18
Y. Kameda et al. / Journal of Molecular Liquids 217 (2016) 17–22
In the present paper, we describe the results of neutron diffraction
experiments on concentrated 9.6 mol% LiPF solutions in DMC. Detailed
The first-order difference function [21–23], ΔLi(Q), was obtained as
the numerical difference in normalized scattering cross sections for
6
+
6
7
structural properties of the first solvation shell of Li was derived from
the least squares fitting analysis of observed difference function obtain-
ed from the difference between scattering cross sections for the solu-
sample solutions with different Li/ Li ratios. Difference function scaled
⁎
at the stoichiometric unit, ( LiPF
6 x 6
) (DMC-d )1 − x, is expressed as the
weighted sum of six partial structure factors relating Li atom.
6
7
tions with different Li/ Li compositions. DMC solvation number
+
4
around the Li in LiClO –DMC solution was also evaluated by Raman
ꢀ
ꢁ
ꢀ
ꢁ
spectra to ensure neutron structure analysis. Subsequently, DFT calcula-
tions were performed to yield further insight into the Li+ solvation by
DMC.
ΔLiðQÞ ¼ ðdσ=dΩÞ for Li sample −ðdσ=dΩÞ fornatLi sample
6
¼
A½aLiOðQÞ–1ꢀ þ B½aLiDðQÞ–1ꢀ þ C½aLiCðQÞ–1ꢀ þ D½aLiPðQÞ–1ꢀ ð1Þ
þE½aLiFðQÞ–1ꢀ þ F½aLiLiðQÞ–1ꢀ;
2
. Materials and methods
where, A = 6x(1 − x)(b6Li–bnatLi)b
O
, B = 12x(1 − x)(b6Li–bnatLi)b
D
, C =
2
2
2
.1. Materials
6x(1 − x)(b6Li–bnatLi)b
C
, D = 2x (b6Li–bnatLi)b
P
, E = 12x (b6Li–bnatLi)b
F
,
and F ¼ x ðb –bnatLi Þ. The distribution function around the Li+,
2
2
2
6
Li
6
6
6
Li-enriched Li
2
CO
3
(95.6% Li, Tomiyama Chemical Co. Ltd.) and
Li
G (r), was obtained by the Fourier transform of observed Δ (Q),
Li
nat
7
natural
Li
2
CO
3
(92.5% Li, natural abundance) were respectively
6
nat
converted to LiCl and LiCl by reacting with HCl in the aqueous solu-
tion. The product solution was dried under vacuum to obtain anhydrous
Q
Z
max
ꢀ
ꢁ−1
−1
2
GLiðrÞ ¼ 1 þ ðA þ B þ C þ D þ E þ FÞ
2π ρr
QΔLiðQÞ sinðQrÞdQ
ð2Þ
6
nat
⁎
⁎
⁎
LiCl ( Li: Li, Li). Enriched LiCl was reacted with AgPF
6
in acetonitrile
0
solution. After filtering precipitated AgCl, the filtrate was evaporated to
dryness under reduced pressure to obtain anhydrous LiPF .
6
¼ ½AgLiOðrÞ þ BgLiDðrÞ þ CgLiCðrÞ þ DgLiPðrÞ þ EgLiFðrÞ þ FgLiLiðrÞꢀ
−1
⁎
ꢁ
ðA þ B þ C þ D þ E þ FÞ
:
⁎
Required amounts of enriched LiPF
6
was dissolved into fully deuter-
ated dimethyl carbonate (DMC-d
LiPF solutions in DMC-d . Density of sample was measured by a vibra-
6 6
tion tube densimeter (Kyoto Electronic, DA-310) at 25 °C. Sample solu-
tions were sealed into cylindrical fused quartz cell with inner diameter
of 12 mm and 1.0 mm in wall thickness. The sample parameters are
listed in Table 1.
6
, 99% D, CIL Inc.) to prepare 9.6 mol%
The upper limit of the integral, Qmax, was set to be 8.41 Å−1.
⁎
Intermolecular parameters concerning the nearest neighbor
+
+
−
Li ⋯DMC and Li ⋯PF
6
interactions were determined through the
least squares fitting for observed ΔLi(Q), employing the model function
model
Li
Δ
[
(Q), involving both the short- and long-range contributions
24–26].
