Lithium transference numbers improving

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... 

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. 

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. 

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. 

Ion receptors are developed to bind to the cations or anions of the lithium salt in the electrolyte to promote ion dissociation and, therefore, increase ionic conductivity of the electrolyte. Although both cation and anion receptors have been shown to improve the ionic conductivity of electrolytes, cationic receptors slow down the mobility of Li" and, hence, reduce the Li ion transference number. In contrast, anion... 

Due to the negative effect on Li ionic transference number, the smdy of cation receptors only lasted a short period of time. Crown ethers, due to their ability to bind Li, were first studied as cation receptors [88, 92, 93]. The addition of crown ethers improved the solubility of lithium salts and ionic conductivity of the resulting electrolyte, especially with less polar solvents. However, these additives have negative effect on cycle life and the high toxicity of crown ethers also discouraged further investigation. 

X 10 S cm". This approach is seemingly espedally useful for battery electrolytes, because the transference number of the lithium ion is increased recently, Weng et al. [519] reported an improved synthesis of tetrafluoro-catechol [105] that is a starting material for the fluorinated boronate ester, 2-(pentafluorophenyl)-tetrafluoro-l,3,2-benzodioxaborole (PFPTFBB). PEPTEBB acts as an anion receptor. In addition, it is an effective redox shuttle for overcharge protection of lithium-ion batteries. Conceptually, this approach is similar to the use of lithium salts with large anions or the immobilization of anions at polymer backbones. 

The easiest way to determine D+ and D is the pfg-NMR method. The method has the advantage that it is not restricted to binary electrolytes even multicomponent systems are realizable. However, it requires expensive instrumentation, is restricted to NMR-visible nuclei, and cannot be compared to electrochemical methods, because it averages ion pairs and isolated ions. In the hterature, many examples of these measurements are found [460-462], also for ILs [411,463,464]. Transference numbers for lithium salts in IL as solvents have already been studied as well as the improvement of the lithium-ion transference number by the choice of a useful anion [411]. 

The matrices of polymers such as poly(vinyl pyrrolidone) (PVP), polysul-fone, poly(trimethylene carbonate) (PTMC), triethylene glycol diacetate-butyl propenoate copolymer [28], and cellulose [29] are different from the mentioned polymers in Sections from 11.1 to 11.5. For example, when porous polysulfone is used as the polymer carrier, the ionic conductivity (3.93 x 10 S/cm at room temperature) and mechanical performance are greatly improved after adding plasticizers. When organic electrolyte is added to PTMC, the uptake ability is greatly improved because its structure is similar to that of the organic electrolyte. Methylcellulose (MC) is prepared easily as a porous polymer membrane, as illustrated in Figure 11.34. It can absorb liquid electrolyte to become a gel polymer electrolyte whose ionic conductivity is 0.2 mS/cm and lithium-ion transference number is 0.29. These results can compare with the commercial separator [29]. 

Single-ion conductors can be obtained by the intercalation of PEO on clay due to the presence of cation charge at the silicate surface. The conductivity values of electrolytes based on POEM with the addition of 2 and 5 wt% clay were found to be around 4 x 10 S/cm at 70 °CF The conductivity obtained can be anisotropic. Molecular dynamic simulation has shown that the Li" ions are solvated preferentially by the silicate oxygen atom rather than PEO. The conductivity is too low for practical applications, even with a cationic transference number equal to one. In order to increase conductivity, but with a cationic transference number different from one, lithium salts were added to PEO/clay nanocomposites. At room temperature, the nanocomposite electrolyte exhibited higher ionic conductivity than unfilled polymer due to the larger content of the PEO amorphous phase. The improvement in conductivity depends on the nature of the clay. Fan et al. have shown that 250-Li-MMT, i.e. Li-MMT heated to 250°C, was more effective in enhancing the conductivity of (PE0)i6LiC104 than Org-MMT, dodecylamine modified Li-MMT, and Li-MMT, since 250-Li-MMT forms an exfoliated structure in the PEO matrix. 

When two different lithium salts are added, the channels for ion transfer and the number of carriers are greatly increased. The introduction of electron-withdrawing groups in the alkyl groups of lithium aluminates increases their ionic conductivities. When lithium aluminates form composites with PEO, the ionic conductivity of PEO is markedly improved. The structures of some lithium aluminates are shown in Figure 10.23. 

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