X. Lin et al. / Journal of Alloys and Compounds 646 (2015) 727e733
729
compounds and 17 three-phase regions within this ternary system.
3
.2.3. Reaction process of LiCoPO
Due to its high operate potential and high capacity, LiCoPO
4
cathode by solid-state synthesis
has
4
been regarded as one of the best candidates of high-voltage cath-
ode materials for Li-ion batteries. Considering the report relating to
the reaction kinetics of LiCoPO
study of the solid-state synthesis (the most convenience method)
of LiCoPO using Li CO , Co and NH PO as the raw materials
4
synthesis is rare, here, the related
4
2
3
3
O
4
4
H
2
4
have been investigated by ex-situ XRD and TG-DTA. Fig. 5 shows the
XRD pattern of the mixed precursor obtained by ball milling and, as
can be observed, this mixed precursor mainly contains the raw
materials Co
terials Li CO
3
O
4
and NH
4
H
2
PO
4
. It is interesting that the raw ma-
PO compound is
2
3
has completely disappeared and Li
3
4
detected in this mixed precursor. The same phenomenon is also
observed by Churikov et al. [40] when they studied the thermolysis
mechanism of LiFePO
NH PO and FeC ·2H
sults indicate that Li CO can react with part of NH
the blending process and form the Li PO phase, which serve as the
4
solid-phase synthesis using Li
O as the raw materials. Both these re-
PO just after
2 3
CO ,
4
H
2
4
2
O
4
2
2
3
4
H
2
4
Fig. 2. XRD patterns of Co
CoOeP system.
2 4 2 2 7 3 2 8
P O12, Co P O and Co P O compounds obtained in
2
O
5
3
4
raw materials for the future reaction. It is believed that these re-
agents can be mixed evenly after the blending process and then
beneficial to the next step of solid phase reaction.
work. According to the previously literature, Co
belongs to monoclinic system with the space group of P2
P2 /b [29]; Co compound has a monoclinic symmetry with
space group of B2 /c [30] or P2 /a [31] or P2 /c [32] or A2/m [33];
Co (PO compound belongs to monoclinic system with the space
group of C2/c [34]. Especially, the Co (PO and Co com-
pounds obtained here belong to the space group of P2 /n and P2 /c,
respectively. Besides, Olbertz et al. [35] reported that CoP
compound can be obtained from acidic melt of P , H PO
at about 400 C and this ultraphosphate com-
pound has a monoclinic system with the space group of P2 /c. Due
to its special synthesized condition, the CoP 11 compound was
failed to be obtained by solid state method within the temperature
3
(PO
4
)
2
compound
1
/n [28] or
The ex-XRD patterns of the samples sintered at different tem-
perature are shown in Fig. 6. All the samples used the same pre-
cursor and the detailed composition of the precursor is discussed in
Fig. 5. It can be seen from Fig. 6 that the reaction temperature of the
1
2 2 7
P O
1
1
1
2
3 4
)
ꢀ
ꢀ
3
4
)
2
2
P
2
O
7
stable LiCoPO
phase is between 350 C and 700 C. At a low re-
4
ꢀ
1
1
action temperature (<350 C), some decomposition of the precur-
4 11
O
sor and formation of the intermediate phase occurs. Firstly, the
2
O
5
3
4
and
diffraction peaks of NH
appeared at the low sintering temperature (200 C) and the
H
PO
compound have completely dis-
4
2
4
ꢀ
2
CoCO
3
·3Co(OH)
2
ꢀ
1
4 2 2 2 7
(NH ) H P O (PDF # 16e0806) compound is detected, indicating
4
O
the decomposition of the raw materials NH
action temperature increases from 200 C to 325 C, (NH
H
PO
. When the re-
4
2
4
ꢀ
ꢀ
4
)
2
H
2
P
2
O
7
ꢀ
ꢀ
range of 350 Ce700 C, where the Li
2 3 3 4 4 2 4
CO , Co O and NH H PO
compound is decomposed and the H
pound is observed. Secondly, compared with the mixed precursor
3 4
obtained after primary grinding, the diffraction peaks of Co O
4 2 7
P O (PDF # 03e0275) com-
powder reagents were used as the raw materials.
