Ordered olivine-type lithium-cobalt and lithium-nickel phosphates prepared by a new precursor method

Article abstract of DOI:10.1002/ejic.201000400

Single phases of olivine-type LiCoPO₄ and LiNiPO₄ were synthesized by thermal treatment of homogeneous lithiummetal-phosphate- formate precursors obtained by freeze drying of aqueous solutions of the corresponding metal formates and LiH₂PO₄. The structure, thermal behavior, and morphology of the precursors were studied by IR spectroscopy, DTA, and SEM. Cobalt and nickel phosphate-formate precursors have a composition LiMH_x(PO₄)(HCOO)_x-yH ₂O, where the formate and phosphate groups are mainly deprotonated. For the Co precursor the formate and phosphates ions are randomly coordinated to both Co and Li cations, for the Ni precursor there is a preferential coordination of the formate and phosphate ions around the Ni²⁺ and Li⁺ ion, respectively. Thermal treatment of the precursors yields single phases of olivine-type LiCoPO4 at 450 °C and LiNiPO⁴ at 700 °C. Structural analysis evidences that both LiCoPO⁴ and LiNiPO⁴ have an ordered olivine-type structure without any Li to M disorder between the metal positions and lithiurm deficiency. The effect of the freeze-dried solution concentration and annealing temperature on the structure, crystallite size, and morphology of LiCoPO⁴ and LiNiPO⁴ has been discussed. The morphology of the cobalt and nickel phospho-olivines comprises isometric particles with mean sizes of 190 and 380 nm, respectively.

Full text of DOI:10.1002/ejic.201000400

FULL PAPER
DOI: 10.1002/ejic.201000400
Ordered Olivine-Type Lithium–Cobalt and Lithium–Nickel Phosphates
Prepared by a New Precursor Method
[a]
[a]
[a]
Violeta Koleva,* Ekaterina Zhecheva, and Radostina Stoyanova
Keywords: Lithium / Cobalt / Nickel / Organic-inorganic hybrid composites / X-ray diffraction
2
+
Single phases of olivine-type LiCoPO
4
and LiNiPO
4
were
tion of the formate and phosphate ions around the Ni and
+
synthesized by thermal treatment of homogeneous lithium–
metal–phosphate–formate precursors obtained by freeze dry-
ing of aqueous solutions of the corresponding metal formates
Li ion, respectively. Thermal treatment of the precursors
yields single phases of olivine-type LiCoPO
LiNiPO at 700 °C. Structural analysis evidences that both
and LiNiPO have an ordered olivine-type structure
4
at 450 °C and
4
and LiH
2
PO
4
. The structure, thermal behavior, and mor- LiCoPO
4
4
phology of the precursors were studied by IR spectroscopy,
DTA, and SEM. Cobalt and nickel phosphate–formate pre-
without any Li to M disorder between the metal positions
and lithium deficiency. The effect of the freeze-dried solution
concentration and annealing temperature on the structure,
cursors have
x 4 x 2
a composition LiMH (PO )(HCOO) ·yH O,
where the formate and phosphate groups are mainly depro-
tonated. For the Co precursor the formate and phosphates
ions are randomly coordinated to both Co and Li cations,
whereas for the Ni precursor there is a preferential coordina-
4 4
crystallite size, and morphology of LiCoPO and LiNiPO has
been discussed. The morphology of the cobalt and nickel
phospho-olivines comprises isometric particles with mean
sizes of 190 and 380 nm, respectively.
Introduction
LiCoPO has been prepared by using solid-state reac-
4
tions[
3,6,7]
and soft chemistry routes.
[8–13]
A common feature
Lithium–transition metal orthophosphates with olivine-
type structure (LiMPO , M = Fe, Mn, Co, and Ni) are the
of all synthetic methods is the relatively high temperature
4
where LiCoPO is formed (600–800 °C and prolonged heat-
4
most intensive studied compounds as cathode materials for
lithium-ion batteries in the last years. This is a consequence
of their high cycling stability, tolerance to overcharge, and
[
4,14]
ing). Concerning LiNiPO , solid-state reactions,
gel, and coprecipitation
sol–
4
[15]
[
9]
methods are used, but in all
cases the synthesis temperature exceeds 775 °C.
[
1,2]
safety.
The best electrochemical performance is achieved
Recently, we developed a phosphate–formate precursor
method for the preparation of nano-sized olivine-type Li-
with the iron analogue LiFePO . The electrochemical reac-
4
tion takes place by reversible lithium intercalation in the
olivine structure concomitantly with the oxidation/re-
[
16]
FePO4. The method is based on the formation of homo-
geneous lithium–iron–phosphate–formate precursors by
freeze drying aqueous solutions containing lithium,
iron(II), phosphate, and formate ions. Thermal treatment
of the lithium–iron–phosphate–formate precursors at tem-
2
+
3+
duction of Fe to Fe , the operating voltage being 3.4 V.
By simple replacement of iron by manganese, cobalt, and
nickel, a higher operating voltage and energy density of the
phospho-olivines can be achieved, which is beneficial for
their electric vehicle applications. Cobalt and nickel phos-
peratures above 300 °C yields nanometric LiFePO (particle
4
sizes between 60 and 100 nm) containing up to 2 mass%
carbon. The advantage of this method is related to the abil-
ity to control the morphology of lithium–iron–phosphate–
formate precursors by a simple variation of the concentra-
tion of the freeze-dried solutions. This, on its turn, has an
impact on the carbon content, particles size distribution,
and electrochemical properties of target LiFePO4.[ The
aptitude of the phosphate–formate precursor method to de-
sign simultaneously precursors and the target product has
pho-olivines exhibit higher operating voltages (4.8 and
+
5
kg).
