Lithium ion solvation

Studies of lithium ion solvation of organolithium compounds are important for a thorough understanding of the behavior of these complex reagents. The chiral lithium amide... 

Watanabe, Y Kinoshita, S. I. Wada, S. Hoshino, K. Morimoto, H. Tobishima, S. I., Electrochemical Properties and Lithium Ion Solvation Behavior of Sulfone-Ester Mixed Electrolytes for High-Voltage Rechargeable Lithium Cells. 7. Power Sources 2008, 179, 770-779. 

The aim of this book is to present the insights of experts on emerging experimental techniques and theoretical concepts that are or will be in the vanguard of the field of electrochemistry in ionic liquids. The two volumes of this book provides the reader with a broad and self-contained account of electrochemical techniques available to work in ionic liquids. It also gathers and critically discusses the important properties of protic ionic liquids, deep-eutectic solvents, task-specific ionic liquids, polymeric ion gels, and lithium-ion solvation, relevant to electrochemistry. 

Umebayashi, Y, Mitsugi, T., Fukuda, S., Fujimori, T., Fujii, K., Kanzaki, R., Takeuchi, M. and Ishiguro, S.-L, Lithium ion solvation in room-temperature ionic liquids involving bis(trifluoromethanesulfonyl) imide anion studied by Raman Spectroscopy and DFT calculations, J. Phys. Chem. B 111, 13028-13032 (2007). 

Chapter 9, by Wang and Balbuena, deals with theoretical studies on the solvent structure and association properties, and on the lithium-ion solvation. SEI layer related phenomena are discussed in relation to lithium-ion solvation in commonly used solvents, co-solvents, and solvent additives. 

The structure of the lithium perchlorate ionic pair has been studied by ab initio quantum mechanical methods by Aroca and co-workers [231]. Surprisingly, the theoretically most stable structure is the bidentate one (Csv), although the experimental evidence [232,242] indicates that the real structure is monodentate (Csv). One reasonable explanation for this discrepancy is that in the ab initio study, lithium ion solvation was not considered. However, the theoretical calculations predict that the Vi(Ai) band of the perchlorate anion in bidentate coordination must appear at lower wave numbers than that of free perchlorate, as observed experimentally. 

Solvation numbers of Li" in mixtures of ethylene carbonate (EC) with propylene carbonate, water, or chloroform were investigated by Hyodo and Okabashi [346] using the Raman active ring breathing mode of (EC). Figure 21 shows the lithium ion solvation number against the mole fraction of EC in the studied mixtures. 

Figure 21 Lithium ion solvation number ( ec) against the mole fraction of EC in several cosolvents. Circles propylene carbonate triangles chloroform squares water. (Reprinted from Ref. 346.)... 

Watanabe Y, Kinoshita S-I, Wada S, Hoshino K, Morimoto H, Tobishima S-I (2008) Electrochemical properties and lithium ion solvation behavior of sulfone-ester mixed electrol3des for high-voltage rechargeable lithium cells. J Power Sources 179 770-779. doi 10.1016/j.jpowsour.2008.01.006... 

Lucht BL, Collum DB (1999) Lithium hexamethyldisilazide a view of lithium ion solvation through a glass-bottom boat. Acc Chem Res 32 1035-1042... 

Additives Modifying the State of Solvation of Lithium Ions... 

Whereas the electrochemical decomposition of propylene carbonate (PC) on graphite electrodes at potentials between 1 and 0.8 V vs. Li/Li was already reported in 1970 [140], it took about four years to find out that this reaction is accompanied by a partially reversible electrochemical intercalation of solvated lithium ions, Li (solv)y, into the graphite host [64], In general, the intercalation of Li (and other alkali-metal) ions from electrolytes with organic donor solvents into fairly crystalline graphitic carbons quite often yields solvated (ternary) lithiated graphites, Li r(solv)yC 1 (Fig. 8) [7,24,26,65,66,141-146],... 

It is now well established that in lithium batteries (including lithium-ion batteries) containing either liquid or polymer electrolytes, the anode is always covered by a passivating layer called the SEI. However, the chemical and electrochemical formation reactions and properties of this layer are as yet not well understood. In this section we discuss the electrode surface and SEI characterizations, film formation reactions (chemical and electrochemical), and other phenomena taking place at the lithium or lithium-alloy anode, and at the Li. C6 anode/electrolyte interface in both liquid and polymer-electrolyte batteries. We focus on the lithium anode but the theoretical considerations are common to all alkali-metal anodes. We address also the initial electrochemical formation steps of the SEI, the role of the solvated-electron rate constant in the selection of SEI-building materials (precursors), and the correlation between SEI properties and battery quality and performance. 

