Formation of metastable Na2CrO4-type LiNiPO 4 from a phosphate-formate precursor

Article abstract of DOI:10.1002/ejic.200900736

High-pressure modification of LiNiPO₄ with a Na ₂CrO₄-type structure was obtained at ambient pressure and low temperature from a mixed LiNi-phosphate-formate precursor, LiNiPO ₄H_x(HCOO)_x.yH₂O (where x ≈ 1.2 and y ≈ 2.5). The structural and thermal characterization of the precursor and the LiNiPO₄ compositions were carried out by powder XRD analysis, IR spectroscopy, and DSC analysis, Thermal treatment of LiNiPO ₄H_x(HCOO)_x.VH₂O precursors between 450 and 650 °C yields a mixture of the two structural modifications of LiNiPO₄: the Na₂CrO₄ type and the olivine type, It was established that: the obtained Na₂C:rO₄-type LiNiPO₄ is a metastable phase, which completely transforms at 700 °C into the olivine-type phase, The enthalpy of the phase transition is ?H = -43,40 kJmol^-1. The mechanism of formation of the two forms of LiNiPO₄ from the LiNi-phosphate-formate precursor is discussed

Full text of DOI:10.1002/ejic.200900736

FULL PAPER
DOI: 10.1002/ejic.200900736
Formation of Metastable Na2CrO4-Type LiNiPO4 from a Phosphate–Formate
Precursor
Violeta Koleva,*[a] Radostina Stoyanova,[a] and Ekaterina Zhecheva[a]
Keywords: Lithium / Nickel / Organic–inorganic hybrid composites / Metastable compounds / X-ray diffraction
High-pressure modification of LiNiPO4 with a Na2CrO4-type
structure was obtained at ambient pressure and low tem-
perature from a mixed LiNi–phosphate–formate precursor,
LiNiPO4Hx(HCOO)x·yH2O (where x ≈ 1.2 and y ≈ 2.5). The
structural and thermal characterization of the precursor and
the LiNiPO4 compositions were carried out by powder XRD
analysis, IR spectroscopy, and DSC analysis. Thermal treat-
ment of LiNiPO4Hx(HCOO)x·yH2O precursors between 450
and 650 °C yields a mixture of the two structural modifica-
tions of LiNiPO4: the Na2CrO4 type and the olivine type. It
was established that the obtained Na2CrO4-type LiNiPO4 is
a metastable phase, which completely transforms at 700 °C
into the olivine-type phase. The enthalpy of the phase transi-
tion is ∆H = –43.40 kJmol–1. The mechanism of formation of
the two forms of LiNiPO4 from the LiNi–phosphate–formate
precursor is discussed.
At an intermediate pressure (between 4 and 20 GPa), a
new structural modification of LiMPO4 with a Na2CrO4-
like structure (denoted also as βЈ-phase in [8]) has been ob-
tained by Amador et al.[7,8] The Na2CrO4-like structure
consists of layers with the composition [(MO6)(LiO4)-
(PO4)]ϱ in the ac plane, where every MO6 octahedron shares
two opposite edges with neighboring MO6 and two apical
oxygen atoms with two LiO4 and two PO4 tetrahedra. Con-
trary to the olivine-type structure, Li+ ions occupy isolated
tetrahedral positions, as a result of which the lithium mo-
bility is hindered. This determines the lack of electrochemi-
cal activity of the Na2CrO4-type structure. On the basis of
first-principle calculations at the DFT level, it is calculated
that, for the Co analogue, both structural modifications
possess close to total energies, with an enthalpy difference
of 0.05 eV/f.u.[7] However, to the best of our knowledge,
there is no data on the preparation of a Na2CrO4-type
modification at ambient pressure of Fe, Ni, or Co ana-
logues.
