Molten salts self-diffusion

Typical values of self-diffusion coefficients and mutual diffusion coefficients in aqueous solutions and in molten salt systems such as (K,Ag)N03 are of the order... 

Some 30 years ago, transport properties of molten salts were reviewed by Janz and Reeves, who described classical experimental techniques for measuring density, electrical conductance, viscosity, transport number, and self-diffusion coefficient. 

While the nuclear magnetic resonance (NMR) technique has widely been used to study diffusion processes of normal liquids, solids, or colloidal systems, there are only a few applications to molten salts. The spin echo self-diffusion method with pulsed field gradients was applied to molten salts by Herdlicka et al. "" There is no need to set up or maintain a concentration gradient. 

Typical values of self-diffusion coefficients and mutual diffusion coefficients in aqueous solutions and in molten salt systems such as (K,Ag)N03 are of the order of 10 m s and the coefficients do not usually vary by more than a factor of 10 over the whole composition range [1, 2, 15]. From measurements in pure ionic liquids we have learned that their self-diffusion coefficients are only of the order of 10 m s From this point of view it is interesting to investigate systems of ordinary and ionic liquids. Figure 4.4-3 shows the results of first measurements in the methanol/[BMIM][PF6] system, which can be seen as a prototype for a system in which an organic and an ionic liquid are mixed. 

The Nernst-Einstein relation shows the dependence between the self-diffusion coefficient Dt and the equivalent conductivity A of molten salts ... 

Several differing simple models of molten salts do indeed give reasonably close calculations of equilibrium properties, e.g., compressibility and surface tension. What these models do not do, however, is to quantitatively rationalize the data on the temperature dependence of conductance, viscous flow, and self-diffusion. The discovery by Nanis and Richards of the fact that simple liquids have heats of activation for all three properties given approximately by 3. lART presents a clear and challenging target for testing models of liquids. 

The determination of transport numbers in aqueous electrolytes is relatively easy (Chapter 3), but in molten salts it poses difficulties of concept, which in turn demand specialized apparatus. Explain why direct determination is difficult. Would it not be better to abandon the direct approach and use the approximate applicability of the Nernst-Einstein equation, relying on self-diffusion determinations Any counter considerations ... 

In liquid salts the isotope factor is in principle measurable by electrotransport (electromigration). Since many such measurements are available (13), it is especially tempting to study thermotransport in molten ionic media. One must bear in mind, however, the possibility of the following complications (a) the mechanisms of electrotransport, thermotransport, and self-diffusion may be non-identical see Reference 16) b) the isotope factors, as determined by electrotransport, are dependent, via transport numbers, on the reference system and (c) severe experimental difficulties may be encountered in liquid salt thermotransport, mainly corrosion and convection effects. 

The Li cation self-diffusion coefficient in a binary EOi2/LiTFSI electrolyte was dominated (90%) by Li" vehicular diffusion of the Li" with an EO12 solvent. In EO12TFSI /Li" molten salt electrolyte, half of the Li" motion was attributed to... 

U Matenaar, J Richter, MD Zeidler. High-temperature-high-pressure NMR probe for self-diffusion measurements in molten salts. J Magn Res A 122 72-75, 1996. 

Table 3.20 The and parameters of the self-diffusion coefficient D/10 s = A exp (—Bo/RT) of molten salts (According to Klemm [265] and Sjoblom [267] or as noted...

Sjoblom CA (1968) Self-diffusion in molten salts. A comparison between diffusion theories and experimental data. Z Natorforsch A 23 933-939... 

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... 

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