Preparation of Pure LiPF6 using fluorine gas at room temperature

Article abstract of DOI:10.1246/cl.2011.360

Pure lithium hexafluorophosphate (LiPF₆) was successfully prepared at room temperature (23 °C) by introducing fluorine gas into a reactor containing LiF and P at -196 °C. The mass fractions of LiPF ₆ and LiF in products prepared at 23 °C were 1.00 and 0.00, respectively, by means of XRD-Rietveld analysis. Namely, the prepared LiPF ₆ was pure enough to be used as an electrolyte salt in lithium ion batteries.

Full text of DOI:10.1246/cl.2011.360

doi:10.1246/cl.2011.360
3
60
Published on the web March 2, 2011
Preparation of Pure LiPF Using Fluorine Gas at Room Temperature
6
1
1
1,2
1
1
Jae-Ho Kim,* Hayato Umeda, Meguru Ohe, Susumu Yonezawa, and Masayuki Takashima
1
Department of Materials Science and Engineering, Faculty of Engineering, University of Fukui,
-9-1 Bunkyo, Fukui 910-8507
Central Glass Co., Ltd., 5253 Oaza Okibu, Ube, Yamaguchi 755-0001
3
2
(
Received January 5, 2011; CL-110009; E-mail: kim@matse.u-fukui.ac.jp)
Pure lithium hexafluorophosphate (LiPF6) was successfully
Figure 1 shows XRD profiles of products obtained through
reaction between LiF and P in F2 gas under various reaction
prepared at room temperature (23 °C) by introducing fluorine
gas into a reactor containing LiF and P at ¹196 °C. The mass
fractions of LiPF6 and LiF in products prepared at 23 °C were
.00 and 0.00, respectively, by means of XRD-Rietveld analysis.
Namely, the prepared LiPF6 was pure enough to be used as an
electrolyte salt in lithium ion batteries.
conditions. After excess F gas was introduced into the reactor
2
at ¹196 °C, the reaction temperature was increased stepwise up
to a maximum of 23 °C. At the temperature of ¹196 °C, the
reaction between the solid mixture of LiF and P and liquid F2
never occurred because of the low temperature. Up to ¹80 °C,
the LiPF6 peak ( ) started to appear in the XRD (Figure 1a).
When the reaction temperature increased to ¹20 °C
(Figure 1b), the peak intensity of LiPF6 increased sharply
with the formation of PF5 gas as shown in the IR spectrum
(Figure 1f), while the LiF peaks ( ) decreased. The IR
spectrum (Figure 1f) indicates the gaseous products formed
from the reaction between the mixture of LiF and P and the
1
Lithium hexafluorophosphate (LiPF ) is the most common
6
1,2
electrolyte salt used in commercial Li-ion cells. Many research
efforts have been to made to obtain highly pure LiPF6 because
even a trace amount of water can deteriorate the performance of
the battery significantly. Generally, liquid anhydrous hydrogen
fluoride (L-AHF) is used as a medium for the preparation
of LiPF6 from LiF and PCl5.3 LiPF6 must be purified by
¹
1
F2 gas at ¹20 °C. The small peak of POF3 at 1416 cm
(Figure 1f) may originate from oxygen adsorbed on the IR cell
recrystallization in a dry organic medium to remove the H O and
wall. Clearly, in the dry gas, PF is the only gas phase product
2
5
HF remaining in the products after preparation. Lithium oxy-
fluorophosphate (LiPOxFy) is also obtained as a by-product
during preparation; it is partially dissolved in the HF solution
and remains as an impurity in LiPF6.4 A previous study
reported a new method for preparing the LiPF6 by reacting LiF
and P with F gas at 300 °C; this method was noted as the F
direct method. The LiPF6 prepared using the F2 direct method
had much higher purity than that prepared using the L-AHF
method. To prevent high exothermicity between P and F2,
however, stepwise introduction of F2 gas into the reactor should
be carried out. Furthermore, to obtain the LiPF6 in high yield,
the reactor must be heated to 300 °C.
detected by FT-IR analysis. All IR data are summarized in
Table 1. Furthermore, the reactor was warmed to room
temperature (23 °C) and allowed to stand for 1 and 10 h. Pure
LiPF6 was successfully prepared at 23 °C after reaction for
10 h. Compared to the typical peak position of LiPF6 by
JCPDS in Figure 1e, only diffraction peaks attributable to
LiPF6 were observed in the XRD pattern of Figure 1d.
Rietveld refinement of XRD data also showed that the mass
5,6
2
2
fractions of LiPF and LiF contained in the product (Figure 1d)
6
were 1.00 and 0.00, respectively as shown in Table 2.
Reaction processes at each step estimated from XRD and IR
spectra (Figure 1), and the mass fractions of mixed compounds
are summarized in Table 2. Concerning the reaction between
LiF and P in excess F2, the PF5 gas is first created by the reaction
between P and F2 at ¹80 °C. The solid­gas reaction between LiF
and PF gas then produces effectively LiPF with increasing
By introducing F gas into the reactor containing a mixture
2
of LiF and P at ¹196 °C, in this paper we tried to prepare pure
LiPF6 at room temperature (23 °C) controlling the exothermicity
between P and F gas.
2
5
6
Fluorine gas (99.5%) was purchased from Daikin Indus-
tries Ltd. LiF and red phosphorus (P) were commercially
provided and guaranteed as 99.9% pure. A mixture (1 g) of LiF
and P with a molar ratio of 1:1 was loaded in the reactor
temperature and reaction time. Particularly, the LiPF6 obtained
at 23 °C after reaction for 10 h was pure enough to be used as an
electrolyte salt in lithium-ion batteries. When LiF was reacted
with preprepared PF5 gas from P and F2 at 23 °C, however,
highly pure LiPF6 was not obtained in this reaction. To acquire
¹
6
3
(3.4 © 10 m ) and maintained at ¹196 to 23 °C under
0.3 MPa (F2 pressure at 23 °C) for 1­10 h. The fluorine gas
LiPF with high purity, it was necessary to increase the reaction
6
was introduced into the reactor at ¹196 °C, the boiling point of
liquid nitrogen (N2). The fluorination apparatus and reactor
were specially designed and made of SUS316L. The product
temperature above 150 °C.
We have reported that LiPF6 can be prepared through
reaction between LiF and P in F2 gas even at a temperature of
¹80 °C. Increasing the temperature to ¹20 °C, the intensity of
the LiPF6 peak increased drastically with the formation of PF5
gas. Pure LiPF6 products could be successfully prepared at room
temperature (23 °C) in 10 h. XRD-Rietveld refinement showed
that the mass fractions of LiPF6 and LiF in products prepared at
23 °C were 1.00 and 0.00, respectively. Namely, the LiPF6
obtained at 23 °C was pure enough to be used as an electrolyte
salt in lithium-ion batteries.
7
has been already confirmed as a mixed compound of LiPF6
and LiF as nonreactive residue using XRD-Rietveld analysis
8
(
Rietan 2000 program ). The mass fraction of each com-
pound was determined using the results for scale factor in the
output file. The crystal structure of LiPF6 was also examined
using XRD-Rietveld analysis. The gaseous products in the
reactor were measured by means of FT-IR analysis
(Nicolet380SPY).
Chem. Lett. 2011, 40, 360­361
© 2011 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