Introduction
Lithium transition-metal orthophosphates with the gene-
ral formula LiMPO4 (M = Fe, Ni, and Co) exhibit three
structural modifications. The stable modification at ambi-
ent pressure adopts the olivine-type structure. This struc-
ture can be described as hexagonal oxygen close packing,
where lithium and transition-metal ions occupy half of the
available octahedral sites with Ci and Cs symmetry, respec-
tively. The MO6 octahedra are ordered in a way to form
zigzag chains in the bc plane, which are cross-linked by
3–
PO4 groups. Lithium ions fall in the rows (running along
the a direction) between the transition metal chains. The
lithium arrangement in the olivine structure favors the 1D
mobility of the Li+ ions along the a direction.[1] The ability
of the olivine structure to intercalate and deintercalate lith-
ium reversibly determines phosphoolivines as potential
cathode materials for lithium-ion batteries.[2,3] The best
electrochemical performance is achieved with the iron ana-
logues, LiFePO4. The reversible electrochemical intercal-
ation of Li+ from the olivine structure takes place concomi-
tantly with the oxidation/reduction of M2+/M3+ ions. This
allows manipulating the potential, where Li intercalation
takes place, replacing iron by manganese, cobalt, and
nickel.[4–6]
In this paper, the preparation of Na2CrO4-type modifica-
tion of the Ni analogue from the LiMPO4 family at ambi-
ent pressure is reported. The method of synthesis is based
on the formation of metal–organic precursors, where Li+
3–
and Ni2+ ions are bonded by PO4 and HCOO– anions.
Metal–organic precursors are isolated by freeze drying
aqueous solutions of Li+, M2+, PO43–, and HCOO– ions.
At ambient pressure the thermal decomposition of metal–
organic precursors yields a Na2CrO4-type modification of
LiNiPO4. This method was recently applied to the prepara-
tion of LiFePO4 compositions. A pure olivine modification
of LiFePO4 is only obtained at ambient pressure.[9] The
structural characterization of the LiNiPO4 samples was car-
ried out by powder X-ray diffraction (XRD) analysis and
The second structural modification of LiMPO4 possesses
a spinel-type structure.[7] While the olivine modification is
stable at ambient pressure, the spinel modification is formed
under high pressures (above 20 GPa) only.
[a] Institute of General and Inorganic Chemistry,
Bulgarian Academy of Sciences,
Acad. G. Bonchev St, bl. 11, 1113 Sofia, Bulgaria
Fax: +359-2-8705024
E-mail: vkoleva@svr.igic.bas.bg
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127

V. Koleva, R. Stoyanova, E. Zhecheva
FULL PAPER
IR spectroscopy. The thermal stability of the Na2CrO4-type HCOOH groups appear: ν(C=O) at about 1700 cm–1, ν(C–
modification of LiNiPO4 was determined by differential O) at 1260 cm–1, and γ(OH) at 975 cm–1.[15] The protonated
–
2–
scanning calorimetry (DSC) analysis.
phosphate ions such as H2PO4 or HPO4 also display
bands at 1230–1260 and 800–900 cm–1 due to out-of-plane
δ(OH) and in-plane γ(OH) bending POH vibrations,
respectively.[16,17] Close inspection of the IR spectra of LiNi
precursors, Ni(HCOO)2·2H2O, and Ni(H2PO4)2·2H2O
shows that the additional three bands at 1720, 1230, and
960 cm–1 are due more to HCOOH than to the protonated
phosphate ions.
Results and Discussion
Freeze drying solutions containing Ni(HCOO)2·2H2O
and LiH2PO4 yields amorphous, pale-green powders with a
LiNiPO4Hx(HCOO)x·yH2O composition (x ≈ 1.2 and y ≈
2.5), where the Ni(Li) to HCOO ratio decreases from 1:2
to 1:1. This indicates that about 0.8 mol of formic acid is
sublimated during the freeze-drying process. The same pro-
cess of sublimation of formic acid was observed when iron
and manganese phosphate–formate precursors are ob-
tained.[9,10]
To rationalize the coordination of formate and phos-
phate groups, IR spectroscopy was undertaken (Figure 1).
For the sake of comparison, the same figure shows the IR
spectra of Ni(HCOO)2·2H2O and Ni(H2PO4)2·2H2O.
In the region of stretching OH vibrations, the IR spec-
trum of the LiNi precursor consists of four bands with posi-
tions close to those of Ni(HCOO)2·2H2O. For the sake of
comparison, we shall mention that the stretching OH vi-
brations for LiHCOO·H2O appear at 3398 and
3109 cm–1.[13] It seems that water molecules are predomi-
nantly coordinated to the nickel ions.
