Anodic stability

Tahle 7. Electrochemical stability ranges or anodic stability limits of several nonaqueous electrolytes... 

Assigned as oxidation voltage, the anodic stability limit is slightly lower. 

Table 7 lists the electrochemical windows or the anodic stability limits of several nonaqueous electrolytes, or their anodic stabilty, the reference electrodes Rref used, the working electrode material Ew, the experimental conditions, and the references. It shows the following features ... 

Comparison of stability limits of low-temperature molten salts and lithium salts with a common anion shows the influence of the solvent, which limits the anodic stability range of solutions based on LiMe or Lilm. The 1,2-dimethyl-3-propylimidazolium methide... 

Substitution of anions by electron-withdrawing substituents, such as F or CF3,increases the anodic stability of anions [78,82,139] Figure7 shows an example. 

The anodic stability of anions on glassy carbon is the following order BR4 < C104 [Pg.476]

Figure 7. Anodic stability limits of lithium ben-zenediolatoborates, Li[B(C6H4, in PC.

Sfeir J. LaCr03-based anodes Stability considerations. J. Power Sources 2003 118 276-285. 

Because of the small ionic radius of lithium ion, most simple salts of lithium fail to meet the minimum solubility requirement in low dielectric media. Examples are halides, LiX (where X = Cl and F), or the oxides Li20. Although solubility in nonaqueous solvents would increase if the anion is replaced by a so-called soft Lewis base such as Br , I , S , or carboxylates (R—C02 ), the improvement is usually realized at the expense of the anodic stability of the salt because these anions are readily oxidized on the charged surfaces of cathode materials at <4.0 V vs Li. 

The anodic stability of the AsFe" anion proved to be high. In proper solvents, such as esters rather than ethers, the electrolyte based on this salt can remain stable up to 4.5 V on various cathode surfaces.The combination of cathodic and anodic stability would have made LiAsFe a very promising candidate salt for both lithium and lithium ion batteries had the toxicity not been a source of concern. Instead, it was never used in any commercialized cells but is still frequently used in laboratory tests even today. ... 

The anodic stability of the Tf anion, as measured on a GC surface, was not found to be particularly high 130 inferior to Bp4 and PFe but better than C104 . Ab initio calculations yielded similar conclusions, and results measured on porous carbon electrodes were consistent with those measured on GC.81... 

Table 4 lists selected electrochemical stability data for various lithium salt anions that are commonly used in lithium-based electrolytes, with the measurement approaches indicated. Although it has been known that the reduction of anions does occur, sometimes at high potentials, the corresponding processes are usually sluggish and a definite potential for such reductions is often hard to determine. The reduction of solvents, occurring simultaneously with that of anions on the electrode, further complicates the interpretation efforts. For this reason, only the anodic stability of salt anions is of interest, while the cathodic limit of the salt in most cases is set by the reduction of its cation (i.e., lithium deposition potential). 

Table 4. Anodic Stability of Electrolyte Solutes Nonactive Electrodes ...

The most noteworthy observation among the entries of Table 4 is that these anodic stability data are widely diversified depending on the conditions under which they were obtained. For example, the anodic stability limits of PFe, AsFe", and SbFe determined in THF solutions are obviously lower than those determined in carbonate solutions, and a possible explanation lies in the fact that THF itself is not an oxidation-resistant solvent therefore, its own decomposition is most likely responsible for these limits. 

It should be pointed out that the above anodic stability order is highly conditional, since the order had been determined by approach 2 above, where the stability limits are defined as the potential at which the decomposition current density reaches an arbitrary value (1.0 mA cm in this case). Any change in this criterion could result in a reversal of the order. 

To exclude the solvent s effect on the anodic stability of salt anions, Koch et al. made a series of salts... 

Figure 8. Determination of anodic stability for various anions in PC solution on GC. (Reproduced with permission from ref 130 (Figure la and b). Copyright 1997 The Electrochemical Society.)...

A more conspicuous example is the anodic stability of linear carbonate (Figure 23). While the anodic limits for both DMC and EMC are above 5.0 V on GC, as observed by similar measurements, the major decomposition current starts before 4.5 V for both solvents when a composite cathode based on spinel LiMn204 is used as working electrode.Apparently, the enormous difference between GC and a composite cathode could not be explained solely by the larger surface area of the latter. It seems more likely that the irreversible oxidative processes were catalyzed by the active mass of the spinel surface, possibly involving spinel lattice disproportionation. Therefore, the stability limits as measured on all of these nonactive and nonporous electrodes can be... 

