Electrolyte Components

Like any electrochemical device, a lithium battery uses two electrodes (anode and cathode) and an electrolyte it is thus obvious that the choice of electrolyte components is dictated by the electrode materials in use. In other words the chemistry of the two electrode-electrolyte interfaces involved in the battery ultimately determines the optimum electrolyte. In principle, however, one may choose to define an ideal electrolyte (which is usually only a wish list ) that would have the following properties (1) a large window of phase stability, i.e., no vaporization or crystallization, (2) non-flammability, (3) a wide electrochemical stability window, (4) non-toxicity, (5) abundant availability, (6) nfui-corrosive to battery components, (7) environmentally friendly, (8) robust against various abuses, such as electrical, mechanical, and thermal ones, and (9) good wetting properties at the electrolyte-electrode interface. 

The vast amount of work published on lithium batteries shows that an ideal electrolyte does not exist. What one hopes to achieve is a workable electrolyte which has enough combination of desirable properties for an acceptable commercial battery. The literature on the electrolytes for lithium batteries is extremely vast, covering perhaps well over a thousand papers and reviews. No attempt is made here to survey all these publications. The approach adopted is to give a brief synopsis of the main points by giving reference and literature entries to some key papers, especially a few critical reviews that survey the huge amount of literature. A most excellent review was published by Xu in 2004 [1] and this chapter draws heavily upon this publication for the earlier work. More recent work, especially on electrolytes involving ionic liquids is drawn from original publications and our own recent review [2]. 

In the electrolytes used in lithium batteries, formulations based on single solvents are very rare. Most batteries employ electrolytes that are based on two or more solvents in which one ore more lithium salts are dissolved. Mixed solvents provide a strategy to meet diverse and often contradictory requirements for battery applications, for example, high fluidity vs. high dielectric constant. Thus, solvents of very different physical and chemical properties are used together to attain various 

For aLiBs or LMBs, the aprotic nonaqueous solvent chosen must be combined with a suitable lithium salt (solute) to obtain an electrolyte appropriate for the anode/ cathode combination chosen for the battery. For an ambient temperature rechargeable battery, an ideal solute should be aimed to meet the following requirements (1) complete dissociation in the solvent at a fair ly high concentration, (2) lithium (Li, Na, Zn, etc.) cation should be able to move with high mobility, (3) anion should be stable against oxidation reaction at the cathode, (4) anion should be inert to the solvent, (5) both anion and cation should remain inert towards all cell components, i.e., separator, (electrode substrate, current collectors the electrode substrate and 

The above criteria limit the choice of solutes for lithium batteries. Owing to a small radius of lithium catimi, simple salts such as halides fail to show minimum 

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The composition, structure, and formation process of the SEI on metallic lithium depend on the nature of the electrolyte. The variety of possible electrolyte components makes this topic very complex it is reviewed by Peled, Golodnitsky, and Penciner in Chapter III, Sec.6 of this handbook. The types and properties of liquid nonaqueous electrolytes, that are commonly used in lithium cells are reviewed by Barthel and Gores in Chapter III, Sec.7. 

Numerous research activities have focused on the improvement of the protective films and the suppression of solvent cointercalation. Beside ethylene carbonate, significant improvements have been achieved with other film-forming electrolyte components such as C02 [156, 169-177], N20 [170, 177], S02 [155, 169, 177-179], S/ [170, 177, 180, 181], ethyl propyl carbonate [182], ethyl methyl carbonate [183, 184], and other asymmetric alkyl methyl carbonates [185], vinylpropylene carbonate [186], ethylene sulfite [187], S,S-dialkyl dithiocarbonates [188], vinylene carbonate [189], and chloroethylene carbonate [190-194] (which evolves C02 during reduction [195]). In many cases the suppression of solvent co-intercalation is due to the fact that the electrolyte components form effective SEI films already at potential which are positive relative to the potentials of solvent co-intercalation. An excess of DMC or DEC in the electrolyte inhibits PC co-intercalation into graphite, too [183]. 