.1 V vs. Li /Li) and energy densities (227.4 and 250.2 Wh/
[3–5]
The main drawback of the phospho-olivines is their
low electronic and ionic conductivity. To overcome this
drawback, specific synthesis methods have to be developed
that aim to minimize the dimensions of the particles and/
or to cover them with conductive substances. In this topic,
a lot of work is carried out on the preparation of LiFePO4
17]
and LiMnPO , whereas the reports on the preparation of
4
also been demonstrated with the preparation of nanocrys-
LiCoPO and LiNiPO are limited.
4
4
[18]
talline manganese phospho-olivine LiMnPO4.
By this
method a new structural modification of LiNiPO (with
[
a] Institute of General and Inorganic Chemistry, Bulgarian
4
[
19]
Academy of Sciences,
Na CrO -type structure) has been prepared.
2
4
Acad. G. Bonchev St, bl. 11, 1113 Sofia, Bulgaria
Fax: +359-2-8705024
E-mail: vkoleva@svr.igic.bas.bg
In this paper we examine the applicability of the phos-
phate–formate precursor method for the preparation of co-
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V. Koleva, E. Zhecheva, R. Stoyanova
FULL PAPER
balt and nickel phospho-olivines, LiCoPO and LiNiPO .
4
4
The precursors and target LiCoPO and LiNiPO composi-
4
4
tions were characterized by XRD, IR spectroscopy, DTA,
and SEM. The mechanism of cobalt and nickel phospho-
olivines formation is discussed on the basis of iron and
manganese analogues.
Results and Discussion
Composition and Structure of the Li–M–Phosphate–
Formate Precursors (M = Co and Ni)
+
2+
Freeze drying of the solution containing Li , Co or
2
+
3–
–
Ni , PO and HCOO in the ratio 1:1:1:2 yields amorph-
4
ous powders with compositions as follows: pink
LiCoH (PO )(HCOO) ·yH O (x ≈ 1 and y ≈ 1.5) and pale
x
4
x
2
green LiNiH (PO )(HCOO) ·yH O (x ≈ 1.2; 2.5 Ͻy Ͻ 2.7).
x
4
x
2
The precursor compositions do not depend on the concen-
tration of freeze-dried solutions. These results evidence for
partial formic acid sublimation during the freeze-drying
process: 1 and 0.8 mol of HCOOH are sublimated in the
case of Co and Ni precursors, respectively. The same results
were established for the iron and manganese phosphate–
[
16,18]
formate precursors,
sublimated.
where only 1 mol of HCOOH is
Figure 1. IR spectra of Co(HCOO)
lithium–cobalt–phosphate–formate precursors.
2 2 2 4 2 2
·2H O, Co(H PO ) ·2H O, and
To examine the form and type of coordination of the
anion groups in the freeze-dried compositions, IR spec-
troscopy was undertaken. Figures 1 and 2 give the IR spec-
tra of Co and Ni freeze-dried precursors. For the sake of
comparison, the IR spectra of M(HCOO) ·2H O and
2
2
M(H PO ) ·2H O (M = Co, Ni) are also presented. The
2
4 2
2
dihydrogen phosphates were synthesized as described else-
[
20]
where.
Three groups of IR bands due to the vibrations of for-
mates, phosphates, and OH groups are resolved in the spec-
tra of the precursors. It is noticeable that the concentration
of the freeze-dried solution does not affect the IR profiles.
The analysis of the IR spectra of both Co and Ni precur-
sors reveals the presence of the characteristic IR vibrations
[
21]
due to deprotonated formate ions:
ν (COO) at 1607–
as
–
1
–1
1
584 cm ; δ(CH) in the range 1399–1393 cm ; ν (COO) in
the range 1378–1355 cm , and δ (OCO) in the range 795–
74 cm . All these characteristic vibrations are clearly seen
s
–
1
s
–
1
7
in the IR spectra of the initial Co and Ni formate salts,
M(HCOO) ·2H O (M = Co, Ni). Close inspection of Fig-
2
2
ure 2 shows that the wavenumbers of δ(CH) and ν (COO)
s
vibrations are the same for the Ni precursor and for pure
Ni(HCOO) ·2H O. Therefore, one can suggest that for the
2
2
Ni precursor the formate ions are predominantly coordi-
2
+
nated to the Ni ions. Contrary to Ni, it appears that for
the Co precursors the formate ions are coordinated simulta-
neously to both Co2 and Li .
+
+
Figure 2. IR spectra of Ni(HCOO)
lithium–nickel–phosphate–formate precursors.
2 2 2 4 2 2
·2H O, Ni(H PO ) ·2H O, and
The vibrations associated with the deprotonated phos-
3
–
phate ion (PO4 ) are also well seen in the IR spectra of the
Co(Ni) precursors (Figures 1 and 2). The two bands in the at 578/530 and 594/514 cm–1 for the Co and Ni precursors,
–
1
1
080–1008 cm range for the Co precursors and the band respectively, are associated with the asymmetric ν bending
4
–
1
3–
[22]
at 1060 cm for the Ni precursors are due to the asymmet- PO4 vibrations.
