Structure of the ambient temperature alkali metal molten salt AlCl3/LiSCN

Article abstract of DOI:10.1063/1.1344609

Neutron diffraction was used for the investigation of structure of the ambient temperature alkali metal molten salt system AlCl₃/LiSCN. Result showed that Aluminum atom maintained a tetrahedral coordination environment and the SCN^- anion coordinates to the Aluminium through the Nitrogen ends. The bond distances obtained from neutron diffraction matched with the calculation results and showed AlCl₃NCS^- was more stable than its isomer AlCl₃SCN^-.

Full text of DOI:10.1063/1.1344609

Structure of the ambient temperature alkali metal molten salt Al Cl 3 / Li SCN
Yi-Chia Lee, David L. Price, Larry A. Curtiss, Mark A. Ratner, and Duward F. Shriver
Citation: The Journal of Chemical Physics 114, 4591 (2001); doi: 10.1063/1.1344609
View online: http://dx.doi.org/10.1063/1.1344609
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JOURNAL OF CHEMICAL PHYSICS
VOLUME 114, NUMBER 10
8 MARCH 2001
Structure of the ambient temperature alkali metal molten
salt AlCl3 ÕLiSCN
Yi-Chia Lee
Department of Chemistry and Materials Research Center, Northwestern University,
Evanston, Illinois 60208
David L. Pricea)
Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439
Larry A. Curtiss
Chemistry and Materials Science Divisions, Argonne National Laboratory, Argonne, Illinois 60439
Mark A. Ratner and Duward F. Shriver
Department of Chemistry and Materials Research Center, Northwestern University,
Evanston, Illinois 60208
͑Received 11 September 2000; accepted 7 December 2000͒
The structure of the ambient temperature alkali metal molten salt system LiSCN/AlCl3 1:1 adduct
was investigated by neutron diffraction, which demonstrates that the aluminum atom is surrounded
by three chlorine atoms and one nitrogen atom, indicating the existence of the AlCl3NCSϪ anion, in
which the NCSϪ coordinates to the Al center through nitrogen. Molecular orbital calculations using
ab initio methods are also performed to study the optimized structures of the AlCl3NCSϪ and its
isomer, AlCl3SCNϪ. The results are consistent with the neutron diffraction data and indicate that
AlCl3NCSϪ is the major anionic complex in the 1:1 LiSCN/AlCl3 adduct. © 2001 American
Institute of Physics. ͓DOI: 10.1063/1.1344609͔
I. INTRODUCTION
glass transition temperature of Ϫ35 °C and ionic conductiv-
ity around 4ϫ10Ϫ6 S/cm at room temperature.5 Liu et al.6,7
have shown that the molten system 1:1 LiSCN/AlCl3 can be
supercooled to produce a material with a glass transition
temperature of Ϫ15 °C and ionic conductivity around 4
ϫ10Ϫ4 S/cm at room temperature. In previous research we
For the applications of high-energy batteries, molten
salts are potentially useful electrolytes.1–4 In the present
work we explore complex formation between a Lewis acid
and the anion which should delocalize the negative charge of
the anion and thereby decrease the Coulombic attractions
between cations and anions. These salt/Lewis acid adducts
usually result in either ionic liquids or crystalline materials
with low melting points. Salts containing large organic cat-
ions, such as butylpyridinium chloride or 1,3-
dialkylimidazolium chloride, interact with AlCl3 to form
ionically conducting liquids at room temperature.1–3 Organic
ionic liquids usually possess high room temperature ionic
conductivity, however, these organic liquids have relatively
high equivalent weights. Furthermore, the instability of these
organic melts when contacted with the electrode surface im-
pedes their applications for energy storage.