2
.2. Neutron diffraction measurements
ꢂ
ꢃ
2
2
Neutron diffraction measurements were carried out at 25 °C using
ΔLimodelðQÞ ¼ Σ2xnLiαðb6Li–bnatLiÞbα exp −l
Q =2 sinðQr Þ=ðQr
Þ
Liα
Liα
2
Liα
ꢂ
ꢃ
the ISSP diffractometer 4G (GPTAS) installed at the JRR-3M research
reactor operated at 20 MW in the Japan Atomic Energy Agency (JAEA),
Tokai, Japan. The incident neutron wavelength λ = 1.281(3) Å, was
determined by Bragg reflections from KCl powder. Beam collimations
were 40′–80′–80′ going from the reactor to the detector. The aper-
ture of the collimated beam was 20 mm in width and 40 mm in
height. Scattered neutrons were collected over the angular range of
2
ð3Þ
þ4πρðA þ B þ C þ D þ E þ FÞ exp −l0 Q =2
−
3
½
Qr0 cosðQr0Þ– sinðQr0ÞꢀQ
:
+
where, nLiα denotes the coordination number of α atom around Li .
Parameters, lLiα and rLiα, are the root-mean-square displacement and
internuclear distance for Li ⋯α pair, respectively. The long-range
+
−
1
3
.0 ≤ 2θ ≤ 118.0°, corresponding to 0.26 ≤ Q ≤ 8.41 Å (Q = 4πsinθ/λ).
Angular step intervals were chosen to be Δ2θ = 0.5° in the range of
.0 ≤ 2θ ≤ 40.0° and Δ2θ = 1.0° in the range of 41.0 ≤ 2θ ≤ 118.0°, respec-
parameter, r
distribution of atoms around Li can be assumed. The parameter, l
describes the sharpness of the boundary at r . Structural parameters,
and r are respectively determined from the least
squares fit to the observed ΔLi(Q). The fitting procedure was performed
0
, corresponds the distance beyond which a continuous
+
0
,
3
0
6
nat
tively. The preset times were 220 s and 140 s for LiPF
6
and LiPF
6
n
Liα, lLiα, rLiα, l
0
0
solutions, respectively. Scattering measurements were carried out for
the vanadium rod (10 mm in diameter), empty cell, and instrumental
background.
−
1
in the range of 0.26 ≤ Q ≤ 8.41 Å with the SALS program [27], assum-
ing that the statistical uncertainties distribute uniformly. In the present
analysis, interatomic distances within DMC molecule was fixed to those
2
.3. Data reduction
reported from gas phase electron diffraction study [28]. The geometry of
−
PF
6
was assumed to be octahedral, with the average P–F distance of
Observed scattering intensities from the sample solutions were
1.597 Å [29].
corrected for instrumental background, absorption [17], and multiple
scattering [18]. The observed count rate for sample solution was con-
verted to the absolute scale by the use of corrected scattering intensities
from the vanadium rod. Details of data correction and normalization
procedures are given elsewhere [19,20].
2.4. Raman sample preparation and measurement
For Raman spectra measurements, LiClO
treating Li CO with HClO in an aqueous solution, followed by repeated
evaporation to remove HClO and solvent H O. Residual HClO was
checked to be negligible by a pH measurement. Thus obtained LiCO
4
salt was prepared by
2
3
4
4
2
4
4
Table 1
salt was dried at 200 °C in a glass tube vacuum oven during a few
days. DMC was stored with a molecular sieve 3A during several weeks
to reduce water content. Sample solutions were prepared in the glove
box of an Ar gas atmosphere with a water content kept less than
Isotopic composition, average scattering length, bLi, of lithium ion, total cross section, and
⁎
number density of samples in the stoichiometric unit ( LiPF
6
)
0.096(DMC-d
6
)
0.904, σ
t
and
ρ, respectively.
Sample
(6
LiPF
6Li/% 7Li/%
95.6 4.4
b
Li/10−12 cm
0.182
σ
t
/barnsa ρ/Å−3
1
ppm. Finally, water content of thus prepared sample solutions were
6
)
0.096(DMC-d
6
)
0.904
0.904
116.866
60.311
0.007942
checked by the Karl Fischer titration. Raman spectra were measured
nat
(
LiPF
6
)0.096(DMC-d
6
)
7.5 92.5 −0.190
by using a JASCO NR-1100 Raman spectrometer with the optical resolu-
a
−1
For incident neutron wavelength of 1.281 Å.
tion of 2.5 cm
.