ric ν stretching vibrations of the PO4 ions. The doublets tions of the ν and ν vibrations for the PO ions in the
It is worth mentioning that the posi-
3
–
3–
3
3
4
4
4092
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Ordered Olivine-Type Lithium–Cobalt and Lithium–Nickel Phosphates
Ni precursors are close to those observed for Li PO at By increasing the concentration of the freeze-dried solu-
3
4
–
1 [23]
1
040 and 592 cm . Hence, it appears that the phosphate tions, there is a slight tendency for increasing the aggregate
ions in the Ni precursors are preferentially coordinated dimensions of the Ni precursors. It is worth noting that
+
around the Li ions.
a flake-like morphology was established for the iron and
In addition, the Ni precursor exhibits three low intensive
–
1
bands at 960, ca. 1230, and 1720 cm (Figure 2). These
bands can be attributed to the protonated forms of the co-
–
2–
ordinated anions – HCOOH and/or H PO or HPO4 . To
2
4
analyze this feature, the characteristic vibrations of proton-
–
2–
ated HCOOH and/or H PO or HPO4 groups are taken
2
4
into account: for HCOOH, the ν(C=O), ν(C–O), and
γ(OH) vibrations are at 1700, 1260, and 975 cm ,
–1 [24]
–
whereas for the protonated phosphate ions (H PO or
2
4
2
–
HPO4 ) the out-of-plane δ(OH) and in-plane γ(OH) bend-
ing POH vibrations are at 1230–1260 and 800–
00 cm– .
1 [25,26]
The comparison shows that the three ad-
9
–
1
ditional bands at 1720, 1230, and 960 cm are due to
HCOOH rather than to protonated phosphate ions. The
presence of small amount of protonated HCOOH is in ac-
cordance with the chemical composition of the Ni precur-
sor, where the amount of formate species is higher than
1
mol.
In the range of the stretching OH vibrations, two bands
–
1
at 3430 and ca. 3250 cm are observed for the Co precur-
sors, whereas there are four bands between 3500 and
–
1
3
200 cm for the Ni precursors. The bending water vi-
–
1
bration is seen at 1650–1670 cm . For the Ni precursors,
the positions of the four OH bands are close to that in
initial Ni(HCOO) ·2H O (Figure 2). For the sake of com-
2
2
parison, for LiHCOO·H O these bands are at 3398 and
2
–
1 [27]
3
109 cm .
This comparison shows that the water mole-
cules are mainly coordinated to the nickel ions in the Ni
precursors. The difference in the positions of OH vibrations
for the cobalt precursors and Co(HCOO) ·2H O (Figure 1)
2
2
[
27]
and LiHCOO·H O
reveals that water molecules are co-
2
ordinated to both cobalt and lithium ions in the cobalt pre-
cursors.
Summarizing, two features concerning the form and the
type of coordination of the anionic groups and water mole-
cules in the Co and Ni precursors can be outlined. First, in
both types of precursors the formate and phosphate groups
are mainly deprotonated. Second, there is a difference in
respect to the type of coordination of the anionic groups
and water molecules. Thus, in the Co precursors the for-
mate and phosphate ions and the water molecules are coor-
dinated to both types of cations without any preference. A
similar feature was established for the Fe and Mn precur-
[
16,18]
sors obtained by the same freeze-drying method.
trary, in the case of Ni precursors the formate ions and the
Con-
2
+
water molecules prefer to coordinate the Ni ions, whereas
the phosphate ions coordinate the Li ions. This is a specific
+
feature of the Ni precursors obtained by freeze-drying of
mixed phosphate–formate solutions.
The Ni and Co precursors exhibit not only a different
structure, but also a different morphology. Figure 3 gives
the SEM images of both precursors. Whereas the mor-
phology of the Co precursors consists of flake-like aggre-
Figure 3. SEM images of LiCo– and LiNi–phosphate–formates
gates, the Ni precursors display “cabbage”-like aggregates. precursors.
Eur. J. Inorg. Chem. 2010, 4091–4099
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4093

V. Koleva, E. Zhecheva, R. Stoyanova
FULL PAPER
manganese phosphate–formate precursors. This result dem- Structural and Morphological Characterization of LiMPO4
onstrates once again the difference in the formation of the (M = Co, Ni)
Ni precursors by freeze drying of solutions. The different
structure and morphology of the Ni precursors as com-
LiCoPO4
pared to that of Co, Mn, and Fe precursors can be related
with the complex equilibria in the metal–phosphate–for-
mate solutions.
The XRD patterns of PrCo-S005 annealed between 350
and 600 °C is shown on Figure 6. The XRD analysis shows
that olivine-type LiCoPO is already formed at 350 °C. In
4
The thermal properties of the Li–M–phosphate–formate
addition, the partially decomposed precursor at 350 °C
(M = Co, Ni) precursors were followed by differential ther-
comprises also impurity phases including CoO, Li PO ,
3
4
mal analysis (Figures 4 and 5). For both compositions, en-
dothermic peaks between 50 and 200 °C accompanied with
a mass loss are clearly observed. These peaks can be related
with dehydration processes, as was observed for the thermal
Co O , Co P O , and an unknown phase with a strong dif-
3
4
2
2
7
fraction peak at 2θ = 22.14°. The phase composition is the
same for the samples obtained from diluted and concen-
trated freeze-dried solutions. Further annealing of the sam-
ples leads to a decrease in the amount of the impurity
phases, culminating at 450 °C in the formation of a single
[
28,29]
decomposition of M(HCOO) ·2H O (M = Co, Ni)
2
2
[
30]
and LiHCOO.