Ambient temperature alkali molten salts generally have
ionic conductivity that is comparable to that of organic ionic
liquids, but with lower equivalent weight and favorable con-
tact with electrodes. Therefore, the development of room
temperature alkali metal molten salt systems has been an
important research goal. In order to obtain ambient tempera-
ture alkali metal molten salts, the choice of alkali metal salts
is crucial, and only a few ambient temperature alkali metal
molten salt systems possess chemical properties that fulfill
the requirements for battery electrolytes.5–9 The 1:1 LiN
͑SO2Cl͒͑SO2F͒/AlCl3 adduct, reported by Xu et al., has a
found that NaN͑CN͒ and AlCl3 also form a 1:1 amorphous
2
adduct with a glass transition temperature at 27 °C and room
temperature ionic conductivity of 6ϫ10Ϫ6 S/cm.8 Zhang
et al.9 prepared a related 1:1 lithium poly͑urea–sulfonyl
imide͒/AlCl3 adduct, which has a glass transition tempera-
ture at Ϫ22 °C and room temperature ionic conductivity of
2ϫ10Ϫ4 S/cm. The formation of molten salts in these sys-
tems may originate from the ambidentate character of the
anions, which offer several configurations when coordinated
to AlCl3 and thereby produce a complicated network that
resists crystallization. Among the ambient temperature alkali
glassy materials mentioned above, the 1:1 LiSCN/AlCl3
complex possesses the advantages of easy preparation and
high ionic conductivity.
In the present research, we use neutron diffraction to
investigate the atomic structure of 1:1 LiSCN/AlCl3 adducts.
Neutron diffraction studies of molten alkali haloaluminates
have been carried out extensively.10,11 These data combined
with vibrational spectroscopic data and molecular orbital cal-
culations provide considerable insight into molten salt sys-
tems. In Sec. II, the neutron diffraction method is described.
The results and discussion are given in Sec. III. Correspond-
ing molecular orbital calculations are presented in Sec. IV.
a͒
Electronic mail: dlp@anl.gov
0021-9606/2001/114(10)/4591/4/$18.00
4591
© 2001 American Institute of Physics
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4592
J. Chem. Phys., Vol. 114, No. 10, 8 March 2001
Lee et al.
II. EXPERIMENTAL SECTION
Anhydrous lithium thiocyanate was prepared by a modi-
fication of a method published in the literature:12 lithium
hydroxide monohydrate was mixed with an equal molar
amount of ammonium thiocyanate and dissolved in water.
After filtration, the mixture was heated under 60 °C at 0.2
torr to remove water and ammonia. The crude product was
dried under high vacuum ͑Ͻ5ϫ10Ϫ5 torr͒ at 150 °C for
three days and then stored and handled in a dry nitrogen-
6
7
filled glove box. LiOH•H2O ͑95 atom%͒, LiOH•H2O ͑97
atom%͒, NH4SCN ͑99.99ϩ%͒, and AlCl3 ͑99.99%͒ were
purchased from Aldrich Chemical Company. The presence
of two Li isotopes was used to separately identify the corre-
lations between lithium, making use of the opposite signs of
the scattering length for 6Li͑ϩ͒ and 7Li͑Ϫ͒. Aluminum chlo-
ride was sublimed three times following the procedure de-
scribed in the literature.13 The AlCl3 and LiSCN in a 1:1
ratio were pulverized, melted at 120 °C under a dry nitrogen
atmosphere, and then loaded into a vanadium container with
dimensions of 9.28 mm i.d., 9.52 mm o.d., and 63 mm in
height. The neutron diffraction measurements were per-
formed at 300 K using the GLAD facility at the Intense
Pulsed Neutron Source at Argonne National Laboratory.
Similar measurements were also performed on an empty va-
nadium container and a vanadium standard was employed
for instrument calibration and data normalization. The data
analysis was carried out following the standard procedures
developed at Argonne National Laboratory for amorphous
materials, including multiple scattering, absorption, and in-
elasticity corrections; an overview is provided in Ref. 14.
The differential cross section per atom d␴/d⍀ can be
deduced from measured intensity I(r) and could be ex-
pressed as
FIG. 1. Experimental average structure factors for 6LiSCN/AlCl3 and
7LiSCN/AlCl3 systems.
where ␳ is the number density. T(r) was obtained using
standard data analysis programs at Argonne, and qualita-
tively discussed in Ref. 14. From this it follows that
T r͒ϭ4␲␳rG r͒.
͑
͑
The G(r) is known as the total pair distribution function and
can be written as
¯ ¯
i, jxixjbi•bj
͚
G r͒ϭ1ϩ
͑
w
g r͒Ϫ1͒, wijϵ
͑
ij
.
