University of South Carolina,Columbia, South Carolina 29208, USA美国，南卡罗来纳
2、，年哥伦比亚29208，南卡罗来纳大学，化学工程系，中心电化学工程ABSTRACT The capacity of a lithium-ion battery decreases during 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
3、overcharge or overdischarge and cause electrolyte decomposition, passive film formation, active 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 mode
4、ls cannot be used to predict cell performance during cycling and under abuse conditions. This 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 l
5、ithium-ion battery models.IntroductionThe typical lithium-ion cell (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.
6、 This secondary (rechargeable) lithium-ion cell has been commercialized only recently.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
7、 since Sony introduced the first commercial cell in 1990.With energy 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 applicationssuch as cellular phone
8、s, portable computers, and camcorders.As more lithium-ion battery 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
9、 Energy Limited, and A&T Battery Corporation have started manufacturing lithium-ion batteries for cellular phones and laptop computers. Yoda1 has considered this advancement and described a future battery society in which the lithium-ion battery plays a dominant role. Several mathematical models of
10、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 current understanding of the mechanisms of capacity fade in lithium-io
11、n 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. Some of the processes that are known to lead to capacity fade in lithium-ion cells are lithium deposition (overcharg
12、e conditions), electrolyte decomposition, active material dissolution, phase changes in the insertion electrode materials, and passive film formation over the electrode and current collector surfaces. Quantifying these degradation processes will improve the predictive capability of battery models ul
13、timately leading to less expensive and higher quality batteries. Significant improvements are required in performance standards such as energy density and cycle life, while maintaining high environmental, safety, and cost standards. Such progress will require considerable advances in our understandi
14、ng of electrode and electrolyte materials, and the fundamental physical and chemical processes that lead to capacity loss and resistance increase in commercial lithium-ion batteries. The process of developing mathematical models for lithium ion cells that contain these capacity fade processes not on
15、ly provides a tool for battery design but also provides a means of understanding better how those processes occur.Present 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.
16、 West developed a pseudo two-dimensional model of a single porous insertion electrode accounting for transport 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
17、 be diffusion limited, and hence charge-transfer resistance at the interface between electrolyte and active material 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 elect
18、rode; thus, they do not have the advantages of a full-cell-sandwich model for the treatment of complex, interacting 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 a
19、ccounted for electrode kinetics in their model for discharge of the TiS2, intercalation cathode. The galvanostatic charge and discharge of a lithium metal/solid polymer separator/insertion positive electrode cell was modeled using concentrated-solution theory by Doyle. The model is general enough to
20、 include a wide range of separator materials, lithium salts, and composite insertion electrodes. Concentrated-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 als
21、o provides advantages over dilute solution theory to account for volume changes. Butler-Volmer-type kinetic 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
22、a constant lithium diffusion coefficient in the active material. The volume changes in the system and film 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
23、 reactions (side reactions) or loss of interfacial contact is not predictable using this model.Fuller developed a general model for lithium ion insertion cells that can be applied to any pair of lithium- ion insertion electrodes and any binary electrolyte system given the requisite physical property
24、 data. Fuller work demonstrated the importance of knowing the dependence of the open-circuit 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
25、 potential functions leading to more uniform current distributions and hence better utilization of active material. Optimization studies were carried out for the Bellcore plastic lithium-ion system. The model was also used to predict the effects of relaxation time on multiple charge-discharge cycles
26、 and on peak power.Doyle modified the dual lithium-ion model to include film resistances on both electrodes and made direct comparisons with experimental cell data for the LiC6-LiPF6, ethylene carbonate/dimethyl carbonate (EC/ DMC), Kynar FLEX-ILiyMn2O4 system. Comparisons between data and the numer
27、ical simulations suggested that there is additional resistance present in the system not 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
28、 or current-collector interfaces. One emphasis of this work was in the use of the battery 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 cau
