Raman and FTIR studies of complex formation in aluminum trichloride-alkali thiocyanate ambient-temperature molten salt solutions

Raman and FTIR spectra of xAlCL 3-(1 − x) MSCN ( M = Li, Na, or K) ambient-temperature molten salt solutions were recorded over the composition range, 0.35 < x < 0.75. Many of these molten salts are glass forming and have high cation conductivities suitable for advanced battery electrolytes. The Raman spectral features of the AlCl 3-MSCN pseudo binaries are similar to those of the AlCl 3-MCl binaries. The internal modes associated with the SCN and AlCl 3 groups indicate that large complex anions such as AlCl 3SCN − and Al 2C1 6SCN − are formed in these melts. Also, complexes in which more than one SCN ligand may substitute for Cl are possible. Therefore, [ Al n ( SCN) m Cl 3 n − m + 1 ] −can be considered as a general formula for complex anions, in which m and n may assume certain integer values that depend on the melt composition. These large complex anions result in a large reduction in the melting point or liquidus temperature. The relative ease of glass formation in the AlCl 3-MSCN pseudo binaries may also be related to the presence of such large complex anion species in the liquid state, with the SCN − anions either terminal or bridging bonded. These complex anions allow for systems with large cation conductivities due to a high degree of decoupling of the M + alkali cations from the anion matrix.