͑
͚
ij
2
¯
i, j
b
͗ ͘
The total pair distribution function G(r) is a linear sum
of various particular pairs, multiplied by weighting factors
wij . Tables I and II list these coefficients for each ion pair in
the 1:1 LiSCN/AlCl3 system. To obtain the coordination
number for each ion pair, the peaks were fit with Gaussian
functions and the coordination numbers calculated as
d␴
2
2
¯
¯
ϭ b S Q͒Ϫ1͒ϩ b
,
͑ ͑
͗ ͘
͗
͘
d⍀
x •b2,
͚
2
2
¯
¯
¯
¯
b
b ϭ
x •b ,
b
͘
ϭ
͗ ͘
͗ ͘
͗
͚
i
i
i
i
i
i
¯ ¯ •A ,
C j͒ϭ
͑
i
ij
xi•bibj
where xi represents the concentration of the ith component,
where Ci(j) is the average number of atoms j around an
atom i, and Aij is the peak area of rT(r) for the ij pair.
Raman spectra were obtained with a Biorad FT spec-
trometer on samples sealed in capillary tubes. The amor-
phous samples display weak and broad bands in contrast
with the more distinct bands of crystalline samples. The
2
¯
and bi and bi are its mean and mean-square neutron scatter-
ing amplitudes, respectively. For a multicomponent system,
the average structure factor S(Q) is the weighted sum of
partial structural factors Sij(Q):
¯ ¯
i, j xixjbi•bjSij Q͒
͚
͑
spectral resolution of the interferometer was 4 cmϪ1
.
S Q͒ϭ
͑
.
2
¯
b
͗ ͘
6
Figure 1 shows the S(Q) values for the LiSCN/AlCl3 and
7LiSCN/AlCl3 systems. The intensity differences for the
TABLE I. Weighting factors for ion pairs in the 1:1 6LiSCN/AlCl3 system.
6
7
various features in the S(Q)’s measured for the Li and Li
samples result from the different Li scattering lengths. The
sharp features originate from a small amount of crystallinity,
which is not expected to affect the conclusions presented
below.
Ion pairs Factor wij Ion pairs Factor wij Ion pairs Factor wij
6Li–S
6Li–C
6Li–N
6Li–Al
6Li–Cl
6Li–Li
S–S
0.004
0.009
0.013
0.005
0.041
0.001
0.003
S–C
0.013
0.019
0.007
0.058
0.016
0.044
0.016
C–Cl
N–N
0.136
0.031
0.023
0.191
0.004
0.070
0.294
S–N
S–Al
S–Cl
C–C
C–N
C–Al
N–Al
N–Cl
Al–Al
Al–Cl
Cl–Cl
The average correlation function T(r) is defined as
Q
2
T r͒ϭ4␲␳rϩ
͑
•
maxQ S Q͒Ϫ1 sin Qr͒dQ,
͓ ͑
͔
͑
͵
␲
0
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4593
J. Chem. Phys., Vol. 114, No. 10, 8 March 2001
Structure of the molten salt AlCl3 /LiSCN
TABLE II. Weighting factors for ion pairs in the 1:1 7LiSCN/AlCl3 system.
TABLE III. Neutron diffraction results for the LiSCN/AlCl3 system.
Ion pairs Factor wij Ion pairs Factor wij Ion pairs Factor wij
Bond Bond length
Coord.
numberb Calc. resultsc
a
6Li system
Pair Cij length
͑calc.͒
7Li–S
7Li–C
7Li–N
7Li–Al
7Li–Cl
7Li–Li
S–S
Ϫ0.005
Ϫ0.012
Ϫ0.017
Ϫ0.006
Ϫ0.054
Ϫ0.002
0.003
S–C
0.016
0.022
0.008
0.069
0.019
0.052
0.019
C–Cl
N–N
0.160
0.037
0.027
0.226
0.005
0.083
0.346
S–N
S–Al
S–Cl
C–C
C–N
C–Al
C–N
C–S
Al–N
Al–Cl
S–N
1.2
1.15
1.62
1.86
2.16
2.77
0.35
0.68
1.36
3.13
3.42
3.96
1
1
1
3
1
N–Al
N–Cl
Al–Al
Al–Cl
Cl–Cl
1.64
1.89
2.17
2.77
2.41
Li–Cl
7Li System
C–N
C–S
Al–N
Al–Cl
S–N
1.17
1.69
1.95
2.17
2.83
2.38
1.15
1.62
1.86
2.16
2.77
0.77
0.87
0.58
2.61
0.56
3.3
1
1
1
3
1
III. RESULTS AND DISCUSSION
The total correlation functions T(r) of 6LiSCN/AlCl3
7
and LiSCN/AlCl3 are shown in Fig. 2. To obtain the aver-
Li–Cl
age bond distances and coordination numbers, a series of
Gaussian functions were used to fit T(r) up to 4.0 Å. A
negative peak, assigned to the 7Li–Cl pair, is observed in the
aCalculated bond lengths ͑in Å͒ are obtained based on the optimized struc-
ture of AlCl3NCSϪ anion at the HF/6-31G level.