According to the TG curves they corre-
spond to the release of 1.5 and 2.7 mol H O molecules from
2
phase of well-crystallized LiCoPO . To the best of our
4
the Co and Ni precursor, respectively. Above 200 °C, the
decomposition of the formate species occurs, which is regis-
tered on the DTA curves with several endo- and exothermic
effects. In addition, the decomposition process is more com-
plex and finishes at higher temperatures for the Ni precur-
sor: the total mass loss for the Ni precursor up to 550 °C is
knowledge this is the lowest synthesis temperature that was
reported for pure olivine-type LiCoPO . The data obtained
4
reveal that the formation of LiCoPO from the phosphate–
4
formate precursors takes place through interaction between
Li PO , Co P O , and cobalt oxides (CoO and/or Co O ).
3
4
2
2
7
3
4
The same mechanism of phospho-olivine formation was es-
[
18]
3
3
8.8%, whereas for the Co precursor the total mass loss is
1.1% up to 400 °C. This is a result of the appearance of
tablished also for LiMnPO4.
Contrary to Co and Mn
phospho-olivines, a single LiFePO phase is formed directly
4
more than one mol of formate species in the Ni precursor.
after the release of the formic acid (i.e., at 350 °C), which is
Figure 4. TG, DTA, and DTG curves for PrCo-S005.
Figure 6. XRD patterns of PrCo-S005 annealed between 350 and
6
00 °C. Symbols: olivine-type LiCoPO
4 3 4
(᭺), Li PO (*); CoO (+),
Figure 5. TG, DTA, and DTG curves for PrNi-S005.
094
Co (᭹), Co (v), unknown phase (?).
3
O
4
2 2 7
P O
4
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Eur. J. Inorg. Chem. 2010, 4091–4099

Ordered Olivine-Type Lithium–Cobalt and Lithium–Nickel Phosphates
accompanied with carbon deposition (up to 2.5 mass%).[16] Table 1. Structural parameters of LiMPO
(M = Co and Ni) deter-
4
mined by Rietveld analysis.
Contrary, no carbon was detected for the LiMnPO and
4
LiCoPO samples. This demonstrates the different mecha-
B (Å2) SOF[a]
4
Atom
Site
x
y
z
nism of formation of LiCo/MnPO4 and LiFePO4 from
phosphate–formate precursors.
LiCoPO
4
-S005, 450 °C
a = 10.2026(3) Å, b = 5.9212(1) Å, c = 4.6995(1) Å
3
The XRD patterns of LiCoPO obtained between 450 V = 283.907(16) Å
4
Rwp = 11.7, R
b
f
= 4.00, R = 3.57
and 600 °C are satisfactory fitted by Rietveld analysis on
the basis of the model of an ideal olivine structure, SG Li
4a
4c
4c
4c
4c
8d
0
0
0
1.1
0.9
1.0
0.9
0.9
0.9
1.0
1.0
1.0
1.0
1.0
2.0
Co
P
O1
O2
O3
0.2788(1)
0.0958(3)
0.0955(7)
0.4547(9)
1/4
1/4
1/4
1/4
0.9791(5)
0.4200(7)
0.743(1)
0.205(1)
Pnma (Figure 7) and the structural parameters are given in
Table 1. The lattice parameters and unit cell volumes seems
to be insensitive towards both the annealing temperature
and the concentration of the freeze-dried solutions. For the
fitting procedure a possible Li/Co disorder between the 4a
and 4c sites was also tried. However, after the refinement
0.1654(8) 0.0458(7) 0.2853(8)
4
LiCoPO -S005, 500 °C
a = 10.2045(2) Å, b = 5.9213(1) Å, c = 4.7002(1) Å
3
the occupancy factors of Li at 4a and Co at 4c tend to 1 V = 284.012(14) Å
Rwp = 10.90, R
b
= 3.56, R
f
= 3.76
(
within the experimental error). For LiCoPO annealed at
4
4
50 °C, the fitting procedure gives R = 3.86 with Li/Co Li
4a
0
0
0
1.2
1.3
1.1
0.9
0.9
0.9
1.0
1.0
1.0
1.0
1.0
2.0
b
Co
4c
4c
4c
4c
8d
0.2787(1)
0.0951(3)
0.0958(6)
0.4561(8)
1/4
1/4
1/4
1/4
0.9789(4)
0.4186(6)
0.745(1)
0.2060(9)
disorder of 0.0035Ϯ0.0015 vs. R = 4.00 without disorder;
for LiCoPO annealed at 500 °C – R = 3.48 with Li/Co
disorder of 0.0018Ϯ0.0014 vs. R = 3.56 without disorder;
for LiCoPO annealed at 600 °C – R = 5.29 with Li/Co
b
P
4
b
O1
O2
O3
b
4
b
0.1670(5) 0.0456(6) 0.2848(7)
disorder of 0.0044Ϯ0.0014 vs. R = 5.30 without disorder.
