High-pressure synthesis of solid solutions between trigonal LiNiO 2 and monoclinic Li[Li1/3Ni2/3]O2

Article abstract of DOI:10.1016/j.jssc.2005.03.007

High-pressure synthesis in an oxygen-rich atmosphere yields solid solutions between LiNiO₂ and Li₂NiO₃ over the whole concentration range. Structural characterization of the high-pressure oxides was performed using powder XRD, SEM analysis, IR spectroscopy, EPR spectroscopy at 9.23 and 115 GHz and magnetic susceptibility measurements. The crystal structure of Li[Li_xNi_1-x]O₂, 0≤x≤13, changes from trigonal R-3m to monoclinic C2/m at Li-to-Ni ratio of 2 (or x=13). The incorporation of Li into NiO₂-layers causes a decrease in the mean Li-O and Ni_1-xLi_x-O bond distance. Li and Ni ions in the mixed Ni_1-xLi_xO ₂-layers display a tendency to order at a short length scale in such a way that mimics the Li_1/3Ni_2/3-arrangment of the end Li[Li_1/3Ni_2/3]O₂ composition. The charge distribution in these oxides proceeds via Ni³⁺ and Ni⁴⁺ ions.

Full text of DOI:10.1016/j.jssc.2005.03.007

ARTICLE IN PRESS
Journal of Solid State Chemistry 178 (2005) 1661–1669
www.elsevier.com/locate/jssc
High-pressure synthesis of solid solutions between trigonal
LiNiO2 and monoclinic Li[Li1/3Ni2/3]O2
Ã
Elitza Shinovaa, Ekaterina Zhechevaa, , Radostina Stoyanovaa, Geoffrey D. Bromileyb,c
aInstitute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, Bldg. 11, BG-1113 Sofia, Bulgaria
bBayerisches Geoinstitut, Universita¨t Bayreuth, D-95440 Bayreuth, Germany
cDepartment of Earth Sciences, University of Cambridge, Cambridge CB2 3EQ, UK
Received 17 January 2005; received in revised form 8 March 2005
Abstract
High-pressure synthesis in an oxygen-rich atmosphere yields solid solutions between LiNiO2 and Li2NiO3 over the whole
concentration range. Structural characterization of the high-pressure oxides was performed using powder XRD, SEM analysis, IR
spectroscopy, EPR spectroscopy at 9.23 and 115 GHz and magnetic susceptibility measurements. The crystal structure of
1
Li[LixNi1ꢀx]O2 ,0pxp3; changes from trigonal R-3m to monoclinic C2/m at Li-to-Ni ratio of 2 (or x ¼ 13). The incorporation of Li
into NiO2-layers causes a decrease in the mean Li-O and Ni1–xLix–O bond distance. Li and Ni ions in the mixed Ni1–xLixO2-layers
display a tendency to order at a short length scale in such a way that mimics the Li1/3Ni2/3-arrangment of the end Li[Li1/3Ni2/3]O2
composition. The charge distribution in these oxides proceeds via Ni3+ and Ni4+ ions.
r 2005 Elsevier Inc. All rights reserved.
Keywords: Lithium nickelates; High-pressure synthesis in oxygen atmosphere; Electron paramagnetic resonance
1. Introduction
LiNiO2 includes mixing of appropriate amounts of Li
and Ni-salts, followed by heat treatment at 700 1C in an
oxidative atmosphere [4–7]. At this temperature, there are
two competitive reactions: solid state interaction between
Li-and Nis-alts and Li evaporation. To compensate for
the Li evaporation during the synthesis procedure, the
initial Li-salt is taken in excess in respect to the
stoichiometric one. Even at these experimental condi-
tions, non-stoichiometric Li1ꢀdNi1+dO2 oxides, where d
varies between 0.01 and 0.05, have been formed [4,7].
By increasing the oxygen pressure, it has been
reported that extra Li ions can be incorporated in the
NiO2-layers. When Li-to-Ni ratio is 2, a layered lithium
nickelate with the composition Li2NiO3 has been
prepared under 150 bar in an oxygen-rich atmosphere
by Migeon et al. [8]. The charge compensation is
achieved by stabilization of highly oxidized Ni4+ ions.
The structure of Li2NiO3 is composed of pure Li layers
and mixed Li/Ni layers. The lithium and nickel ions are
ordered in the mixed layers, leading to a reduction in the
space group symmetry from R-3m to C2=c [8]. The main
Lithium–nickel oxides (LiNiO2) with layered crystal
structure belong to the main family of electrode
materials used in lithium-ion batteries, where reversible
oxidation/reduction of the Ni3+/Ni4+ ions takes place
in the course of electrochemical lithium deintercalation/
intercalation [1–3]. The intercalation properties of
LiNiO2 have been shown to depend strongly on the
structural characteristics of the LiO2-layers. Based on X-
ray diffraction (XRD) and neutron diffraction studies, it
has been found that Ni ions in an oxidation state of 2+
fill the depleted LiO2-layers in nearly stoichiometric
Li1ꢀdNi1+dO2. The appearance of extra Ni ions in the
LiO2-layers complicates the extraction of Li, as a result
of which non-stoichiometric lithium nickelates display
poor electrochemical performance as cathode materials
in lithium ion batteries. The synthesis procedure of
Ã
Corresponding author. Fax: +359 2 870 50 24.
E-mail address: zhecheva@svr.igic.bas.bg (E. Zhecheva).
0022-4596/$ - see front matter r 2005 Elsevier Inc. All rights reserved.
doi:10.1016/j.jssc.2005.03.007