*
bCoordination number are for the pair i– j the average number of atoms j
around an atom i.
7
T(r) of the LiSCN/AlCl3 sample. Table III lists all the ex-
perimental results for bond distances and coordination num-
bers, as well as the results of the ab initio calculation for the
AlCl3NCSϪ anion discussed below. From Table III, we can
see that bond distances and coordination numbers for each
pair of atoms are in general reasonable agreement with the
ab initio calculation results. A disagreement in the coordina-
cBased on the AlCl3NCSϪ anion.
6
tron absorption of Li nuclei. The peak at 1.9 Å, assigned to
the Al–N bond distance, indicates that the thiocyanate anion
is coordinated to the AlCl3 molecule through the N instead of
the S. Atwood et al.15,16 investigated a series of adducts
made of Al͑CH3͒3 and the thiocyanate ion and they con-
cluded that the Al͑CH3͒3 molecule coordinates to the N end
with an Al–N bond distance around 1.94 Å. The Al–S bond
distances reported in the literature17,18 are longer ͑about 2.32
Å͒. These observations are in accord with the concept of
hard and soft acids and bases ͑HSAB͒.19 The S end of thio-
cyanate anion is a soft base while the N end is hard. When
complexed with Al3ϩ, the hard–hard Al–N bond should be
preferred. The present data show the coordination numbers
CAl͑Cl͒ϳ3 and CAl͑N͒ϳ1, indicating a tetrahedral coordina-
tion environment for each Al atom.20 The tetrahedral coordi-
nation geometry of aluminum in molten haloaluminates has
been observed and reported in the literature.10,11 The CLi͑Cl͒
value of ϳ3.6 indicates the predominance of tetrahedral co-
ordination environment for the lithium cation as well.
6
tion numbers of the C–N and S–N pairs in the Li system
probably arises from complications caused by the high neu-
IV. MOLECULAR ORBITAL CALCULATIONS
Ab initio methods were employed to determine the opti-
mized structures of AlCl3NCSϪ and AlCl3SCNϪ anions. Ini-
tial calculations were performed at the Hartree–Fock level
*
and split-valence plus polarization 6-31G basis sets were
used. Local minima were obtained by full geometrical opti-
mization and both anionic structures have all positive fre-
quencies. All calculations were carried out using the com-
puter program GAUSSIAN 94.21
Two isomers, AlCl3NCSϪ and AlCl3SCNϪ anions, were
studied and geometry optimizations were performed at the
Ϫ
*
HF/6-31G level and are shown in Fig. 3. The AlCl3NCS
FIG. 2. The total correlation functions T(r) for 6LiSCN/AlCl3 and
7LiSCN/AlCl3 systems. The correlation functions have units of ÅϪ2. Each
Gaussian fitted curve corresponds to a certain ion pair in the LiSCN/AlCl3
systems. The dotted line represents the deviation between the experimental
result and the total sum of all the fitted Gaussian curves.
anion, in which N is bonded to the Al atom, has a linear
Al–N–C structure, while the AlCl3SCNϪ anion, in which S
is bonded to the Al atom, has a bent Al–S–C structure with
a bond angle of 101°. Selected bond distances and bond
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4594
J. Chem. Phys., Vol. 114, No. 10, 8 March 2001
Lee et al.
ment with the calculation results on the AlCl3NCSϪ anion.