Reactivity of e ol with Electrolyte Components - a Tool for the Selection of Electrolyte Materials... 

According to the Marcus theory [64] for outer-sphere reactions, there is good correlation between the heterogeneous (electrode) and homogeneous (solution) rate constants. This is the theoretical basis for the proposed use of hydrated-electron rate constants (ke) as a criterion for the reactivity of an electrolyte component towards lithium or any electrode at lithium potential. Table 1 shows rate-constant values for selected materials that are relevant to SE1 formation and to lithium batteries. Although many important materials are missing (such as PC, EC, diethyl carbonate (DEC), LiPF6, etc.), much can be learned from a careful study of this table (and its sources). 

Carbon dioxide as additive improves the behavior of (Li02C0CH2)2 films formed above intercalation potentials in EC/DEC-based electrolytes due to increased formation of Li 2 CO 3 [200], It is interesting to note that SO2 reduction occurs at quite high potentials, before the reduction of other electrolyte components films contain inorganic and organic lithium salts [201]. 

When a solid electrolyte component is interfaced with two electronically conducting (e.g. metal) films (electrodes) a solid electrolyte galvanic cell is formed (Fig. 3.3). Cells of this type with YSZ solid electrolyte are used as oxygen sensors.8 The potential difference U R that develops spontaneously between the two electrodes (W and R designate working and reference electrode, respectively) is given by ... 

As shown on Fig. 4.1, the counter and reference electrodes are deposited on the opposite side of the gas-impervious sohd electrolyte component, which is typically 500 pm to 2 mm thick. The electrolyte thickness is not crucial, but it is preferable to keep it low, so that the ohmic drop in it is small during operation, preferably below 100-600 mV. 

A number of authors have suggested various mixing rules, according to which the quantity a could be calculated for a measured electrolyte in a mixture, starting from the known individual parameters of the single electrolytes and the known composition of the solution. However, none of the proposed mixing relationships has found broad application. Thus, the question about the dependence of the mean activity coefficients of the individual electrolytes on the relative contents of the various electrolytic components was solved in a different way. 

If the electrolyte components can react chemically, it often occurs that, in the absence of current flow, they are in chemical equilibrium, while their formation or consumption during the electrode process results in a chemical reaction leading to renewal of equilibrium. Electroactive substances mostly enter the charge transfer reaction when they approach the electrode to a distance roughly equal to that of the outer Helmholtz plane (Section 5.3.1). It is, however, sometimes necessary that they first be adsorbed. Similarly, adsorption of the products of the electrode reaction affects the electrode reaction and often retards it. Sometimes, the electroinactive components of the solution are also adsorbed, leading to a change in the structure of the electrical double layer which makes the approach of the electroactive substances to the electrode easier or more difficult. Electroactive substances can also be formed through surface reactions of the adsorbed substances. Crystallization processes can also play a role in processes connected with the formation of the solid phase, e.g. in the cathodic deposition of metals. 

Carbon, Lithium tetrachloroaluminate, Sulfinyl chloride Kilroy, W. P. et al., J. Electrochem. Soc., 1981, 128, 934-935 In electric battery systems, lithium is inert to the electrolyte components in absence of carbon, but in presence of over 10% of carbon (pre-mixed by grinding with the metal), contact with the electrolyte mixture leads to ignition or explosion. 

Santner H. J., Moller K. C., Ivanco J., Ramsey M. G., Netzer F. P., Yamaguchi S., Besenhard J. O., Winter M., Acrylic acid nitrile, a film-forming electrolyte component for lithium-ion batteries, which belongs to the family of additives containing vinyl groups, J. Power Sources, (2003) 119, 368-372. 

There appears to be a more adequate approach when a local polarization characteristic is obtained as a result of analysis of the processes in the elementary cell and the local section of the electrode. This characteristic depends on the state transformation of the solid reagents and the concentrations of the electrolyte components. It further may be introduced into the equations describing the macrokinetic processes in an electrode, and may be used to model the behaviour of the system as a whole. 