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structural difference between LiNiO2 and Li2NiO3 is
associated with the nickel-rich layers. For layered
LiNiO2, pure NiO2-layers exist, whereas for monoclinic
Li2NiO3 mixed [Li1/3Ni2/3]O2-layers are developed, in
addition to the pure LiO2-ones. In addition, the layered
oxide with Li2NiO2 composition has been prepared by
electrochemical insertion of extra Li in the LiO2-layers
of LiNiO2 [9]. The incorporation of Li leads to the
collective transition of all Li+ ions from octahedral to
tetrahedral positions. Contrary to Li2NiO3, the charge
compensation is achieved by Ni2+ ions.
against the quartz–coesite and kyanite–sillimanite transi-
tions, as well as the melting point of diopside. Temperature
was measured with a Pt90Rh10–Pt thermocouple. Experi-
ments were performed using the ‘‘hot-piston in’’ technique.
In this method, the pressure is increased to approximately
10% below the final run pressure, then the temperature is
increased to the desired run temperature, and finally, the
pressure is increased to the final value. Samples were
quenched isobarically by turning off the power whilst
maintaining a pressure within 0.02 GPa of the run
pressure. Quench rates were of the order of 751 per
second. The lithium content of the samples, the mean
oxidation state of nickel and the total nickel content were
determined by atomic absorption analysis, iodometric
titration and complexometric titration, respectively.
X-ray phase analysis was performed using a Philips X0
Pert powder diffractometer with monochromatic CoKa1
radiation, and Si being the internal standard. The scan
range was 15p2yp120 with a step increment of 0.021. A
Fullprof computer program was used for the calculations
[17]. The diffractometer point zero, the Lorentzian/
Gaussian fraction of the pseudo-Voigt peak function,
scale factor, the unit cell parameters (a and c), the oxygen
parameter (z), the thermal factor for the 3a, 3b and 6c
positions, the line half-width parameters and the preferred
orientation were determined. The cationic occupancy
factors were refined taking into account that the total
occupancies of the 3a, 3b and 6c sites are equal to unity.
SEM analyses on powder samples coated with gold
were carried out on a JEOL-100 B microscope with
10 kV accelerating voltage.
The IR spectra were recorded with a Nicolet Avatar-32
spectrometer using KBr pellets. Magnetic susceptibility
was determined by the Faraday method at 100–300 K.
EPR measurements at 9.23 GHz (X-band) were
carried out using a ERS 220/Q spectrometer over the
temperature range 85–410 K. The g-factors were estab-
lished with respect to a Mn2+/ZnS standard. The signal
intensity was determined by double integration of the
experimental EPR spectrum. The high-frequency EPR
spectra were recorded on a single-pass transmission
EPR spectrometer built in the High-Magnetic Field
Laboratory, Grenoble, France. The frequencies were
changed from 95 to 475 GHz using Gunn diodes and
their multipliers. The detection of absorption was
performed with a bolometer. The recording tempera-
tures were varied from 5 to 300 K using a variable
temperature insert (Oxford Instruments).
For pressures between 2 and 4 GPa, formation of
Li[LixNi1ꢀx]O2 with xE0.12 has been reported [10].
These compositions display different electrochemical
properties compared to layered LiNiO2. Moreover, the
electrochemical behavior of Li[LixNi1ꢀx]O2 is rather
close to that of electrodes based on ‘‘LiNiO2–Li2MnO3’’
oxides. Recently, complex ‘‘LiNiO2–Li2MnO3’’ oxides
have been considered as most perspective electrode
materials for lithium-ion batteries [3,11–16]. The struc-
ture of ‘‘LiNiO2–Li2MnO3’’ oxides is still unclear. Based
on the careful XRD analysis, Dahn et al. have been
suggested that LiNiO2 and Li2MnO3 oxides form solid
solution phases [14]. Analysis of the MAS-NMR spectra
and lattice imaging by TEM have been interpreted in
terms of the structure integration of monoclinic
Li2MnO3 into layered LiNiO2 leading to complex
domain structure of ‘‘LiNiO2–Li2MnO3’’ oxides [16].
The main goal of this paper is to study the formation
of a solid solution between trigonal LiNiO2 and
monoclinic Li[Li1/3Ni2/3]O3. For the preparation of
new yLiNiO2.(1ꢀy)Li[Li1/3Ni2/3]O2 compositions, we
have considered a synthetic procedure involving a
solid-state reaction between Li2O2 and NiO under
high-pressure in an oxygen-rich atmosphere, intended
to stabilize Li+ ions in the nickel layers. XRD powder
analysis, scanning electron microscopy (SEM) and IR
spectroscopy were used for structural characterization
of the mixed [LixNi1ꢀx]O2-layers. The oxidation state of
nickel ions was determined by chemical analysis,
electron paramagnetic resonance (EPR) of Ni3+ and
magnetic susceptibility measurements.
2. Experimental
Compositions of Li[LixNi1ꢀx]O2 were prepared by a
solid state reaction between NiO (obtained by thermal
decomposition of Ni(OH)2 at 550 1C ) and Li2O2 (Aldrich)
at 3 GPa and 700 1C for 2.5 h. High-pressure synthesis was
performed using a 12 in end-loaded piston-cylindrical
apparatus at the Bayerisches Geoinstitut. Samples were
encapsulated in 1 cm long, 5 mm diameter welded Pt
capsules. The capsules were surrounded by a pyrophyllite
sample holder and inserted into talc–pyrex cells with a
tapered graphite-resistance heater. Pressure was calibrated
3. Results and discussion
3.1. Structural characterization of Li[LixNi1ꢀx]O2
Varying the Li/Ni ratio in the initial mixture of NiO
and Li2O2 from 1.0 to 2.05 at high pressure resulted in

