LiPFs

Anhydrous silver hexafluorophosphate [26042-63-7] AgPF, as well as other silver fluorosalts, is unusual in that it is soluble in ben2ene, toluene, and xylene and forms 1 2 molecular crystalline complexes with these solvents (91). Olefins form complexes with AgPF and this characteristic has been used in the separation of olefins from paraffins (92). AgPF also is used as a catalyst. Lithium hexafluorophosphate [21324-40-3] LiPF, as well as KPF and other PF g salts, is used as electrolytes in lithium anode batteries (qv). 

The electrolyte used is 1 molar LiPF dissolved in a mixture of 30% ethyl carbonate (EC) and 70% diethyl carbonate (DEC) by volume. This electrolyte IS easy to use because it will self-wet the separator and eleetrodes at atmospheric pressure. The electrolyte is kept under an argon atmosphere in the glove-box. The moleeules of electrolyte solvents, like EC and DEC, have in-plane dimensions of about (4 A x 5 A) to (6 A x 7 A). These molecules are normally larger than the openings of the micropores formed in the region 3 carbons (Fig. 2) as described in section 5. 

The addition of 2-me-thylfuran, thiophene, 2-methylthiophene, pyrrole, and 4—methylthiazole to propylene carbonate-LiPF6 or propylene carbonate-THF-LiPF, improved the cycling efficiency [109]. THF-2MeTHF- Li AsF6 with an additional of 2-methylfuran showed the longest cycle life [110, 111]. 

Figure 5. Morphology of deposited lithium on lithium after immersion of lithium in I molL"1 LiPF(, - PC with HF (3 vol. %) for three days five cycles with I mAh cm 2 in 1 molL-1 LiPF6-PC.

State-of-the-art thin film Li" cells comprise carbon-based anodes (non-graphitic or graphite), solid polymer electrolytes (such as those formed by solvent-free membranes, for example, polyethylene oxide, PEO, and a lithium salt like LiPFe or LiCFsSOs), and metal oxide based cathodes, in particular mixed or doped oxides... 

Table 2. Electrochemical characteristics of natural graphite LBG1025 in half cells Electrolyte EC-DMC (1 1) LiPF (1M).

Very similar to the case of LiC104, an SEI formed from LiAsFe-based electrolytes, either on a lithium or carbonaceous anode, mainly consists of alkyl carbonates or Li2COs rather than LiF, as one would expect from the behavior of its close structural brothers LiPFe or LiBF4. This can be attributed to the much less labile As—F bond that is resistive to hydrolysis. 

On the other hand, sulfonate (—SOsLi) became the anion of choice because it is highly resistant to oxidation, thermally stable, nontoxic, and insensitive to ambient moisture as compared with LiPFe or LiBp4. As the simplest member of this category (Rp = CFs), lithium triflate (LiTf) received extensive research as a candidate for lithium/lithium ion cells. Other similar salts studied include perfluoroethyl sulfonate (Rp = C2F5), perfluorobutylsulfonate (Rp = and the oligomeric versions that are based on polyether linkages. 

In this sense, Lilm favors solvents with a low dielectric constant. Electrochemical stability tests were carried out on a GC electrode, and Im was found to be stable against oxidation in EC/DMC up to 2.5 V vs a Ag+/Ag reference, which translates to 5.0 V vs Li, an oxidation limit lower than those for L1BF4 and LiPFe,but still high enough to be practical. The morphology of cycling lithium in Lilm-based electrolytes is apparently superior to that in other salt-based electrolytes. 

Among the numerous salts vying for lithium/ lithium ion batteries, LiPFe was the obvious winner and was eventually commercialized. The success of LiPFe was not achieved by any single outstanding property but, rather, by the combination of a series... 

LiPFe was proposed as an electrolyte solute for lithium-based batteries in the late 1960s, and soon its chemical and thermal instabilities were known. Even at room temperature, an equilibrium exists ... 

On the other hand, the P—E bond is rather labile toward hydrolysis by even trace amounts of moisture in nonaqueous solvents, producing a series of corrosive products (Scheme 5). Thermal gravimetric analysis (TGA) reveals that, in a dry state, LiPFe loses 50% of its weight at >200 °C but that, in nonaqueous solutions, the deterioration occurs at substantially lower temperatures, for example, as low as 70 X. 

In nonaqueous solvents based on mixed alkyl carbonates, LiPFe remains one of the most conducting salts. For example, in EC/DMC (1 1) the conductivity is 10.7 mS cm only fractionally lower than that of... 

LiAsFe. According to the ionics studies on the limiting properties in various solvents, this excellent conductivity results from the combination of its ionic mobility and dissociation constant, although in neither category does LiPFe stand at the most outstanding position " ... 

The reversed order in the above two properties clearly demonstrates the conflicting nature of the requirements and the advantage of the well-balanced properties of LiPFe. 

Electrochemical studies on a GC electrode and various metal oxide-based cathode surfaces confirm that the solution of LiPFe in mixed carbonates can effectively resist oxidation up to 5.1 thus... 

The above merits made LiPFe the salt of choice when lithium ion technology leaped from concept into product. In 1990, it was used by Sony in the first generation lithium ion cell, and since then, its position in the lithium ion industry has remained unchallenged. Like EC as an indispensable solvent component, LiPFe has become the indispensable electrolyte solute for almost all lithium ion devices manufactured in the past decade. 

After more than a decade of exploration, the skeletal components of the electrolyte for the commercialized lithium ion devices have been identified. Within the various brands of lithium ion cells, the exact electrolyte composition differs from manufacturer to manufacturer, and the formulas remain proprietary information however, the overwhelming majority of these are apparently based on two indispensable components EC as the solvent and LiPFe as the solute. In most cases, one or more linear carbonates, selected from DMC, DEC, or EMC, are also used as cosolvents to increase the fluidity and reduce the melting point of the electrolyte, thus forming the popular composition consisting of LiPFe/ EC/linear carbonate (s). 

It must be pointed out that, in the state-of-the-art electrolytes, the actual high-temperature limits for application in cells are usually not set by the upper boundary of the liquid range because other factors might push the limits far lower. For example, the upper boundary of the liquid range is 90 °C for DMC-, 110 °C for EMC-, and 120 °C for DEC-based electrolytes, all of which are far above the high-temperature limit set by the salt LiPFe (70 °C). ... 

Even if LiPFe is replaced by more thermally stable salts, the thermal stability of passivation films on both the anode and the cathode would still keep the high-temperature limits lower than 90 °C, as do the thermal stability of the separator (<90 °C for polypropylene), the chemical stability of the insulating coatings/sealants used in the cell packaging, and the polymeric binder agents used in both cathode and anode composites. 

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