Alkali-metal chloroaluminates

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... 

The binary mixture of A1C13 and NaCl is the alkali metal chloroaluminate most commonly used as an electrochemical solvent. The preparation, purification, and general properties of this and several related inorganic chloroaluminate systems have... 

The useful potential window of the equimolar AlCl3-NaCl melt extends from about 2.2 to 0 V vs. the A1(III)/A1 couple in NaCl(satd) melt. Plambeck [30] summarizes the physical properties of the equimolar (mp = 151 °C) and 63-37 mol% eutectic melts. Pyrex cells are satisfactory for use with the AlCl3-NaCl melt. Teflon has also been used in these melts, but it slowly decomposes. Experimentation with the alkali metal chloroaluminates requires the use of an inert-atmosphere glove box or a gas-tight cell. 

Sodium and lithium Both sodium [15] and lithium [16] electrodeposition was successful in neutral chloroaluminate ionic liquids that contained protons. These elements are interesting for Na- or Li-based secondary batteries, where the metals would serve directly as the anode material. The electrodeposition is not possible in basic or acidic chloroaluminates, only proton-rich NaQ or LiQ buffered neutral chloroaluminate liquids were feasible. The protons enlarged the electrochemical window towards the cathodic regime so that the alkali metal electrodeposition became possible. For Na the proton source was dissolved HQ that was introduced via the gas phase or via 1-ethyl-3-methylimidazolium hydrogen dichloride. Triethanolamine hydrogen dichloride was employed as the proton source for Li electrodeposition. For both alkali metals, reversible deposition and stripping were reported on tungsten and stainless steel substrates, respectively. 

The acidity of chloroaluminate can be neutralized and buffered by addition of a base such as the chloride anion, introduced as MCI (M being an alkali metal) [Eq. (6)]. Buffered neutral chloroaluminates exhibit a latent acidity . Surprisingly, the chloride anion could be displaced from the AlClf anion, which does not react in... 

Latent acidity of ionic liquid arises when weak bases are added to buffered neutral chloroaluminate ionic liquid. These neutral ionic liquids are formed when excess alkali metal chloride (MCI) is added to an acidic chloroaluminate ionic liquid (Equation... 

The alkali metal chloride (MCI) reacts with the acidic chloroaluminate dimers... 

In chloroaluminate melts, chloride ion donors (e.g., alkali metal chlorides) are bases and substances increasing AICI3 (Al2Cl 7) concentration, in comparison with the initial pure solvent, are acids. The pK values of reaction [9.2.5] in NaAlCl4 melt decrease with the temperature increase from 7.1 (175°C) to 5.0 (400°C). This corresponds to observation that stabihty of chloroaluminates decreases with temperamre. The increase in radius of alkali metal cation results in increase of the pK values, in Lr-Na -K -Cs sequence it grows as follows 3.8-5.0-5.8-7.4. The explanation of such a decrease may be obtained within the framework of HSAB principle. Indeed, both chloride ion and alkali metal cations are referred to as hard bases and the hardness of the latter decreases from Li to Cs. Therefore, the strength of fixation of chloride ion formed on the right side of reaction... 

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. 

In contrast, when small alkali ions such as lithium are introduced in the melt, oxidation state II becomes stable and disproportionation reactions appear giving rise to a colloidal precipitate together with titanium deposition. As for other refractory metals, the stability range of titanium (II) is greatly enlarged in alkali chloroaluminates. This effect is due to the... 

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