Room temperature inorganic "quasi-molten salts" as alkali-metal electrolytes

Room temperature inorganic liquids of high ionic conductivity have been prepared by reacting Lewis acid AlCl₃ with sulfonyl chlorides. The mechanism is not clear at this time since a crystal structure study of the 1:1 complex with CH₃SO₂Cl (T_m = 30°C) is not consistent with a simple chloride transfer to create AlClO₄^- anions. The liquid is in a state somewhere between "ionic" and "molecular". A new term "quasi-molten salt" is adopted to describe this state. A comparably conducting liquid can be made using BCl₃ in place of AlCl₃. Unlike their organic counterparts based on ammonium cations (e.g., pyridinium or imidazolium) which reduce in the presence of alkali metals, this inorganic class of cation shows great stability against electrochemical reduction (ca. -1.0 V vs. Li⁺/Li), with the useful consequence that reversible lithium and sodium metal deposition/stripping can be supported. The electrochemical "window" for these quasi-salts with AlCl₃ ranges up to 5.0 V, and their room temperature conductivities exceed 10^-4 S/cm. They dissolve lithium and sodium tetrachloroaluminates up to mole fraction ∼0.6 at 100°C and intermediate compositions are permanently stable at ambient. The resultant lithium or sodium salt solutions exhibit electrochemical windows of 4.5 ∼ 5.0 V vs. Li⁺/Li or Na⁺/Na and show room temperature conductivities of 10^-3.0 ∼ 10^-2.5 S/cm. In preliminary charge/discharge tests, the cell Li/"quasi-ionic liquid electrolyte"/Li_1+xMn₂O₄ showed a discharge capacity of ca. 110 mAh/(g of cathode) and sustained 80% of the initial capacity after 60 cycles, indicating that these quasi-molten salt-based electrolytes are promising candidates for alkali-metal batteries.