Molten salt electrolytes

The molten salt electrolytes project involves both experimental and theoretical studies of molten salt systems possibly suitable for batteries. It started in 1998 at Theoretical Chemistry at Northwestern University. The systems studied include LiI/AlCl₃ and LiSCN/AlCl₃.

LiI/AlCl₃

Two compositions, LiI/AlCl₃ = 1:1 and 1:2, were simulated, each for about 8 ns in decreasing temperature. By clicking you will get pictures animated in time resolution about 100 ps:

• LiI/AlCl₃ = 1:1, T=600K, time=4ns (676kB)
• LiI/AlCl₃ = 1:1, T=500K, time=2ns (342kB)
• LiI/AlCl₃ = 1:1, T=350K, time=2ns (297kB)
• LiI/AlCl₃ = 1:2, T=600K, time=4ns (594kB)
• LiI/AlCl₃ = 1:2, T=500K, time=2ns (354kB)
• LiI/AlCl₃ = 1:2, T=350K, time=2ns (293kB)
• Normal mode vibrations of Al₂Cl₆ The pictures on this page are the final configurations of the 1:1 and 1:2 runs, respectively.

  
  Lithium is white, aluminium magenta (purple), chlorine green, iodine yellow. The Al-Cl and Al-I bonds are shown as bars instead of balls. Atoms in molecules and complex ions (AlCl₃I^-, Al₂Cl₆, ...) are shifted by the appropriate multiple of the box size not to be broken because of the periodic boundary conditions.

  [J. Kolafa professional homepage] [J. Kolafa private homepage]

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  Yi-Chia Lee, Jiri Kolafa, Larry A. Curtiss, Mark A. Ratner, and Duward F. Shriver
  Molten Salt Electrolytes. I. Experimental and Theoretical Studies of LiI/AlCl₃
  J. Chem. Phys. 114, 9998-10009 (2001)

  Molten LiI/AlCl₃ with different molar ratios of LiI to AlCl₃ were prepared and characterized by Raman spectroscopy, thermal analysis, and impedance measurements. The LiI/AlCl₃ adducts melt at 70-80^oC and the Raman spectrum indicates that a variety of haloaluminates exist in the system. The 1:1 adduct has the highest ionic conductivity, 2e-6 S/cm at 25^oC, and the conductivity increases dramatically as the temperature is increased. Molecular dynamics simulations suggest that several haloaluminates are present in the adducts. Ab initio calculations were carried out on the species that were predicted by MD simulations and these results were compared with Raman spectra, and good agreement was obtained. Several ns long MD simulations allowed us to study the conductivity and relaxation processes in the 1:1 and 1:2 melts at higher temperatures.
  

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  Yi-Chia Lee, Jiri Kolafa, and Larry A. Curtiss
  Force field and molecular dynamics of molten AlCl₃/LiI and AlCl₃/LiSCN
  Poster presented at The CCP5 Annual Meeting 2000 - Molecular Simulation in the 21st Century, University of Surrey, Guildford, UK, 2nd-5th July 2000

  Adducts of aluminimum chloride with different alkali metal salts are crystalline materials with low melting points (LiI/AlCl₃) or glasses (LiSCN/AlCl₃). Complexing of anions with AlCl₃ delocalizes the electric charge and thus increases the ionic conductivity. These systems could therefore be useful as electrolytes in high energy density batteries. The proposed force field is based on the exp-6 (Buckingham) potential with the Busing combining rules for the exp-term and the r^-6 attraction for anions. The Li⁺-anion interactions are modified by an r^-12 term. The anions are polarizable and the induced dipole is damped at close cation-anion separations by the shell-core model [1]. The model of the thiocyanate anion (*-S-*--C--*-N-*) uses four auxiliary charges (*) and polarizability tensors at S and N. The parameters for Al³⁺, Cl^-, and I^- are taken from [1], the model of Li⁺ is based on the LiCl and LiI crystals and the properties of the LiCl pair calculated at the MP2/6-31G^* level, and the model of SCN^- is fitted to geometry and vibrational spectra (MP2/6-31G^*) of SCN^- [2] and different conformers of AlCl₂SCN, AlCl₃SCN^-, and LiSCN [3]. The MD calculations are performed using MACSIMUS [4] implementing a novel predictor-corrector scheme for polarizable force fields [5]. Various structural characteristics (correlation functions, coordination numbers, oligomer analysis) as well as diffusion coefficients and conductivity are measured and analyzed.

  [1] Z. Akdeniz, G. Pastore, M. P. Tosi, Phys. Chem. Liq. 32, 191 (1996); Z. Akdeniz, G. Pastore, M. P. Tosi, Nuovo Cimento 20, 595 (1998).
  [2] P. W. Schultz, G. E. Leroi, J. F. Harrison, Molec. Phys. 88, 217 (1996); S. T. Howard, Molec. Phys. 85, 395 (1995).
  [3] T. Veszpremi, T. Pasinszki, M. Feher, J. Am. Chem. Soc. 116, 6303 (1994).
  [4] MACSIMUS.
  [5] J. Kolafa, Molec. Simul. 18, 193 (1996).