Lithium transference number

To improve the lithium transference number, a typical approach has been the preparation of a polymer/salt hybrid,8 17 in which an ionic group is immobilized in... 

The lithium transference number (l,) of these organoboron polymer electrolytes was evaluated by combination of dc polarization and ac impedance methods, as reported by Evans et al44 (Table 1). The observed t+ at 30°C was 0.50-0.35, indicating that anions were significantly trapped in these systems. Owing to the stronger Lewis acidity of the alkylborane unit, alkylborane-type polymers showed relatively higher t+. 

Table 1 Lithium Transference Number t+ for Organoboron Polymer Electrolytes...

The lithium transference number (t+) of a polymer bearing PEO550 side chain/LiCF3S03 was found to be 0.38 at 30°C. This value implies that anions were effectively trapped by organoboron units, similar to linear organoboron polymer electrolytes. 

The lithium transference number calculated for 7/LiTFSI was 0.47 at 30°C, showing that anions were effectively trapped by methoxyboron unit. 

To immobilize such anions as borate, organoboron polymers were reacted with aryllithium reagents.31,32 The reaction of alkylborane polymers with n-BuLi was examined first however, the ionic conductivity of the resulting material was very low. Moreover, complicated peaks were observed in the H-NMR spectrum. Conversely, selective lithium borate formation was observed in the nB-NMR spectrum when PhLi was employed (scheme 6). An ionic conductivity of 9.45 X 10 7Scm 1 was observed at 50°C. The observed ionic conductivity was relatively low because of the decreased number of carrier ions compared with dissolved salt systems. However, the lithium transference number of this polymer was markedly high (0.82 at 30°C). 

A variety of organoboron polymer electrolytes were successfully prepared by hydroboration polymerization or dehydrocoupling polymerization. Investigations of the ion conductive properties of these polymers are summarized in Table 7. From this systematic study using defined organoboron polymers, it was clearly demonstrated that incorporation of organoboron anion receptors or lithium borate structures are fruitful approaches to improve the lithium transference number of an ion conductive matrix. 

Interestingly, the nonpolyether-type polymer electrolyte 7 showed a relatively high lithium transference number of 0.47 in the presence of LiTFSI. This is possibly due to the absence of strong binding of ether oxygen to the lithium cation. Moreover, anion trapping of the boron atom is not retarded by coordination of oxygen to the... 

Monomer II is also a polymerizable IL composed of quatemized imidazoliimi salt, as shown in Figure 29.1. This monomer is liquid at room temperature and shows a Tg only at —70°C. Its high ionic conductivity of about 10 S cm at room temperature reflects a low Tg. Although the ionic conductivity of this monomer decreased after polymerization as in the case of monomer I, it was considerably improved by the addition of a small amount of LiTFSI. Figure 29.3 shows the effect of LiTFSI concentration on the ionic conductivity and lithium transference number ( Li ) for polymer II. The bulk ionic conductivity of polymer II was 10 S cm at 50°C. When LiTFSI was added to polymer 11, the ionic conductivity increased up to 10 S cm After that, the ionic conductivity of polymer II decreased gradually with the increasing LiTFSI concentration. On the other hand, when the LiTFSI concentration was 100 mol%, the of this system exceeded 0.5. Because of the fixed imidazolium cations on the polymer chain, mobile anion species exist more than cation species in the polymer matrix at this concentration. Since the TFSI anions form the IL domain with the imidazolium cation, the anion can supply a successive ion conduction path for the lithium caiton. Such behavior is not observed in monomeric IL systems, and is understood to be due to the concentrated charge domains created by the polymerization. 

Froemling T, Kunze M, Schoenhoff M, Sundermeyer J, Roling B (2008) Enhanced lithium transference numbers in ionic hquid electrolytes. J Phys Chem B 112 12985-12990... 

While additives meant to improve the SEl certainly constitute a major portion of the additive research for Li-ion batteries, many other additives aim to improve different aspects of the Li-ion batteries, such as the safety characteristics, ionic conductivity of the electrolyte, and high/low temperature performance of the electrolyte. For example, researchers have developed redox shuttle and overcharge shutdown additives to protect the battery from overcharge and the resulting thermal mnaway, flame retardant additives to reduce the flammability of the electrolyte, and anion receptors to enhance the ionic conductivity and increase the lithium transference number. These additives may not be critical to the cell performance, but may be very important and possibly necessary in commercial batteries. 

However, an excess C3H3FO3 of 5% leads to a decreased lithium transference number, large concentration polarization and a reduced discharge capacity, in particular at a high current. 

The idea was to increase the lithium transfer number, which is low in most of ILs, IL-salt mixtures, or IL-acid mixtures. Zwitterions with imidazolium cations covalently bound to anions, such as sulfonate, carbonate, imide, or borate, are solids at room temperature, despite their IL-like structure mixing with bis(trifluoromethylsulfonyl) imide, however, leads to blends that are liquid at room temperature. The organoborate-containing zwitterion-lithium salt mixture (Figure 29.3) displays a high lithium transfer number of 0.69, a high-ionic conductivity of 3.0 x 10 S cm at 50°C, and a low glass transition temperature (-35°C). 

CIOSEK, M., SIEKIERSKI, M. and WIECZOREK, w., 2005. Determination of lithium transference number in PE0-DME-LiC104 modified with alumina powders of various surface acidity. Electrochimica Acta, 50(19), 3922-3927. 

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