b
4
LiCoPO -S005, 600 °C
On the basis of the Rietveld refinement of neutron and X-
a = 10.2057(1) Å, b = 5.9228(1) Å, c = 4.7009(1) Å
ray powder diffraction, Ehrenberg et al.[ reported a Li V = 284.161(7) Å
31]
3
R
wp = 10.30, R
b
= 5.29, R
f
= 6.84
deficiency at 4a site (92% occupancy) for LiCoPO ob-
4
tained by the citric-assisted Pechini method at 600 °C. We Li
4a
0
0
0
1.1
0.9
1.0
0.9
0.9
0.9
1.0
1.0
1.0
1.0
1.0
2.0
Co
4c
4c
4c
4c
8d
0.2787(1)
0.0955(2)
0.0978(5)
0.4543(7)
1/4
1/4
1/4
1/4
0.9785(4)
0.4204(6)
0.744(1)
0.2069(9)
have also tested a Li deficiency at 4a, but the refinement
P
results rejected this possibility: for LiCoPO annealed at
4
O1
O2
O3
4
50 °C R is 4.06 for 4a site occupancy of 0.980Ϯ0.020; for
b
LiCoPO annealed at 500 °C R is 3.53 for 4a site occu-
4
b
0.1658(4) 0.0456(6) 0.2823(7)
pancy of 1.010Ϯ0.010; for LiCoPO annealed at 600 °C Rb
is 5.39 for 4a site occupancy of 1.020Ϯ0.020. This means
4
4
LiNiPO -S005, 700 °C
a = 10.0384(1) Å, b = 5.85945(5) Å, c = 4.68020(5) Å
3
that LiCoPO without any Li/Co disorder and Li deficiency V = 275.288(5) Å
4
R
wp = 8.32, R
b
f
= 3.37, R = 4.83
is formed from phosphate–formate precursors between 450
and 600 °C. For comparison, LiMnPO samples prepared Li
4a
4c
4c
4c
4c
8d
0
0
0
1.1
1.2
1.1
0.7
0.7
0.7
1.0
1.0
1.0
1.0
1.0
2.0
4
Ni
0.2757(1)
0.0952(2)
0.0996(3)
0.4503(4)
1/4
1/4
1/4
1/4
0.9821(2)
0.4199(4)
0.7450(7)
0.2019(6)
by the same method at 500–600 °C are characterized by an
ordered olivine-type structure, whereas at a lower tempera-
P
O1
O2
ture (450 °C) a small Li/Mn disorder of 2.6% between 4a
and 4c sites was found.[
18]
O3
0.1647(3) 0.0447(4) 0.2760(4)
[
a] The site occupation factors (SOF) are given as number of atoms
per formula unit.
LiCo precursors is completed for a short heating time at
50 °C (3 h) after the release of formic acid. The IR spec-
trum does not display any IR bands due to the formate ions
vibrations. There are only bands due to the stretching PO
and bending OPO vibrations. By extending the heating time
from 3 to 10 h, the characteristic PO43– ion vibrations be-
come more resolved. The LiCo samples annealed at 450 °C
exhibit the typical IR spectrum for well crystalline olivine-
3
[
32]
type LiCoPO4:
ν (PO ) modes in the region 1149–976
3 4
–1
and ν (PO ) modes in the region 646–550 cm . The bands
at 521 and 471 cm are suggested to originate mainly from
Li translations. The bands positions are independent on
Figure 7. Rietveld refinement of the XRD pattern of PrCo-S005
annealed at 450 °C.
4
4
–1
+
[32]
The results from XRD analysis are confirmed by the IR the annealing temperature, which evidences the good ther-
spectroscopic data (Figure 8). The decomposition of the mal stability of the obtained LiCoPO powders.
4
Eur. J. Inorg. Chem. 2010, 4091–4099
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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4095

V. Koleva, E. Zhecheva, R. Stoyanova
FULL PAPER
Figure 9. SEM images (left column) and corresponding particles
4
size distributions (right column) of LiCoPO .
Figure 8. IR spectra of PrCo-S005 annealed between 350 and
600 °C.
The influence of the initial solution concentration and
the annealing temperature on the LiCoPO morphology is
4
demonstrated by SEM analysis (Figure 9). The correspond-
ing particles size distributions as determined from SEM
analysis are also given (Figure 9, right). The morphology
of LiCoPO consists of isometric particles with close size
4
distribution and does not depend on the solution concen-
tration: the mean particles dimensions are around 185Ϯ35
and 190Ϯ30 nm for powders obtained at 500 °C from 0.05
and 0.10  solutions, respectively (Figure 9A and B). By in-
creasing the annealing temperature from 500 to 600 °C,
there is a particle growth (around 230Ϯ40 nm) and particle
size distribution is slightly disturbed (Figure 9C). In ad-
Figure 10. Dependence of the crystallite size D200 on the synthesis
temperature for LiCoPO and LiNiPO prepared from two freeze-
dried solutions. Symbols: LiCoPO prepared from 0.05  solution
4
4
4
dition, the crystallite sizes of LiCoPO determined from the (᭺) and 0.10  solution (᭹), LiNiPO prepared from 0.05  solu-
4
4
tion (ᮀ) and 0.10  solution (᭿).
X-ray diffraction line broadening fall in the nanometric
range (between 70 and 100 nm, Figure 10). On increasing
the annealing temperature, the crystallite sizes tend to in- LiCoPO obtained by microwave heating the particles were
4
crease. Furthermore, the crystallite sizes slightly increase about 400 nm–2 µm, and for LiCoPO prepared from cit-
[13]
4
with the concentration of the freeze-dried solutions. It is ric acid assisted sol–gel method the particles were about
[
9]
worth mentioning that the crystallite sizes are slightly lower 50 nm. The particles sizes of LiCoPO were reduced to
4
than the particle size dimensions determined from SEM about of 100–150 nm by addition of conductive carbon dur-
[
6,13]
analysis.
ing the synthesis
or by subsequent high-energy ball
[
7]
LiCoPO obtained from phosphate–formate precursors milling. The comparison shows that the phosphate–for-
4
is characterized with particles having small dimensions. In mate precursor method allows preparing LiCoPO4 with
the literature, there are several reports on the formation of small particles, which will be favorable for its electrochemi-
LiCoPO with particle dimensions varying from 50 nm to cal performance.