In summary, two features concerning the manner of co-
ordination of the phosphate and formate groups and water
molecules in LiNiPO4Hx(HCOO)x·yH2O precursors can be
outlined. It appears that the formate and phosphate groups
are mainly deprotonated. The formate groups and water
molecules prefer to coordinate around the Ni2+ ions,
whereas the phosphate groups prefer to coordinate around
the Li+ ions. This is a specific feature of LiNi precursors
obtained by freeze drying of mixed formate–phosphate
solutions. When the same freeze drying method is used for
the preparation of LiM–phosphate–formate (M = Fe, Mn,
Co) compositions, both formate and phosphate groups are
protonated, and they are coordinated to the Fe2+ and Li+
ions in a nonpreferential way.[9,10,18] The different types of
3–
coordinations of PO4 and HCOO– to M2+ and Li+ in the
phosphate–formate precursors determines their different
thermal behavior.
Thermal decomposition of LiNi–phosphate–formate pre-
cursors leads to the formation of a target LiNiPO4 compo-
sition at temperature higher than 450 °C. To elucidate the
mechanism of LiNiPO4 formation, Figure 2 shows the
XRD patterns of the lithium–nickel–phosphate–formate
precursor heated at selected temperatures in the range of
350 to 750 °C. At 350 °C, the sample is still amorphous.
However, XRD peaks due to Li3PO4 (JCPDS 25–1030) and
hexagonal Ni metal (JCPDS 45–1027) become visible. Go-
ing from 350 to 400 °C, new phases such as NiO (JCPDS
Figure 1. IR spectra of LiNi–phosphate–formate precursor, Ni-
(HCOO)2·2H2O, and Ni(H2PO4)2·2H2O.
In the IR spectra of LiNi–phosphate–formate precursors, 4–835) and Li4P2O7 (JCPDS 87–409) also crystallize. This
the IR modes due to the formate, phosphate, and OH means that at 350–400 °C the thermal decomposition of
groups are clearly resolved. The characteristic IR bands for freeze-dried LiNi–formate–phosphate precursors yields a
the formate ion vibrations are assigned on the basis of IR mixture between Ni/NiO and lithium phosphate phases. For
studies of formate salts.[11–13] These vibrations are: νas- comparison, in this temperature range, pure LiFePO4 with
(COO) at 1585 cm–1; δ(CH) at 1399 cm–1; νs(COO) at 1378/ an olivine-type structure is obtained. This results from the
3–
1355 cm–1, and δs(OCO) at 795/774 cm–1. Further on, the different structures of the precursors. When PO4 and
δ(CH) and νs(COO) vibrations in the Ni precursor are the HCOO– ions are preferentially coordinated around the Li+
same as those detected for pure Ni(HCOO)2·2H2O. There- and Ni2+ ions, respectively, then mixtures of lithium phos-
fore, one can suppose that the formate ions are preferen- phates and metal phases are obtained during thermal de-
tially coordinated around the Ni2+ ions in LiNi precursors. composition.
In addition, the low intensity of the IR bands at 960,
Structural parameters of both modifications were deter-
1230, and 1720 cm–1 cannot be unambiguously attributed. mined by Rietveld analysis of the XRD pattern of lithium
In this region, the characteristic bands for protonated nickel phosphate obtained at 500 °C (Figure 3).
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Metastable Na2CrO4-Type LiNiPO4 from a Phosphate–Formate Precursor
worth mentioning that the lattice parameters of the
Na2CrO4 form obtained by us at ambient pressure are close
to those for the phase obtained under high pressure
(6.5 GPa).[8] In addition, the lattice volume is expanded by
2.9% during the transformation from the Na2CrO4 form to
the olivine form at ambient pressure. Under high pressure
the change in the lattice volumes is 2.8%.[8]
Table 1. Structural parameters of olivine-type and Na2CrO4-type
LiNiPO4 determined from Rietveld analysis of the XRD pattern
of a sample annealed at 500 °C.
