8
84
Chemistry Letters Vol.33, No.7 (2004)
Preparation of Lithium Hexafluorophosphate from LiF and P in Fluorine Atmosphere
Jae-Ho Kim, Kazushi Nagahara, Susumu Yonezawa, and Masayuki Takashima
Department of Materials Science and Engineering, Faculty of Engineering, University of Fukui,
3-9-1 Bunkyo, Fukui-shi 910-8507
(Received November 27, 2003; CL-031161)
Pure lithium hexafluorophosphate (LiPF6) has been success-
fully prepared by the reaction between the elemental fluorine and
the equi-molar mixture of LiF and P (F2 direct method). The
product was pure enough to be used as an electrolyte salt of lithi-
um secondary battery. Especially, the stepwise introducing of
fluorine gas into a reaction system was effective to prepare LiPF6
in a high yield. The results of Rietveld refinement of XRD data
ꢀ
revealed that the structure of LiPF6 was trigonal (R3, Z = 3, a0 =
0
1
.4932(2), c0 = 1.2641(5) nm, cell volume; 2.663(2) Â
À28
3
0
m ). The cell constants of LiPF6 prepared by the F2 direct
method were almost the same as those of LiPF6 prepared by the
reaction between LiF and PF5 in a liquid anhydrous hydrogen
fluoride (L-AHF method).
The most popular electrolyte salt used in the Li-ion battery
is lithium hexafluorophosphate (LiPF6) which has good solubil-
ity in various solvents, such as PC (propylene carbonate). A lot
of efforts has been done to obtain highly pure LiPF6 because
even the trace amount of water can critically deteriorate the bat-
tery performance. Generally liquid anhydrous hydrogen fluoride
Figure 1. XRD patterns of the products obtained at various
À3
1
steps, n. n means the number of steps in which (3.9 Â 10 )/n
À5
3
mol F2 was introduced into the reactor (1.57 Â 10 m ) for each
step. The reaction temperature and time were 573 K and 5 min,
respectivery. ((a), n = 1; (b), n = 2; (c), n = 3; (d), n = 4;
(e), the product prepared by L-AHF) and mean the peaks
due to LiPF6 and LiF, respectively.
(
L-AHF) is used as a media for the preparation reaction of LiPF6
2
,3
between LiF and PF5. Acetonitrile can also be used as a media
for the preparation reaction. In both cases, LiPF6 must be puri-
fied by recrystallization in a dry organic media to remove H2O
and HF remained in the products after the preparation reaction.
Lithium oxyfluorophosphate (LiPOxFy) must also be produced
as a by-product which is partially dissolved into HF solution
tion between LiF and P in F gas has not been completed under
2
this condition. Increasing the steps, n, these peaks diminished
and finally no peak corresponding to LiF was detected for n =
4 (Figure 1d). The procedure of the stepwise introduction of flu-
orine gas may be more important to carry out the reaction effi-
ciently and homogeneously than the other factors such as tem-
perature, F2 pressure and reaction time in this case. There
must be a certain equibrium which conducts this phenomenon
6
4
and remains as an impurity in LiPF6. The trace amount of water
5
in AHF has been tried to be removed by using F2 gas. In order to
decrease the amount of HF remained in the product, the process
is proceeded by the way of forming the adduct, Li(CH3CN)4PF6,
with highly dried acetonitrile. It is, however, very difficult to re-
move them sufficiently. In this study, the preparation method of
LiPF6 by the reaction of the mixture of LiF and P(red phospho-
rus) with elemental fluorine was developed (F2 direct method).
Fluorine gas (99.5% pure) was supplied from Daikin Industries,
Ltd. LiF and red phosphorus were commercially provided and
was guaranteed as 99.9% pure. LiPF6 has been successfully pre-
pared by the reaction between the mixture of LiF and P in an
during the reaction. It has been known that LiPF dissociates into
ꢀ
LiF and PF at a temperature higher than 220 C. But here, LiPF
5
6
6
ꢀ
was efficiently been prepared at 300 C. F might prevent LiPF
2
and PF from dissociation and hydrolysis, respectively or play an
5
important role to promote the reaction of LiF + PF = LiPF .
5
6
Rietveld refinement of XRD data for LiPF was shown in
6
Figure 2. The refinement were carried out by referring the liter-
6
–9
ature. R and S value for LiPF prepared by direct method
F
6
ꢀ
(n = 4, at 300 C) were 7.94 and 2.09, respectively. Because
the product is so hygroscopic that some impurities such as LiP-
O F must remained in it, the quality of XRD data was not so
ꢀ
equi-molar ratio and F2 gas at 300 C under 0.4 MPa (F2 pres-
sure) within 5 min. The amounts of LiF, P, and F2 were 1.3 Â
x
y
À3
À3
À3
10
, 1.3 Â 10 , and 3.9 Â 10 mol, respectively in this case.
high to proceed the further refinement. While the refinement
has not been sufficiently finished, several parameters can be
available to be used for discussion. They were summarized in
À5
3
The volume of reactor made of nickel is 1.57 Â 10 m . It was
found that stepwise introducing of F2 into the reactor was effec-
tive to obtain LiPF6 in a high yield. F2 of p=n (n, steps) was in-
troduced for each step where p corresponds to its stoichiometric
amount to complete the reaction.
Table 1. The Rietveld refinement of XRD data of LiPF prepared
6
in liquid AHF (L-AHF method) were carried out in the same
manner as that for LiPF prepared by direct method and the re-
6
Figure 1 shows the results of XRD pattern of the sample pre-
ꢀ
sults were also summarized in Table 1. It may be suggested that
LiPF prepared by L-AHF method has slightly larger values of
co and cell volume. It is, however, reasonable that the lattice pa-
pared at 300 C at various steps, n. The peaks corresponding to
ꢀ
6
ꢀ
LiF appeared at 39 and 45 in Figure 1a. That means the reac-
Copyright Ó 2004 The Chemical Society of Japan
Kim, Jae-Ho