4
5
µm: for LiCoPO obtained by solid-state reactions the
Because the iron, manganese, and cobalt phosphate–
4
[6]
[7]
particles were about 200–300 nm and 0.5–5 µm, for formate precursors have a close structure and mor-
4096
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Eur. J. Inorg. Chem. 2010, 4091–4099

Ordered Olivine-Type Lithium–Cobalt and Lithium–Nickel Phosphates
phology,[16,18] it deserves to compare the particle dimen- structure of the XRD pattern of LiNiPO4 annealed at
sions and the crystallite sizes of the target LiMPO olivines. 700 °C. As in the case of LiCoPO , the structural analysis
4
4
The results show that the particle dimensions and crystallite reveals the formation of an olivine-type LiNiPO phase, SG
4
sizes increase in the following order: LiFePO Ͻ LiMnPO4 Pnma, without any Li/Ni disorder (between 4a and 4c posi-
4
Ͻ LiCoPO . The observed order can be related with the tions) and Li deficiency: R = 3.18 with Li/Ni disorder of
4
b
specific features of their formation: LiFePO4 is directly 0.004Ϯ0.002 vs. Rb = 3.29 for 4a site occupancy of
formed after decomposition of the phosphate–formate pre- 1.020Ϯ0.020 vs. R = 3.37 without Li/Ni disorder and Li
b
cursor, whereas both LiMnPO and LiCoPO are a result deficiency. In addition, the unit cell parameters of LiNiPO4
4
4
of the solid-state reaction between the decomposition prod- do not depend on the concentration of the freeze-dried
ucts. Further, the formation of LiCoPO proceeds in air, solutions.
4
whereas an argon atmosphere is needed for LiMnPO4.
LiNiPO4
The XRD patterns of PrNi-S005 annealed at tempera-
tures between 400 and 750 °C for 10 h are depicted in Fig-
ure 11. Thermal treatment of the LiNi–formate–phosphate
precursors at 400 °C yields a phase mixture of metal Ni,
NiO, and lithium phosphates phases like Li PO4 and
3
Li P O . LiNiPO is formed at a temperature higher than
4
2
7
4
400 °C through a solid-state reaction between nickel oxide
and lithium phosphates. A characteristic feature is that the
LiNiPO composition at 500 °C represents a phase mixture
4
between two structural modifications: olivine-type and
Na CrO -type modifications (denoted also as βЈ-phase
2
33]
4
in[ ). The mechanism of formation of the two forms of
LiNiPO4 from a LiNi–phosphate–formate precursor was
discussed in our previous communication.[ The appear-
ance of the metastable Na CrO -type modification of
19]
Figure 12. Rietveld refinement of the XRD pattern of PrNi-S005
annealed at 700 °C.
2
4
LiNiPO at 500 °C is a specific feature of the phosphate–
4
formate precursor method for the Ni analogue. This reveals,
most probably, once again the different structure and mor-
phology of the nickel-phosphate–formate precursor.
Figure 13 shows the IR spectra of the samples annealed
between 400 and 700 °C. The IR spectrum at 400 °C is
dominated by the absorption bands of phosphate groups in
the regions of 1150–900 and 650–500 cm . The weak bands
around 1630, 1400, and 750 cm–1 are related with residual
formate species. In accordance with the XRD data, the IR
–1
Figure 11. XRD patterns of PrNi-S005 annealed between 400 and
7
00 °C. Symbols: Li
unknown phase (?), Na
PO (᭺), common peak (x).
3
PO
4
(*), metal Ni (+), NiO (᭹), Li
4 2 7
P O (v),
2
CrO
4
-type LiNiPO (β), olivine-type LiNi-
4
4
The temperature-induced transformation of the metasta-
ble Na CrO -type to the olivine-type LiNiPO is finished at
2
4
4
700 °C (Figure 11). Figure 12 and Table 1 give the results
Figure 13. IR spectra of PrNi-S005 annealed between 400 and
of the Rietveld refinement based on an ideal olivine-type 700 °C.
Eur. J. Inorg. Chem. 2010, 4091–4099
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
4097

V. Koleva, E. Zhecheva, R. Stoyanova
FULL PAPER
spectrum of the sample annealed at 500 °C displays absorp- type and Na CrO -type structural modifications is formed
2
4
tion bands due to the two LiNiPO phases. The bands at at 500 °C. The Na CrO -type modification of LiNiPO is
4
2
4
4
–
1
9
34, 609 and 501 cm (ν , ν , and ν modes, respectively) transformed to olivine-type LiNiPO at 700 °C.
1
4
2
4
are attributed to the Na CrO -type form only, whereas the
2
4
residual bands originate from both forms. The IR spectrum
of the sample at 700 °C fully coincides with that of pure Experimental Section
[
32]
olivine-type LiNiPO
(Figure 13): ν (PO ) vibration at
1 4
4
–
1
–1
Preparation of Lithium–Cobalt–Phosphate–Formate and Lithium–
Nickel–Phosphate–Formate Precursors: The starting reagents are
9
43 cm , ν (PO ) vibrations in the region 1147–980 cm ,
ν (PO ) vibration in the region 662–550 cm , and Li -
translations in the range 525–476 cm .
3 4
–
1
+
4
4
M(HCOO)
metal formate dihydrates, M(HCOO)
neutralization of dilute formic acid solutions with the correspond-
2
·2H
2
O (M = Co, Ni) and LiH
2 4
PO (Aldrich). The
–
1
2
·2H O, were prepared by
2
Figure 14 depicts the SEM image of the olivine-type Li-
NiPO4 prepared by the phosphate–formate method at ing carbonates at 60–70 °C. The solutions were then filtered and
00 °C. As in the case of LiCoPO , the morphology of concentrated. The crystals M(HCOO) ·2H O (M = Co, Ni) grew
7
4
2
2
LiNiPO comprises isometric particles with a higher mean by cooling at room temperature. They are very stable upon storage,
size of 380Ϯ70 nm and a broader particle size distribution.