Olivine-type
LiNiPO4
(this work)
Na2CrO4-type
LiNiPO4,
ambient pressure
(this work)
Na2CrO4-type
LiNiPO4,
high pressure
(6.5 GPa)[8]
a [Å]
b [Å]
c [Å]
V [Å3]
SG
10.0426(3)
5.8582(2)
4.6804(2)
275.364(17)
Pnma
5.3628(3)
8.1344(3)
6.1279(2)
267.325(17)
Cmcm
4
5.3580(2)
8.1272(3)
6.1241(3)
266.7(1)
Cmcm
4
Z
4
RB
RF
0.043
0.033
0.041
0.031
0.033
Rexp
Rwp
χ2
0.024
0.10
2.49
0.024
0.10
2.49
0.028
0.09
5.7
Two structural modifications are, further on, charac-
terized by IR spectroscopy (Figure 4). The sample annealed
at 700 °C exhibits a typical IR spectrum for well-crys-
tallized olivine-type LiNiPO4.[14,19] The band at 943 cm–1 is
due to ν1(PO4), the bands in the region of 1147–980 corre-
spond to ν3(PO4), and those in the region of 662–550 cm–1
correspond to ν4(PO4) vibrations. The bands at 525 and
476 cm–1 are suggested to originate mainly from Li+ trans-
lations.[19] In the IR spectra of the samples annealed at
500 °C three additional bands are observed at 934, 609, and
501 cm–1, which are attributed to the Na2CrO4-type form.
According to factor group analysis, these bands are as-
signed as follows: the band at 934 cm–1 to the ν1 mode, that
at 609 cm–1 as a component from ν4(PO4) vibrations, and
that at 501 cm–1 as originating from ν2(PO4) and/or Li+ and
Ni2+ translations.
Figure 2. XRD patterns of LiNi–phosphate–formate precursor an-
nealed at different temperatures. Symbols: Li3PO4 (*), metal Ni (+),
NiO (᭹), Li4P2O7 (v), unknown phase (?), Na2CrO4-type LiNiPO4
(β), olivine-type LiNiPO4 (o), common peak (x).
The formation of the Na2CrO4-like modification at am-
bient pressure was unexpected if we take into account the
previously reported data on high-pressure synthesis of the
LiNiPO4 modification.[8] Evidently, under the conditions of
our experiments the Na2CrO4-type modification obtained
is a metastable phase, which transforms upon heating at T
Ͼ 650 °C into thermodynamically stable olivine modifica-
tion. To prove the phase transformation, a DSC experiment
was performed on a sample annealed at 500 °C (Figure 5).
A broad exothermic peak between 580 and 650 °C was
clearly resolved due to the Na2CrO4-typeǞolivine-type
transformation. The experimentally measured enthalpy of
this process was –19.1 kJmol–1 for the mixture containing
Na2CrO4-type and olivine-type LiNiPO4 in a ratio of
Figure 3. Rietveld refinement of the XRD pattern of lithium nickel
phosphate obtained at 500 °C. Bragg reflections for olivine-type
LiNiPO4 (o) and Na2CrO4-type LiNiPO4 (β) are shown. The differ-
ence between the observed and calculated profiles is plotted.
The atomic coordinates of olivine-type LiNiPO4 and the 0.44:0.56 (determined by Rietveld analysis). The recalcul-
high-pressure form of Na2CrO4-type LiNiPO4 determined ated enthalpy for the phase transformation of pure
in ref.[8] were used as a starting model for Rietveld refine- Na2CrO4-type to olivine-type LiNiPO4 was ∆H
ment. The structural parameters are listed in Table 1. It is –43.40 kJmol–1.
=
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129

V. Koleva, R. Stoyanova, E. Zhecheva
FULL PAPER
occupied either by Ni (Na2CrO4 phase, Figure 6a) or by
Li (olivine phase, Figure 6b). Irrespective of this structural
similarity, the formation of the Na2CrO4-type phase is ob-
served for the Ni analogues only.
Figure 4. IR spectra of LiNi–phosphate–formate precursor an-
nealed at different temperatures.
Figure 6. Structure of the metal sheets in Na2CrO4-type LiNiPO4
(a) and olivine-type LiNiPO4 (b).