These morphological changes (hill and blister formation) were attributed to the intercalation of solvated lithium ion into graphite interlayers and to the accumulation of its decomposition products (some of them gases), respectively. On the other hand, rapid exfoliation and rupturing of graphite layers were observed in ImolL-1... 

In conclusion, it seems that solvents appropriate for lithium-ion batteries employing a graphite anode must have high solvation energy, high E°, and high /0 for reduction in order to slow the cointercalation of the solvated ion, and to enhance the formation of the SEI at the most positive potential (far from the Li/Li+ potential). 

One of the most important factors affecting Qsei [76, 78, 87] is graphite-anode exfoliation, as a result of intercalation of solvated lithium ions. Factors that are reported to decrease (9lR are increasing the EC content in organic carbonates or di-oxolane solutions [98, 991 addition of C02 [31, 87, 99] or crown ethers [8, 71, 78] and increasing the current density [73] (this also lowers <2SE [14] as a result of decrease in (2s P ) ... 

It is worth mentioning that single-ion conductivities of lithium ions and anions at infinite dilution, and transference numbers of ligand-solvated lithium ions estimated therefrom, increase due to the replacement of more than one (generally four) solvent molecules. Table 6 demonstrates this beneficial feature. 

Table 6. Single-ion conductivities of solvated lithium ions and anions at 25 °C in PC at infinite dilution [13]...

Very little work has been done in this area. Even electrolyte transport has not been well characterized for multicomponent electrolyte systems. Multicomponent electrochemical transport theory [36] has not been applied to transport in lithium-ion electrolytes, even though these electrolytes consist of a blend of solvents. It is easy to imagine that ions are preferentially solvated and ion transport causes changes in solvent composition near the electrodes. Still, even the most sophisticated mathematical models [37] model transport as a binary salt. 

If HMPA is included in the solvent, the Z-enolate predominates.236,238 DMPU also favors the Z-enolate. The switch to the Z-enolate with HMPA or DMPU is attributed to a looser, perhaps acyclic TS being favored as the result of strong solvation of the lithium ion. The steric factors favoring the -TS are therefore diminished.239 These general principles of solvent control of enolate stereochemistry are applicable to other systems.240 For example, by changing the conditions for silyl ketene acetal formation, the diastereomeric compounds 17a and 17b can be converted to the same product with high diastereoselectivity.241... 

Organolithium compounds can add to a, (3-unsaturated ketones by either 1,2- or 1,4-addition. The most synthetically important version of the 1,4-addition involves organocopper intermediates, and is discussed in Chap 8. However, 1,4-addition is observed under some conditions even in the absence of copper catalysts. Highly reactive organolithium reagents usually react by 1,2-addition, but the addition of small amounts of HMPA has been found to favor 1,4-addition. This is attributed to solvation of the lithium ion, which attenuates its Lewis acid character toward the carbonyl oxygen.111... 

The ion solvating polymers have found application mainly in power sources (all-solid lithium batteries, see Fig. 2.19), where polymer electrolytes offer various advantages over liquid electrolyte solutions. 

Benkeser and Tincher 128>, on the other hand, reduced acetylenes preferentially to trans olefins using solvated electrons generated at a platinum cathode by electrolytic reduction of lithium chloride in methylamine [lithium metal is formed from lithium ion at the cathode in this electrolysis its dissolution in methylamine generates the solvated electron and regenerates lithium... 

The HRTEM observation of the cross section of a coated fiber showed that the core is constituted of aromatic layers highly misoriented, whereas they are preferentially oriented in parallel for the thin coating pairs of stacked layers form mainly Basic Structural Units (BSUs) in which the average interlayer distance is smaller than between the aromatic layers in the bulk of the fiber. Since the nanotexture is more dense for the pyrolytic carbon than for the fiber itself, it acts as a barrier which prevents the diffusion of the large solvated lithium ions to the core of the fiber, allowing the passivation layer to be less developed after this treatment. Hence, the major amount of lithium inserted is involved in the reversible contribution therefore this composite material is extremely interesting for the in-situ 7Li NMR study of the reversible insertion. 

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