The particle size derived from SEM is larger than the crys-
tallite size derived from XRD (about 90 nm, Figure 10).
This means that the SEM particles are not single crystals.
It is worth mentioning that there are no data in the litera-
ture for morphological characterization of LiNiPO4.
4
which ensures an exact stoichiometry of the target products. Trans-
parent phosphate–formate solutions of lithium and cobalt
nickel, respectively) were obtained by mixing the solutions of
M(HCOO) ·2H O (M = Co or Ni) and LiH PO keeping a stoi-
chiometric ratio M/Li = 1:1. The pH value of both solutions before
mixing was adjusted to pH ≈ 3 with addition of a small quantity
of HCOOH (1:1). In comparison with the iron and manganese
(
2
2
2
4
2 2
salts, the solubility of Co(HCOO) and Ni(HCOO) in water is
limited up to 2.7% at 25 °C. Therefore, in these cases the concentra-
tions of freeze-dried solutions were varied between 0.05 and 0.10 
(
with respect to the metal ions). The solutions thus prepared were
frozen instantly with liquid nitrogen and dried in vacuo (20–
0 mbar) for about 17 h at –20 °C with an Alpha-Christ Freeze
3
Dryer. For the sake of simplicity, the precursors obtained from
solutions with different concentrations will be further on denoted
as PrCo-S005 and PrCo-S01, PrNi-S005 and PrNi-S01, i.e., ob-
tained from 0.05 and 0.10  solutions, respectively. After drying,
the solid precursors were predecomposed at 350 and 400 °C for
Figure 14. SEM image and particles size distribution of LiNiPO
annealed at 700 °C.
4
3
h for cobalt and nickel precursors, respectively. The samples were
further annealed at different temperatures from 350 to 750 °C for
0 h under static air atmosphere. The LiCoPO and LiNiPO sam-
ples obtained from the different precursors are denoted as LiC-
oPO -S005 and LiCoPO -S01, LiNiPO -S005 and LiNiPO -S01,
respectively.
1
4
4
Conclusion
4
4
4
4
Homogeneous lithium–metal–phosphate–formate pre-
cursors with a composition LiMH (PO )(HCOO) ·yH O Methods for Sample Characterization: The metal content in starting
x
4
x
2
(
2 2 4
M = Co, Ni) were prepared by freeze drying of aqueous M(HCOO) ·2H O, the precursors, and LiMPO (M = Co, Ni) sam-
solutions of the corresponding metal formates and ples was determined complexometrically. The lithium content was
LiH PO . Cobalt and nickel precursors have different struc-
ture and morphology. Whereas in the Co precursors the
formate and phosphates ions are randomly coordinated to
both Co and Li cations, in the Ni precursors the formate
ions are preferentially around Ni and the phosphate ions
are around Li . The morphology of the Co precursors con-
sists of flake-like aggregates, whereas the “cabbage”-like ag-
determined by atomic absorption analysis. Elemental analysis (C
2
4
and H) was performed by using Elementar Analysensysteme
GmbH (VarioEL analyzer). X-ray structural analysis was per-
formed with a Bruker Advance 8 diffractometer with LynxEye de-
tector by using Cu-K
refinement by the Rietveld method were carried out by using 0.02°
θ steps of 10 s duration. The computer program FULLPROF was
used in the calculations. The diffractometer point zero, the Lo-
α
radiation. Step-scan recordings for structure
2
+
+
2
gregates are formed for the Ni precursors. Nickel phos- rentzian/Gaussian fraction of the pseudo-Voigt peak function, scale
phate–formate precursors are decomposed at higher tem- factor, the unit cell parameters (a, b, and c), the thermal factor for
peratures in comparison with the Co analogues. Thermal the 4a, 4c, and 8d positions and the line half-width parameters
were determined. The crystallites size of LiMPO
4
was calculated
treatment of the precursors yields single olivine-type LiCo-
by the Scherrer equation from the line width of the (200) reflection
PO at 450 °C and LiNiPO at 700 °C with crystallite sizes
4
4
2
2
1/2
peak: D200 = λ/[(β – β
o
)
α
cosθ200], where λ(Cu-K ) = 0.15418 nm,
of 65 and 90 nm, respectively. LiCoPO4 and LiNiPO4
phases display isometric particles with mean SEM particle
dimensions of 185 and 380 nm, respectively. For both Co
and Ni systems, the phosphate–formate precursor method
yields olivine-type phase without any Li to M disorder (be-
β is the peak width at the half height corrected with instrumental
broadening, and θhkl is the Bragg angle. The line width was deter-
mined by profile analysis using a WinPlotr program. The IR spec-
tra were recorded with a Fourier transform Nicolet Avatar-320 in-
–1
strument using KBr pellets (resolution Ͻ 2 cm ). The thermal
tween 4a and 4c positions) and Li deficiency. A specific analysis (simultaneously obtained DTA, TG, and DTG curves) of
feature of LiNiPO is that a phase mixture between olivine- the precursors was carried out by a “Stanton Redcroft” apparatus
4
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Eur. J. Inorg. Chem. 2010, 4091–4099

Ordered Olivine-Type Lithium–Cobalt and Lithium–Nickel Phosphates
in the temperature range up to 650 °C, a heating rate of 5 °C/min,
[15] D. Shanmukaraj, R. Murugan, Ionics 2004, 10, 88–92.