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1663
the formation of new Li[LixNi1ꢀx]O2 compositions with
1
0pxp3: Thermal decomposition of Li2O2 under the
experimental conditions used (700 1C and 3 GPa)
ensured an oxygen-rich atmosphere which was necessary
for the stabilization of the nickel ions at high oxidation
states
ð1 ꢀ xÞ NiO þ 1=2ð1 þ xÞ Li2O2 þ sLi2O2
! Li½LixNi1ꢀxꢁO2 þ s Li2O þ s=2 O2:
To compensate for the lithium volatility at the
relatively high synthesis temperature, a small excess of
Li2O2, s, was used. As in the case of synthesis under
atmospheric pressure, the Li/Ni ratio in the oxides
obtained under high pressure was a little below that of
the initial mixture (Table 1). The Li content in the oxides
obtained was determined both by chemical analysis and
Rietveld analysis of the XRD patterns. In accordance
with the literature data on LiNiO2 [7,18], the Rietveld
analysis gives systematically higher value for the Li-to-
Ni ratio. However, there is a similar trend in the changes
of the Li/Ni ratio (Table 1). In addition, in contrast to
solid state synthesis under atmospheric pressure, at high
pressure, oxides with a Li/Ni41 can be obtained. For
oxides prepared at atmospheric pressure and 700 1C, the
Li-to-Ni ratio is always lower than 1, irrespective of the
ratio used in the precursor mixture. For example, Arai et
al. [4] reported the preparation of nearly stoichiometric
LiNiO2 with L/NiE1 by using 3 moles of Li excess.
Fig. 1 shows XRD patterns of oxides obtained under
high pressure, with varying initial Li/Ni ratios from 1.05
to 2.05. For all the samples obtained, the main
diffraction reflections match well with the layered-type
crystal structure (R-3 m). However, the compositions
can be divided in three groups with respect to the initial
Li-to-Ni ratio: 1:05pLi=Nip1:20; 1:2oxo2:05 and
x ¼ 2:05: Chemical analysis and X-ray diffraction
studies show that with 1:05pLi=Nip1:20 layered non-
stoichiometric oxides, (Li1ꢀdNid)NiO2 with do0.05, are
obtained (Table 1). The diffraction lines of all samples
are best described by a hexagonal symmetry, space
group (R-3 m). The structural model used comprises Li
in 3b sites (000.5), Ni in 3a sites (000) and oxygen in 6c
sites (00z). The amount of Ni in the LiO2 layer (d)
depends on the initial Li/Ni ratio, the best stoichiometry
corresponding to Li/Ni ¼ 1.1. Moreover, samples of
[Li1ꢀdNid]NiO2 obtained at high-and atmospheric
pressure do not differ in their parameters and cationic
arrangement.
For samples with initial Li/Ni41.25, low-intensity
diffuse peaks in the range 22o2Yo281 appear in
addition to the main diffraction lines. For the end
composition with initial Li/Ni ¼ 2.05, the structural
refinement has been carried out with a monoclinic
layered C2/c space group according to Migeon et al. [8].
However, the fitting procedure is unsatisfactory

