Nonaqueous lithium salt electrolyte

It is worth mentioning that the use of microelectrodes (dimensions of micrometers or less) [248] allows the investigation of the electrochemical stability range of solvents without addition of a salt [249]. These studies allow a distinction between chemical and electrochemical reactions at the electrodes. In contrast to the results of Ue et al. [247], THF is not reduced at potentials down to —2 V vs Li/Li+, but oxidizes already at +4 V vs Li/Li+, whereas PC is stable up to 5 V, but already reduces at potentials of less than 1 V vs Li/Li+. Nonetheless, electrochemical stability ranges from CV experiments may be used for screening useful electrolytes. Table 17.9 shows the electrochemical stability limits of several nonaqueous lithium salt electrolytes versus different REFs. Furthermore the used WEs and the experimental conditions, like scan rate v and the onset current density io, with their references are listed. 

Table 17.9 Electrochemical stability ranges of several nonaqueous lithium salt electrolytes.

Pure salts, including ILs, are not generally expected to be good conductors at moderate temperatures because of electrostatic interactions between their ions, which causes a decrease in mobility. Usually the maximiun ionic conductivity is obtained for electrolyte concentrations around 1 M. Angell et al. proved this is not always trae, showing that the neat PlLs of BAN, dimethylammonium nitrate, and ethylammonimn formate have good conductivities, which are less than those for lithimn salts in aqueous solutions but higher than those for nonaqueous lithium salts. 

This section reports on the current state of knowledge on nonaqueous electrolytes for lithium batteries and lithium-ion batteries. The term electrolyte in the current text refers to an ion-conducting solution which consists of a solvent S and a salt, here generally a lithium salt. Often 1 1-salts of the LiX type are preferred for reasons given below only a few l 2-salts Li2X have attained some importance for batteries, and 1 3-salts Li3X are not in use. 

The available choice of lithium salts for electrolyte application is rather limited when compared to the wide spectrum of aprotic organic compounds that could make possible electrolyte solvents. This difference could be more clearly reflected in a comprehensive report summarizing nonaqueous electrolytes developed for rechargeable lithium cells, in which Dahn and co-workers described over 150 electrolyte solvent compositions that were formulated based on 27 basic solvents but only 5 lithium salts. ... 

As a result, the acid strength of the proton is approximately equivalent to that of sulfuric acid in nonaqueous media. In view of the excellent miscibility of this anion with organic nonpolar materials, Armand et al. proposed using its lithium salt (later nicknamed lithium imide , or Lilm) in solid polymer electrolytes, based mainly on oligomeric or macro-molecular ethers. In no time, researchers adopted its use in liquid electrolytes as well, and initial results with the carbonaceous anode materials seemed promising. The commercialization of this new salt by 3M Corporation in the early 1990s sparked considerable hope that it might replace the poorly... 

For lithium electrolytes, the only variable in salt structure is the anion. In a given nonaqueous solvent system, the dissociation of a lithium salt would be... 

Few significant efforts were made on this issue before a new lithium salt (Lilm) was found to cause serious A1 corrosion in nonaqueous electrolytes during the early lOOGs. " Only in recent years has an in-depth understanding of this phenomenon been obtained, a direct result from the increased research interest driven by the expectation that this new salt may replace the thermally unstable... 

This section reviews these research efforts in the past decade on developing new solvents and lithium salts for nonaqueous electrolytes of lithium ion cells, but the cosolvents or additives developed for nonflammable electrolytes, most of which are phosphorus or fluorinated molecules, are not included, since their presence is intended for improvement in safety rather than performance. They will be reviewed in section 8.5. 

On the basis of the findings on LiBOB performance in nonaqueous solvents and other advances made to improve the low-temperature performance of lithium ion electrolytes. Jow and co-workers proposed that an electrolyte with a much wider temperature range could be formulated using LiBOB alone or in combination with other salts. The following section (8.4) will be dedicated to this topic. 

Tetraalkylammonium tosylates [74] and trifluoromethanesulfonates [72] are also excellent electrolytes. Although tetraalkylammonium ions are favored as the cations for supporting electrolytes because of their wide potential range, other cations are sometimes used for special applications—for example, methyltri-phenyl phosphonium, whose tosylate is freely soluble in methylene chloride, and other fairly nonpolar solvents [74] or metal ions (lithium salts tend to have the best solubility in organic solvents) where undesirable reactions of the tetraalkylammonium ion might occur [13,75]. The properties of many electrolytes suitable for nonaqueous use have been surveyed [76]. 

A unique approach in nonaqueous electrochemistry which may be applicable to several fields, especially for batteries, was recently presented by Koch et al. (private communication). They showed that it is possible to use solid matrices based on lithium salts contaminated with organic solvents as electrolyte systems. These systems demonstrate several advantages over liquid systems based on the same solvents and salts as solutions. Their electrochemical windows are larger, especially in the anodic direction (oxidation reactions), and it appears that their reactivity toward active electrodes (e.g., Li, Li—C) is much lower than that of the liquid electrolyte systems. 

The conductivity of nonaqueous electrolyte solution increases with increasing concentration of lithium salt and then decreases through a maximum, as shown in Fig. 2. Such behavior has been explained by the ionic dissociation process of LiA (molecule in solution) as described by the equation. A high dissociation of... 

Similar to other bis(perfluoraIkylsulfonyl)imides, known to be very strong protic acids (currently (C4F9S02)2NH holds the record as the strongest acid in gas phase ), cychc imides form stable salts with variety of counter cations, such as ammonium, potassium, sodium, lithium, etc. Lithium salts of 74-76 were patented as conductive salts for nonaqueous electrolytes of hthium batteries. The DesMarteau group reported synthesis and isolation of stable benzenediazonium salt containing anion of imide 75 and unusual reactions of this material. ... 

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