16] V. Koleva, E. Zhecheva, R. Stoyanova, J. Alloys Compd. 2009,
76, 950–957.
[
and sample mass of 10 mg. The morphology of the precursors and
LiMPO powders was observed by JEOL (JSM-5300 and JSM-
510) scanning electron microscopes.
4
4
[
[
[
[
[
[
17] E. Zhecheva, Ml. Mladenov, P. Zlatilova, V. Koleva, R. Stoy-
anova, J. Phys. Chem. Solids 2010, 71, 848–853.
18] V. Koleva, R. Stoyanova, E. Zhecheva, Mater. Chem. Phys.
2010, 121, 370–377.
19] V. Koleva, R. Stoyanova, E. Zhecheva, Eur. J. Inorg. Chem.
2010, 127–131.
5
Acknowledgments
Authors are grateful for the financial support of the National Sci-
ence Fund of Bulgaria (Ch 1701/2007) and the National Centre for
New Materials (DO-02-82/2008).
20] V. Koleva, H. Effenberger, J. Solid State Chem. 2007, 180, 956–
967.
21] K. G. Kidd, H. H. Mantsch, J. Mol. Spectrosc. 1981, 85, 375–
389.
[
[
[
1] A. K. Padhi, K. S. Nanjuindaswamy, J. B. Goodenough, J.
22] K. Nakamoto, Infrared and Raman Spectra of Inorganic and
Electrochem. Soc. 1997, 144, 1188–1194.
2] M. S. Whittingham, Y. Song, S. Lutta, P. Y. Zavalij, N. Cher-
nova, J. Mater. Chem. 2005, 15, 3362–3379.
3] K. Amine, H. Yasuda, M. Yamachi, Electrochem. Solid State
Lett. 2000, 3, 178–179.
Coordination Compounds, 4th ed., Wiley, New York, 1978, p.
138.
[23] A. Ait Salah, P. Jozwiak, K. Zaghib, J. Garbarczyk, F. Gend-
ron, A. Manger, C. M. Julien, Spectrochim. Acta 2006, 65A,
1007–1113.
[
[
4] J. Wolfenstine, J. Allen, J. Power Sources 2005, 14, 389–390.
5] W. F. Howard, R. M. Spotnitz, J. Power Sources 2007, 165,
[24] A. Novak, Struct. Bonding (Berlin) 1974, 18, 177–216.
[25] J. Baran, T. Lis, M. Drozd, H. Ratajczak, J. Mol. Struct. 2000,
887–891.
516, 185–202.
[
[
[
[
[
6] J. M. Loris, C. Pérez Vicente, J. L. Tirado, Electrochem. Solid
[
[
26] J. Xu, D. F. R. Gilson, I. S. Butler, Spectrochim. Acta 1998,
State Lett. 2002, 5, A234–A237.
A54, 1869–1878.
7] B. Jin, H.-B. Gu, K.-W. Kim, J. Solid State Electrochem. 2008,
27] K. Mouaïne, P. Becker, C. Carabatos-Nédelec, Phys. Stat. Sol.
12, 105–111.
1997, 200, 273–287.
8] N. N. Bramnik, K. Nikolowski, D. M. Trots, H. Ehrenberg,
Electrochem. Solid State Lett. 2008, 11, A89–A93.
[
[
28] P. Baraldi, Spectrochim. Acta 1979, 35A, 1003–1007.
29] V. Rosenband, A. Gany, J. Mater. Proc. Technol. 2004, 153–
9] Gangulibabu, D. Bhuvaneswari, N. Kalaiselvi, N. Jayaprakash,
P. Periasamy, J. Sol-Gel Sci. Technol. 2009, 49, 137–144.
10] P. Deniard, A. M. Dulac, X. Rocquefelte, V. Grigorova, O. Le-
bacq, A. Pasturel, S. Jobic, J. Phys. Chem. Solids 2004, 65, 229–
154, 1058–1061.
[
30] T. Meisel, Z. Halmos, K. Seybold, E. Pungor, J. Thermal Anal.
1975, 7, 73–80.
[
31] H. Ehrenberg, N. N. Bramnik, A. Senyshyn, H. Fuess, Solid
State Sci. 2009, 11, 18–23.
233.
[
[
[
[
11] R. Vasanthi, D. Kalpana, N. G. Renganathan, J. Solid State
Electrochem. 2008, 12, 961–969.
[32] M. Th. Paques-Ledent, P. Tarte, Spectrochim. Acta 1974, 30A,
12] X. Huang, J. Ma, P. Wu, Y. Hu, J. Dai, Z. Zhu, H. Chen, H.
Wang, Mater. Lett. 2005, 59, 578–582.
13] H. H. Li, J. Jin, J. P. Wei, Z. Zhou, J. Yan, Electrochem. Com-
mun. 2009, 11, 95–98.
673–689.
[33] O. García-Moreno, M. Alvarez-Vega, F. García-Alvarado, J.
García-Jaca, J. M. Gallardo-Amores, M. L. Sanjuán, U. Am-
ador, Chem. Mater. 2001, 13, 1570–1576.
Received: April 12, 2010
14] K. Rissouli, K. Benkhouja, J. R. Ramos-Barrado, C. Julien,
Mater. Sci. Eng., B 2003, 98, 185–189.
Published Online: July 30, 2010
Eur. J. Inorg. Chem. 2010, 4091–4099
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
4099