The simultaneous appearance of the two structural modi-
fications of LiNiPO4 after the solid-state reaction between
lithium phosphates and Ni/NiO at 450 °C allows the
mechanism of their formation to be explained. On the basis
of the available reports in the literature,[6,8,20,21] the prepara-
tion of the thermodynamically stable olivine-type LiNiPO4
is realized at temperatures higher than 750 °C, even in the
case when solution-based methods are used. This tempera-
ture is significantly higher than that of the Fe- (350 °C[9]),
Mn- (450 °C[10]), and Co- (450 °C[18]) analogues. The for-
mation of the olivine-type LiNiPO4 at higher temperature
implies some kinetics limitations. It appears that these ki-
netic difficulties give rise to the formation of the Na2CrO4-
type metastable LiNiPO4 at low temperature and ambient
pressure.
Figure 5. DSC curve of lithium nickel phosphate obtained at
500 °C (mixture of olivine-type LiNiPO4 and Na2CrO4-type LiN-
iPO4 in a ratio of 0.56:0.44).
Conclusions
For the iron, cobalt, and nickel members of the LiMPO4
family, it is worth mentioning that both Na2CrO4-type and
Freeze drying solutions containing Ni(HCOO)2 and
olivine-type forms exhibit common structural features with LiH2PO4 yields precursors with compositions LiNiPO4-
respect to the “metallic” framework arrangement (Li, M, Hx(HCOO)x·yH2O (x ≈ 1.2 and y ≈ 2.5). In the precursor,
and P).[8] In Figure 6 the arrangement of the cations in the the formate and phosphate ions are mainly deprotonated
two types of structures is depicted for LiNiPO4. In both and exhibit preferential coordination: Li+ prefers phosphate
structures, hexagonal sheets composed of P atoms and Li/ ions, whereas Ni2+ prefers formate ions. Below 400 °C the
Ni atoms are ordered in an ABAB sequence. In the olivine thermal decomposition of the precursors results in the for-
phase, the hexagonal sheets consist of P and Ni atoms, mation of a mixture of lithium phosphates and Ni/NiO. Af-
whereas in the Na2CrO4 phase these sheets are formed by ter further annealing, two structural modifications of
P and Li atoms (Figure 6). In both cases, the P atoms from LiNiPO4 are simultaneously formed. A Na2CrO4-type
two neighboring sheets build distorted octahedra, which are LiNiPO4 is stabilized as a metastable phase at 450 °C,
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Eur. J. Inorg. Chem. 2010, 127–131

Metastable Na2CrO4-Type LiNiPO4 from a Phosphate–Formate Precursor
whereas pure olivine-type LiNiPO4 is obtained at 700 °C.
Acknowledgments
The lattice parameters of Na2CrO4-type LiNiPO4 obtained
at ambient pressure are close to that of Na2CrO4-type LiN-
iPO4 obtained at high pressure (6.5 GPa). The enthalpy of
the phase transition Na2CrO4-type Ǟ olivine-type LiNiPO4
Authors are grateful for the financial support of the National Sci-
ence Fund of Bulgaria (Ch 1701/2007) and to the National Centre
for New Materials (DO-02-82/2008). Thanks to Prof. T. Spassov
is ∆H = –43.40 kJmol–1. Within the LiMPO4 family (M = (Sofia University, Bulgaria) for the DSC measurement.
Fe, Co, Mn, Ni), the appearance of the metastable
Na2CrO4-type modification at ambient pressure is a specific
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The starting Ni(HCOO)2·2H2O was prepared by dissolving nickel
carbonate with dilute formic acid at 60–70 °C. The solution was
then filtered and concentrated. Crystals were obtained by cooling
to room temperature. Ni(HCOO)2·2H2O is very stable during stor-
age, which ensures an exact stoichiometry in the final product.
The chemical composition of the precursors and of the LiNiPO4
samples was determined by using data from the chemical analyses
of Ni (complexometrically), Li (atomic absorption analysis), C and
H (Elementar Analysensysteme GmbH), and thermal analysis
(“Stanton Redcroft” apparatus). X-ray structural analysis was
made with a Bruker Advance 8 diffractometer (Cu-Kα radiation).
Step-scan recordings for structure refinement by the Rietveld
method were carried out using by 0.02° 2θ steps of 10 s duration.
The computer program FULLPROF was used in the calculations.
The DSC curve was recorded using DSC 20 of Mettler TA 3000
system at a heating rate of 10 °Cmin–1.
Published Online: December 1, 2009
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