Ionically Conducting Melts

A special class of ionically conducting melts are the oxide-based systems (usually, mixtures of a metal oxide and a nomnetal oxide, e.g., CaO and Si02) with melting points between 1200 and 2500°C. Such melts are often formed in the high-temperature processes of metallurgy. 

The ionic conductivities of most solid crystalline salts and oxides are extremely low (an exception are the solid electrolytes, which are discussed in Section 8.4). The ions are rigidly held in the crystal lattices of these compounds and cannot move under the effect of applied electric fields. When melting, the ionic crystals break down, forming free ions the conductivities rise drastically and discontinuously, in some cases up to values of over 100 S/m (i.e., values higher than those of the most highly conducting electrolyte solutions). 

When crystals with covalent bonds (e.g., AICI3 or TiCy melt, the melt conductivity remains low (e.g., below 0.1 S/m), which implies that the degree of dissociation of the covalent bonds after melting is low. The covalent crystals also differ from the ionic crystals by their much lower melting points. The differences between these two types of crystal are rather pronounced, whereas there are few crystalline solids with intermediate properties. 

A typical special feature of the melts of ionic crystals (ionic liquids) are their high concentrations of free ions, of about 25 M. Because of the short interionic distances, considerable electrostatic forces act between the ions, so that melts exhibit pronounced tendencies for the formation of different ionic aggregates ion pairs, triplets, complex ions, and so on. 

Another special feature of ionic liquids is the lack of a foreign ( inert ) molecular medium, particularly a solvent, between the ions. Hence, they lack ion-molecule and the many types of nonelectrostatic interactions. 

---

The most direct effect of defects on tire properties of a material usually derive from altered ionic conductivity and diffusion properties. So-called superionic conductors materials which have an ionic conductivity comparable to that of molten salts. This h conductivity is due to the presence of defects, which can be introduced thermally or the presence of impurities. Diffusion affects important processes such as corrosion z catalysis. The specific heat capacity is also affected near the melting temperature the h capacity of a defective material is higher than for the equivalent ideal crystal. This refle the fact that the creation of defects is enthalpically unfavourable but is more than comp sated for by the increase in entropy, so leading to an overall decrease in the free energy... 

Lithium Nitride. Lithium nitride [26134-62-3], Li N, is prepared from the strongly exothermic direct reaction of lithium and nitrogen. The reaction proceeds to completion even when the temperature is kept below the melting point of lithium metal. The lithium ion is extremely mobile in the hexagonal lattice resulting in one of the highest known soHd ionic conductivities. Lithium nitride in combination with other compounds is used as a catalyst for the conversion of hexagonal boron nitride to the cubic form. The properties of lithium nitride have been extensively reviewed (66). 

Molten vanadate ashes (melts) can exhibit both semiconducting and ionic conduction and experiments have shown that semiconducting melts are more coiTosive than those exhibiting ionic conduction. Application of this knowledge as a corrosion control technique is not yet feasible, and a more complete discussion will not be attempted in this article. 

Addition of co-solvents can also change the co-miscibility characteristics of ionic liquids. As an example, the hydrophobic [BMIM][PFg] salt can be completely dissolved in an aqueous ethanol mixture containing between 0.5 and 0.9 mole fraction of ethanol, whereas the ionic liquid itself is only partially miscible with pure water or pure ethanol [13]. The mixing of different salts can also result in systems with modified properties (e.g., conductivity, melting point). 

The ionic conductivity of alkali-metal chloroaluminates was also investigated by Weppner and Huggins [37] but also only in the temperature range between room temperature and just above the melting point. At room temperature the ionic conductivity... 

As outlined in chapter 1 the term electrode is used - contrary to the suggestion of W. Nemst - to designate the electronically conducting phase only, the term electrolyte solution covers all types of ionically conducting phases (solutions, melts, solids) being in contact with the former phase. 

During the production of the chapter, a current review of the RFOT theory has appeared in print [V. Lubchenko and P. G. Wolynes, Annu. Rev. Phys. Chem. 58, 235 (2007)]. In addition, microscopic descriptions of the onset of activationless reconfigurations [J. D. Stevenson, J. Schmalian, and P. G Wolynes, Nat. Phys. 2, 268 (2006)] and prefactors for viscosity and ionic conductivity of deeply supercooled melts [V. Lubchenko, J. Chem. Phys. 126, 174503 (2007)] are now available. 

For this reason, other types of electrolytes are used in addition to aqueous solutions (i.e., nonaqueous solutions of salts (Section 8.1), salt melts (Section 8.2), and a variety of solid electrolytes (Section 8.3). More recently, a new type of solid electrolyte is being employed more often (i.e., water-impregnated ionically conducting polymer films more about them in Chapter 26). 

