Ambient temperature molten salt electrolyt

Singh P., Rajeshwar K., Dubow 1. and lob R. (1980a), Photoelectrochemical behavior of n-GaAs electrodes in ambient-temperature molten-salt electrolytes , J. Am. Chem. Soc. 102, 4676-4681. 

KouraN, Matsumoto S, Idemoto Y (1998) Electrodeposition of amorphous Co-Zn alloy from ambient-temperature molten salt electrolytes of EMIC system. J Surf Fin Soc Jpn 49 1215-1220 (in Japanese)... 

Koura, N., Ejiri, H., and Takeishi, K. (1993). Potyaniline secondaiy cells with ambient temperature molten salt electrolytes. ]. Electrochem Soc., 140, pp. 602-605. 

Bonhote, R Dias, A.-R Papageorgiou, N. Kalyanasundaram, K. Gratzel, M. Hydrophobic, highly conductive ambient-temperature molten salts, Inorg. Chem. 1996, 35, 1168-1178. Ohno, H. Functional design of ionic hquids, Bull. Chem. Soc. Jpn. 2006, 79, 1665-1680. Susan, M. A. B. H. Kaneko, T. Noda, A. Watanahe, M. Ion gels prepared by in situ radical polymerization of vinyl monomers in an ionic liquid and their characterization as polymer electrolytes, J. Am. Chem. Soc. 2005, 127, 4976 983. 

Many think the future moves toward solvent free systems Scrosati presents a chapter on polymer electrolytes, most of which are solvent-containing gel-polymers in practical systems, and Nishi discusses gel-polymer battery properties and production. Webber and Blomgren give extensive treatment of ionic hquids (otherwise known as ambient-temperature molten salts) and their use in lithium-ion and other battery systems. 

Polymer and gel electrolyte systems are discussed in Chapters 7 and 8 by Nishi and Scrosati, respectively. Ionic liquids (ambient temperature molten salts) are discussed in Chapter 6. 

The classical example of a soUd organic polymer electrolyte and the first one found is the poly(ethylene oxide) (PEO)/salt system [593]. It has been studied extensively as an ionically conducting material and the PEO/hthium salt complexes are considered as reference polymer electrolytes. However, their ambient temperature ionic conductivity is poor, on the order of 10 S cm, due to the presence of crystalUne domains in the polymer which, by restricting polymer chain motions, inhibit the transport of ions. Consequently, they must be heated above about 80 °C to obtain isotropic molten polymers and a significant increase in ionic conductivity. 

Solid polymer and gel polymer electrolytes could be viewed as the special variation of the solution-type electrolyte. In the former, the solvents are polar macromolecules that dissolve salts, while, in the latter, only a small portion of high polymer is employed as the mechanical matrix, which is either soaked with or swollen by essentially the same liquid electrolytes. One exception exists molten salt (ionic liquid) electrolytes where no solvent is present and the dissociation of opposite ions is solely achieved by the thermal disintegration of the salt lattice (melting). Polymer electrolyte will be reviewed in section 8 ( Novel Electrolyte Systems ), although lithium ion technology based on gel polymer electrolytes has in fact entered the market and accounted for 4% of lithium ion cells manufactured in 2000. On the other hand, ionic liquid electrolytes will be omitted, due to both the limited literature concerning this topic and the fact that the application of ionic liquid electrolytes in lithium ion devices remains dubious. Since most of the ionic liquid systems are still in a supercooled state at ambient temperature, it is unlikely that the metastable liquid state could be maintained in an actual electrochemical device, wherein electrode materials would serve as effective nucleation sites for crystallization. 

In this case, we have an electrolyte identical to that which is present in lithium-polymer batteries, made of poly(ethylene oxide) (or PEO) in the presence of a lithium salt, solid at ambient temperature, and which needs to be heated above ambient temperature in order for the battery to work (T > 65°C for PEO). Thus, the electrolyte, in its molten state, exhibits sufficient ionic conductivity for the lithium ions to pass. This type of electrolyte can be used on its own (without a membrane) because it ensures physical separation of the positive and negative electrodes. This type of polymer electrolyte needs to be differentiated from gelled or plasticized electrolytes, wherein a polymer is mixed with a lithium salt but also with a solvent or a blend of organic solvents, and which function at ambient temperature. In the case of a Li-S battery, dry polymer membranes are often preferred because they present a genuine all solid state at ambient temperature, which helps limit the dissolution of the active material and therefore self-discharge. Similarly, in the molten state (viscous polymer), the diffusion of the species is slowed, and there is the hope of being able to contain the lithium polysulfides near to the positive electrode. In addition, this technology limits the formation of dendrites on the metal lithium... 

Another method for the estimation of the intrinsic volumes of electrolytes, independent of values of the ionic radii, was proposed by Pedersen et al. [53], who employed the molar volume of the molten alkali metal halides, extrapolated to ambient temperatures, as a measure of their intrinsic volumes in aqueous solutions, but the extrapolation is quite long. A variant of this idea is to use the molar volumes of molten hydrated salts, proposed by Marcus [54], where the temperature extrapolation to 25°C is much shorter. It is then necessary to subtract the volume of the water of hydration, which is n times the molar volume of electrostricted water, 15.2 cm mok at 25°C [55], from the extrapolated molar volume of the undercooled molten hydrated salt containing n water molecules per formula unit of the salt. A cogent method, applicable to highly soluble salts, was proposed by Marcus [56]. The volumes considered, applied to aqueous solutions, are intrinsic, so they should be independent of the concentration c and to a certain extent also of the temperature T. The partial molar volume of an electrolyte, V c, T), describes the volume that it actually occupies in the solution and does not include the volume of the water. Therefore, a fairly short extrapolation of the hnear 25°C) from c = 3M to such high concentrations at which all of the solvent is as closely packed as possible (completely electrostricted) is equivalent to considering the electrolyte as an undercooled molten hydrated salt... 

Thermal batteries are primary reserve batteries that employ inorganic salt electrolytes. These electrolytes are relatively nonconductive solids at ambient temperatures. Integral to the thermal battery are pyrotechnic materials scaled to supply sufficient thermal energy to melt the electrolyte. The molten electrolyte is highly conductive, and high currents may then be drawn from the cells. 

Several molten salts (Carlo Erba 99.995%) were used as electrolyte solvent LiF-KF (51-49mol%), LiF-NaF (60-40 mol%), NaF-KF (40-60 mol%), NaF-MgF2 (78-22 mol%), NaF-CaFj (69-31 mol%) and LiF-CaF2 (80-20 mol%). All the solvents were initially dehydrated by heating under vacuum (7 x 10 atm) from ambient temperature up to their melting point for one week. Silicon ions were introduced into the bath in the form of potassium or sodium hexafluorosilicate (K2SiFg or Na2SiFg Alfa Aesar 99.99%) powder. 

A smaller number of binary lithium systems have also been investigated at lower temperatures. This has involved measurements using LiN03-KN03 molten salts at about 150 °C [44] as well as experiments with organic solvent-based electrolytes at ambient temperatures [45, 46]. Data on these are included in Table 14.4. 

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