When the poor anodic stability of DMC or EMC alone on a similar cathode surface is considered, the role of EC in stabilizing the solvent system becomes obvious. A conclusion that could be extracted from these studies is that the existence of EC not only renders the electrolyte system with superior cathodic stability by forming an effective SEI on the carbonaceous anode but also acts as a key component in forming a surface layer on the cathode surface that is of high breakdown potential. It is for its unique abilities at both electrodes that EC has become an indispensable cosolvent for the electrolyte used in lithium ion cells. 

Another issue closely related with the anodic stability of electrolytes is the interaction between electrolyte components and the commonly used cathode substrate A1 in lithium ion cells. 

A1 is thermodynamically unstable, with an oxidation potential at 1.39 V. Its stability in various applications comes from the formation of a native passivation film, which is composed of AI2O3 or oxyhydroxide and hydroxide.This protective layer, with a thickness of 50 nm, not only stabilizes A1 in various nonaqueous electrolytes at high potentials but also renders the A1 surface coating-friendly by enabling excellent adhesion of the electrode materials. It has been reported that with the native film intact A1 could maintain anodic stability up to 5.0 V even in Lilm-based electrolytes. Similar stability has also been observed with A1 pretreated at 480 °C in air, which remains corrosion-free in LiC104/EC/ DME up to 4.2 However, since mechanical... 

It should be mentioned that the commercially available VC usually contained a small percentage (<2%) of prohibitors that stabilized the reactive VC from polymerizing upon storage. These compounds were usually radical scavengers such as 2,6-di-tert-butyl-p-cresol (DBC) or butylated hydroxy toluene (BHT). Recently, reports pointed out that the presence of these prohibitors actually had a negative impact on the anodic stability of VC on various cathode materials, and VC of high purity was of vital importance in obtaining reproducible performance in lithium ion cells. 

Choquette et al. investigated the possibilities of using a series of substituted sulfamides as possible electrolyte solvents (Table 12). These compounds are polar but viscous liquids at ambient temperature, with viscosities and dielectric constants ranging between 3 and 5 mPa s and 30 and 60, respectively, depending on the alkyl substituents on amide nitrogens. The ion conductivities that could be achieved from the neat solutions of Lilm in these sulfamides are similar to that for BEG, that is, in the vicinity of 10 S cm Like BEG, it should be suitable as a polar cosolvent used in a mixed solvent system, though the less-than-satisfactory anodic stability of the sulfamide family might become a drawback that prevents their application as electrolyte solvents, because usually the polar components in an electrolyte system are responsible for the stabilization of the cathode material surface. As measured on a GC electrode, the oxidative decomposition of these compounds occurs around 4.3—4.6 V when 100 fik cm was used as the cutoff criterion, far below that for cyclic carbonate-based solvents. 

Figure 48. Anodic stability as measured on a spinel LL-Mn204 cathode surface for EMS-based electrolytes (a) Lilm (b) LiC104 (c) LiTf. In all cases, 1.0 m lithium salt solutions were used, and slow scan voltammetry was conducted at 0.1 mV s with lithium as counter and reference electrodes and spinel LiJV[n204 as working electrode. (Reproduced with permission from ref 75 (Figure 3). Copyright 1998 The Electrochemical Society.)...

Thermal decomposition temperature determined by TGA. Anodic stability limit determined by cyclic voltammetry. Stainless steel working electrode. Number of fluorine substituents on the aromatic ring. 

In addition to the above thermodynamic consideration, kinetics also play an important role in determining the anodic stability of these salts. For example, some salts whose decomposition products are polymeric moieties were found to passivate the electrode surface effectively." Therefore, although the intrinsic oxidation potentials for these anions were not as high ( 4.0 V), they showed stability up to 4.50 V in subsequent scans. It should be cautioned here, though, as the passivation was only observed on an inert electrode surface, whether similar passivations would occur on an actual cathode surface... 

S.3.2.3. Lithium Borates with Nonaromatic Ligands. The presence of aromatic ligands in Barthel s salts was believed to be responsible for the high melting points and basicity of the borate anions, which in turn translate into moderate or poor solubilities and ion conductivities as well as low anodic stabilities. To avoid use of these bulky aromatic substituents, Xu and Angell synthesized a series of borate anions that are chelated by various alkyl-based bidentate ligands, which serve as electron-withdrawing moieties by the presence of fluorine or carbonyl functionalities. Table 13 lists the... 

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