Shown in Figure 1.1 is the oxygen ion conductivity of selected oxides. Among these oxides, only a few materials have been developed as SOFC electrolytes due to numerous requirements of the electrolyte components. These requirements include fast ionic transport, negligible electronic conduction, and thermodynamic stability over a wide range of temperature and oxygen partial pressure. In addition, they must... 

Data reported by van Krevelen et al.(7) on the NH3-CO2-H2O subsystem (including data of Pexton and Badger) and data of Frohlich(63) were analyzed during an early period of our investigation. (11) We plan to revise our correlation with use of Meissner s(64) treatment of ionic activity coefficients, which is better suited than van Krevelen s for application to mixtures of three or more electrolyte components. 

The objective of this chapter is to study some essential practical aspects, which have to be considered. First, as necessary background information, the different alternatives for electrochemical cell operation are discussed in general. Then follows an overview of properties of electrode materials, electrolyte components, and cell separators. Finally, examples of cell constructions are shown. 

Electrolyte solvents decompose reductively on the carbonaceous anode, and the decomposition product forms a protective film. When the surface of the anode is covered, the film prevents further decomposition of the electrolyte components. This film is an ionic conductor but an electronic insulator. 

For the convenience of this discussion, a somewhat arbitrary demarcation was drawn between state-of-the-art (SOA) and novel electrolyte systems, with the former referring to the ones currently used in commercialized lithium ion cells and the latter to the ones improved over the SOA systems but still under development. It should be pointed out that the exact electrolyte compositions in commercialized devices are usually proprietary knowledge, but publications from the affiliated researchers normally disclose sufficient information to reveal the skeletal electrolyte components employed. The distinction made in this review concerning the previously mentioned demarcation is based on such open literature. 

The efforts to improve ion conductivity have revolved around eq 1, that is, aiming at increasing either the salt dissociation degree riij or the ionic mobility (mj). Since these two factors are decided simultaneously by the physicochemical natures of the salt and solvents, different approaches involving either of these electrolyte components have been adopted. 

The cycle life of a rechargeable battery depends on the long-term reversibility of cell chemistries, and the electrochemical stability of the electrolyte plays a crucial role in maintaining this reversibility. In electrochemistry, there have been numerous techniques developed to measure and quantify the electrochemical stability of electrolyte components, and the most frequently used technique is cyclic voltammetry (CV) in its many variations. 

In voltammetric experiments, the oxidative or reductive decompositions of the investigated electrolyte components (solvents or salts) are made to occur on an electrode whose potential is controlled, and the corresponding decomposition current recorded as the function of the potential is used as the criterion for... 

The second approach is an adaptation of the voltammetry technique to the working environment of electrolytes in an operational electrochemical device. Therefore, neat electrolyte solutions are used and the working electrodes are made of active electrode materials that would be used in an actual electrochemical device. The stability limits thus determined should more reliably describe the actual electrochemical behavior of the investigated electrolytes in real life operations, because the possible extension or contraction of the stability window, due to either various passivation processes of the electrode surface by electrolyte components or electrochemical decomposition of these components catalyzed by the electrode surfaces, would have been... 

Passivation is a process where the products from the initial decomposition of electrolyte form a dense, protective film that covers up the pristine surface of the electrode and prevents any sustained decomposition. The electrolyte components that are sacrificed to form such a protective film would have a deter-... 

Because of the similar potentials between fully lithiated graphite and lithium metal, it has been suggested that the chemical nature of the SEIs in both cases should be similar. On the other hand, it has also been realized that for carbonaceous anodes this formation process is not expected to start until the potential of this anode is cathodically polarized (the discharge process in Figure 11) to a certain level, because the intrinsic potentials of such anode materials are much higher than the reduction potential for most of the solvents and salts. Indeed, this potential polarization process causes one of the most fundamental differences between the SEI on lithium metal and that on a carbonaceous anode. For lithium metal, the SEI forms instantaneously upon its contact with electrolytes, and the reduction of electrolyte components should be indiscriminate to all species possible,while, on a carbonaceous anode, the formation of the SEI should be stepwise and preferential reduction of certain electrolyte components is possible. 

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