In a number of general properties, such as viscosity and thermal conductivity, melts differ little from solutions. Their surface tensions are two to three times higher than those of aqueous solutions. This leads to poorer wetting of many solids, including important electrode materials such as carbon and graphite, by the ionic liquids. 

Crystals with Frenkel or Schottky defects are reasonably ion-conducting only at rather high temperatures. On the other hand, there exist several crystals (sometimes called soft framework crystals ), which show surprisingly high ionic conductivities even at the room or slightly elevated temperatures. This effect was revealed by G. Bruni in 1913 two well known examples are Agl and Cul. For instance, the ar-modification of Agl (stable above 146°C, sometimes denoted also as y-modification ) exhibits at this temperature an Ag+ conductivity (t+ = 1) comparable to that of a 0.1m aqueous solution. (The solid-state Ag+ conductivity of a-Agl at the melting point is actually higher than that of the melt.) This unusual behaviour can hardly be explained by the above-discussed defect mechanism. It has been anticipated that the conductivity of ar-Agl and similar crystals is described... 

Every ionic crystal can formally be regarded as a mutually interconnected composite of two distinct structures cationic sublattice and anionic sublattice, which may or may not have identical symmetry. Silver iodide exhibits two structures thermodynamically stable below 146°C sphalerite (below 137°C) and wurtzite (137-146°C), with a plane-centred I- sublattice. This changes into a body-centred one at 146°C, and it persists up to the melting point of Agl (555°C). On the other hand, the Ag+ sub-lattice is much less stable it collapses at the phase transition temperature (146°C) into a highly disordered, liquid-like system, in which the Ag+ ions are easily mobile over all the 42 theoretically available interstitial sites in the I-sub-lattice. This system shows an Ag+ conductivity of 1.31 S/cm at 146°C (the regular wurtzite modification of Agl has an ionic conductivity of about 10-3 S/cm at this temperature). 

In many ways, chloroaluminate molten salts are ideal solvents for the electrodeposition of transition metal-aluminum alloys because they constitute a reservoir of reducible aluminum-containing species, they are excellent solvents for many transition metal ions, and they exhibit good intrinsic ionic conductivity. In fact, the first organic salt-based chloroaluminate melt, a mixture of aluminum chloride and 1-ethylpyridinium bromide (EtPyBr), was formulated as a solvent for electroplating aluminum [55, 56] and subsequently used as a bath to electroform aluminum waveguides [57], Since these early articles, numerous reports have been published that describe the electrodeposition of aluminum from this and related chloroaluminate systems for examples, see Liao et al. [58] and articles cited therein. 

Recently, there has been considerable interest in developing molten salts that are less air and moisture sensitive. Melts such as l-methyl-3-butylimidazolium hexa-fluorophosphate [211], l-ethyl-3-methylimidazolium trifluoromethanesulfonate [212], and l-ethyl-3-methylimidazolium tetrafluoroborate [213] are reported to be hydro-phobic and stable under environmental conditions. In some cases, metal deposition from these electrolytes has been explored [214]. They possess a wide potential window and sufficient ionic conductivity to be considered for many electrochemical applications. Of course if one wishes to take advantage of their potential air stability, one loses the opportunity to work with the alkali and reactive metals. Further, since these ionic liquids are neutral and lack the adjustable Lewis acidity common to the chloroaluminates, the solubility of transition metal salts into these electrolytes may be limited. On a positive note, these electrolytes are significantly different from the chloroaluminates in that the supporting electrolyte is not intended to be electroactive. 

Substitution of Rb for Ag has led to an ordered Rblj array within which Ag ions are disordered at room temperature over an array of face-shared tetrahedral sites although the E = in this compound is a little larger, stabilisation of fast ionic conduction to room temperature with the elimination of a first-order phase change between room temperature and the melting point was a major technical accomplishment (Bradley and Greene, 1966 Owens and Argue, 1967). Unfortunately there are few technical applications other than in electronic components that can use Ag as the working ion. 

It usually takes place close to the melting temperature of the polymer when the pores collapse turning the porous ionically conductive polymer film into a nonporous insulating layer between the electrodes. At this temperature there is a significant increase in cell impedance and passage of current through the cell is restricted. This prevents further electrochemical activity in the cell, thereby shutting the cell down before an explosion can occur. 

Electrical conductivity measurements on silicate melts indicate an essentially ionic conductivity of unipolar type (Bockris et al., 1952a,b Bockris and Mellors, 1956 Waffe and Weill, 1975). Charge transfer is operated by cations, whereas anionic groups are essentially stationary. Transference of electronic charges (conductivity of h- and n-types) is observed only in melts enriched in transition elements, where band conduction and electron hopping phenomena are favored. We may thus state that silicate melts, like other fused salts, are ionic liquids. 

---