3
61
10
20 30 40 50 60 10 20 30 40 50 60
/degree
/degree
2
θ
2
θ
(c)
(d)
LiPF
LiF
6
10 20 30 40 50 60
/degree
(
(
d)
e)
2
θ
(b)
IR analysis
10
20 30 40 50 60
/degree
2
θ
(a)
(f)
−
196°C, 1h
10
20
30
40
/degree
50
60
1
700 1500 1300 1100-1 900
Wavenumber/cm
2θ
Figure 1. XRD patterns of products obtained at various reaction conditions [(a): ¹80 °C, 1 h; (b): ¹20 °C, 1 h; (c): 23 °C, 1 h;
d): 23 °C, 10 h] and IR spectra (f) of gaseous products obtained at ¹20 °C. And comparison of (d) XRD profile of LiPF6 and (e) the
(
typical peak position of LiPF6 by JCPDS.
Table 1. IR frequency assignments of gaseous products
obtained in the reactor at ¹20 °C as shown in Figure 1f
Consequently, our work presents a new synthesis method of
pure LiPF6 using fluorine gas at room temperature.
Vibrational
Assignment
¹
1
References
frequencies/cm
1
S. S. Zhang, T. R. Jow, K. Amine, G. L. Henriksen, J. Power
Sources 2002, 107, 18; R. Oesten, U. Heider, M. Schmidt,
Solid State Ionics 2002, 148, 391.
1
9
1
018
45
416
PF5 (PF stretching mode)
PF (PF stretching mode)
5
POF3 (P=O stretching mode)
2
3
M. S. Ding, J. Chem. Eng. Data 2003, 48, 519.
W. N. Smith, J. E. Pa, U.S. Patent 3607020, 1971; D. J.
Salmon, D. W. Barnette, U.S. Patent 5378445, 1993.
R. A. Wiesboeck, U.S. Patent 365330, 1972.
J.-H. Kim, K. Nagahara, S. Yonezawa, M. Takashima, Chem.
Lett. 2004, 33, 884.
Table 2. Reaction processes and products obtained at each
reaction conditions
4
5
Mass fractions of
Reaction processes at reaction conditions
productsa
6
7
8
J.-H. Kim, S. Yonezawa, M. Takashima, J. Fluorine Chem.
Conditions Process
LiPF6
LiF
2
006, 127, 1054.
¹
196 °C, 1 h LiF(s) + P(s) + F2(l)
0.00
1.00
M. Takashima, Y. Nosaka, T. Unishi, Eur. J. Solid State
Inorg. Chem. 1992, 29, 691.
F. Izumi, T. Ikeda, Mater. Sci. Forum 2000, 321­324, 198.
LiF(s) + PF5(l or g) +
F2(g) + LiPF6(s)
20 °C, 1 h LiF(s) + PF5(g) + F2(g) + LiPF6(s)
¹
80 °C, 1 h
0.08
0.92
¹
0.52
0.84
1.00
0.48
0.16
0.00
2
2
3 °C, 1 h LiF(s) + PF5(g) + F2(g) + LiPF6(s)
3 °C, 10 h LiPF6(s) + F2(g)
aMass fractions of products were evaluated from XRD-Retveld
results in Figure 1.
Chem. Lett. 2011, 40, 360­361
© 2011 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/