29、se either irreversible side reactions or melting of metallic lithium, Chen and Evans carried 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 o
30、f heat transport and the existence of highly localized heat sources due to battery abuse 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
31、high temperature. Pals and Newman developed a model to predict the thermal behavior of lithium 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 emphas
32、ized the importance of considerations of heat removal and thermal control in lithium polymer battery systems.Verbrugge and Koch developed a mathematical model for lithium intercalation processes associated with a cylindrical carbon microfiber. They characterized and modeled the lithium intercalation
33、 process in single-fiber carbon microelectrodes including 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 fo
34、r a Li/TiS2 cell using redox additives has been theoretically analyzed in terms of a finite linear diffusion model by Narayanan .Darling and Newman modeled a porous intercalation cathode with two characteristic particle sizes.They reported that electrodes with a particle size distribution show modes
35、tly inferior capacity-rate behavior and relaxation on open circuit is substantially faster when the particles are uniformly sized. Nagarajan modeled the effect of particle size distribution on the intercalation electrode behavior during discharge based on packing theory. They observed that during pu
36、lse discharge, an electrode consisting of a binary mixture displays higher discharge capacity than an electrode consisting of single sized 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
37、 same-sized particles. Recently Darling and Newman 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 mak
38、e several interesting conclusions about self-discharge processes in these cells and their impact on positive electrode state-of-charge。 A number of models having varying degrees of sophistication have been developed for lithium rechargeable batteries. For the most part, these models consider the ide
39、al 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 are less common because these processes are not as well understood. Also, models of failure modes in batterie
40、s do not usually have general applicability to a wide range of systems. However, the importance of these phenomena in the safe and efficient operation of high-energy lithium-ion batteries requires that they be in corporated into future battery models。Capacily Fading PhenomenonSide reactions and degr
41、adation processes in lithium-ion batteries may cause a number of undesirable effects leading to capacity loss. Johnson and White have shown that the capacities of commercial lithium-ion cells fade by ca.10-40% during the first 450 cycles.A flow chart describing many of the processes leading to capac
42、ity fade is shown in Fig. 2. In Fig. 3, the capacity fade processes are shown on half-cell discharge curves. This gives a clearer picture of the processes by demonstrating where each is expected to manifest itself during operation of the battery Below, we discuss each of these processes in some deta
43、il, after first discussing the general topic of capacity balance.Capacity Balancing in Lithium-Ion CellsLithium-ion cells operate by cycling lithium ions between two insertion electrode hosts having different insertion energies.For optimum performance, the ratio of the lithium-ion capacities of the
44、two host materials should be balanced. Capacity balancing refers to the optimization of the mass loading in the two electrodes to achieve the maximum capacity (or energy) from the battery under conditions of steady cycling. Due to the practical importance of this subject for maximizing cell performa
45、nce, as well as the safety implications with poorly balanced cells, this subject has been discussed in the literature by several authors. The condition for balanced capacities in a lithium-ion cell can be written in terms of a ratio of active masses in the electrodes. Written as a ratio of positive
46、to negative electrode masses, this expression isThis equation says that the desired mass ratio depends on the relative coulombic capacities of the two electrodes (C is in units of mAh/g) and the amount of cyclable lithium in each. The cyclable lithium is quantified in terms of the range of lithium s
47、toichiometry in the insertion electrode that can be cycled reversibly, with the notation that x refers to the range of negative electrode stoichiometry and y to the positive electrode. For some insertion materials, which have several plateaus over which lithium can be inserted and deinserted, one ma
48、y choose to cycle over only a limited range of stoichiometry for reversibility or safety reasons. In these cases, the stoichiometric range entered in the above formula would be reduced from its maximum value. For example, consider the case of a lithium-ion cell having a petroleum coke negative elect
49、rode and a lithium manganese oxide spinel positive electrode. By choice, we can assign useful ranges of stoichiometries for the two electrode materials of 0.61 for the coke and 0.83 for the lithium manganese oxide. These stoichiometric rangescorrespond to the following electrochemical processes The active mass ratio needed to cycle these two materials in the manne