3
3
.2. Li
2
OeCoOeP
2
O
5
ternary system
become weaker and weaker with the increases of the sintering
temperature and then they completely disappear when the sin-
tering temperature increases to 300 C. During the decomposition
ꢀ
.2.1. Ternary compounds
In the Li OeCoOeP
reported previously: LiCoPO
37], LiCo 10 [38] and Li
results, LiCoPO compound forms a monoclinic system with the
space group of Pnma [10] or P2 a [36]; Both the Li CoP [17]
compound and LiCo 10 compound [38] form a monoclinic sys-
tem but belong to the space group of P2 /c and P2 /m, respectively.
LiCo(PO compound [37] forms an orthorhombic system with the
space group of P2 and Li Co (P [39] compound forms a
2
2
O
5
ternary system, five compounds were
[10,36], Li CoP [17], LiCo(PO
Co (P [39]. Base on previously
of Co O compound, two intermediate compounds Co N (ICSD #
3
4
2
4
2
2
O
7
3
)
3
16894) and CoP2 (ICSD # 38316) have been detected. Meanwhile,
ꢀ
[
2
P
3
O
6
5
2
O
7
)
4
LiCoPO phase begins to appear at 300 C and the diffraction peaks
4
ꢀ
ꢀ
4
detected at 2
q
¼ 36.0 and 36.9 are more likely to be that of
1
2
2 7
O
LiCoPO compound, which has been reported to be formed even at
4
ꢀ
2
P
3
O
a low sintering temperature (300 C) using the NH CoPO ·H O and
4 4 2
1
1
LiOH·H O as the raw materials [41]. However, the well-crystalline
2
ꢀ
3
)
3
LiCoPO4 compound can finally be formed at 350 C in this paper
1
2
1
2
1
6
5
2
O
7
)
4
and no other intermediate products can be detected by XRD. All the
diffraction peaks of LiCoPO4 compound can keep no change but
become slight sharper when the sintering temperature increase
triclinic system with the space group of P1. As shown in Fig. 3, the
existence of these five compounds have been confirmed success-
fully by solid-state method and especially, LiCoPO
ꢀ ꢀ
4
compound ob-
from 350 C to 700 C, indicating the subsequent growth of LiCoPO
4
tained here has formed a monoclinic system with the space group
of Pnma. Except these five compounds, no other new lithium
crystal within this temperature range. Later, at the sintering tem-
ꢀ
perature of 750 C, part of the LiCoPO sample decomposed into the
4
cobaltous phosphates can be found within the Li
ternary system under the reduction atmosphere.
2
OeCoOeP
2
O
5
impurity phase of Li P O (ICSD # 59243) and Co P (ICSD # 94379),
4
2
7
2
as shown in Fig. 6.
In order to know more about the reaction process of the LiCoPO
synthesis, TG-DTA curve measured under the blowing gas (95%
Ar þ 5% H ) is presented in Fig. 7. As can be observed, the TG curve
shows two significant weight losses in the temperature ranges of
4
3.2.2. The subsolidus phase relations
The phase relations within the Li
2
OeCoOeP
2
O
5
ternary system
2
under the reduction atmosphere have been systematically inves-
tigated by the means of XRD patterns. Fig. 4 shows the subsolidus
ꢀ
ꢀ
20e270 C and 270e420 C. The first significant weight loss can be
mainly attributed to the decomposition of NH PO and degra-
dation of Co , which has formed several intermediate com-
pounds and release the ammonia gas and water vapour. This
phase relations of the Li
2
OeCoOeP
2
O
5
system according to the
4
H
2
4
results of the powder XRD data of 52 specimens. The final results
have confirmed that there exist 6 binary compounds, 5 ternary
3 4
O