The AlCl3NCSϪ is more stable than its isomer AlCl3SCNϪ
*
by 16.6 kcal/mol at the HF/6-31G level. The calculated
frequencies of the AlCl3NCSϪ are also consistent with the
optical data.
ACKNOWLEDGMENTS
We appreciate the support of this research by ARO/
ARPA award DAAH-95-1-0524. Partial support from the
Office of Science, U. S. Department of Energy, under Con-
tract W-31-109-ENG-38 is also acknowledged.
FIG. 3. Optimized structures of AlCl3NCSϪ and AlCl3SCNϪ anions at the
*
HF/6-31G level.
angles are also illustrated in Fig. 3. From Table III, we can
see that the neutron diffraction experimental data obtained
from the condensed phases are in general agreement with the
ab initio results calculated in the gas phase. This indicates
the structural perturbation of the anions by the surrounding
1 E. I. Cooper and E. J. M. Sullivan, Proceedings of the 8th International
Symposium on Molten Salts ͑The Electrochemical Society, Pennington,
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Ϫ
4 H. A. Hjuler, R. W. Berg, and N. J. Bjerrum, J. Electrochem. Soc. 136,
901 ͑1989͒.
*
HF/6-31G level, the AlCl3NCS anion is more stable than
AlCl3SCNϪ by 16.6 kcal/mol. At a higher level of theory,
5 K. Xu and C. A. Angell, Mater. Res. Soc. Sym. Proc. 393, 505 ͑1995͒.
6 C. Liu and C. A. Angell, Solid State Ionics 86–88, 467 ͑1996͒.
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*
MP2/6-31G , in which electron correlation is included by
Mo”ller–Plesset perturbation theory to second order, the en-
ergy difference between two anions is 15.5 kcal/mol. These
results suggest that the N-bonded complex is the dominant
species, consistent with the experimental data, which showed
no Al–S coordination peak.
In addition to geometrical optimization of both anions,
we also calculated the vibrational frequencies of both anions
and compared with experimental data. FT Raman spectra
show peaks at 2138 cmϪ1 in the CϵN stretching region and
at 344 cmϪ1 when LiSCN is complexed with AlCl3. The
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12, 1634 ͑2000͒.
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͑2000͒.
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12 D. A. Lee, Inorg. Chem. 3, 289 ͑1964͒.
13 C. A. Thomas, Anhydrous Aluminum Chloride in Organic Chemistry ͑Re-
inhold, New York, 1941͒.
*
theoretical results show that, at the HF/6-31G level scaled
14 F. R. Trouw and D. L. Price, Annu. Rev. Phys. Chem. 50, 571 ͑1999͒.
15 K. S. Seale and J. L. Atwood, J. Organomet. Chem. 64, 57 ͑1974͒.
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135 ͑1979͒.
by a factor of 0.89, the AlCl3NCSϪ anion has Raman active
bands at 2121 cmϪ1 and 338 cmϪ1, in reasonable agreement
with the experimental data.
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L23 ͑1986͒.
V. CONCLUSIONS
18 N. Burford, B. W. Royan, R. E. v. H. Spence, and R. D. Rogers, J. Chem.
Soc. Dalton Trans. 2111 ͑1990͒.
The structures of ambient temperature molten salt sys-
tem LiSCN/AlCl3 were investigated using neutron diffrac-
tion. Molecular orbital calculations based on ab initio meth-
ods were also performed. The neutron diffraction results
show that the Al atom maintains a tetrahedral coordination
environment and the SCNϪ anion coordinates to the Al
through the N end instead of S end. The bond distances
obtained from neutron diffraction are in reasonable agree-
19 Hard and Soft Acids and Bases, edited by R. G. Pearson ͑Dowden, Hutch-
inson & Ross, Stroudsburg, PA, 1973͒.
20 The coordination deduced from the 7Li data is less accurate than from the
6Li data due to the positive/negative cancellation of scattering intensity.
However, the raw data of Table III give a net coordination around Al as
4.49 and 3.19 for 6Li and 7Li, respectively. Only a coordination number of
4 is consistent.
21
GAUSSIAN 94, Revision C3, M. J. Frisch, G. W. Trucks, H. B. Schlegel,
et al., Gaussian, Inc., Pittsburgh, PA, 1995.
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