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1664
Li/Ni=1.10
9000
6000
3000
0
Si
-3000
8000
6000
4000
2000
0
Li/Ni=1.25
Si
-2000
6000
C2/m
C2/c
Li/Ni=2.05
Li/Ni=2.05
4000
2000
0
Li/Ni=1.5
24
26
28
30
Si
-2000
20
30
40
50
60
70
80
90
100
110
120
Two theta,o
Fig. 1. XRD patterns of lithium-nickel oxides prepared at 3 GPa from precursors with Li-to-Ni from 1.05 to 2.05. The difference between the
observed and calculated profiles is plotted. Bragg reflections for layered trigonal and monoclinic structures and Si standard are indicated. For
comparison, the inset gives the range where the low-intensity peaks due to the monoclinic C2=c and C2=m appear.
Table 2
Position and structural parameters determined from XRD Rietveld
and 3 Ni ions as nearest neighbors. It appears that high
pressure (3 GPa) favors the formation of monoclinic
refinement for Li[LixNi1ꢀx]O2 prepared from precursor with Li/
oxides with C2=m space group, while oxides with C2=c
Ni ¼ 2.05
are obtained at lower pressure (150 bar). The influence
Atom Site
x
y
z
Biso (A2) Occupancy
of the experimental conditions on the space group
symmetry of oxides has been established already for
Li2MnO3 [19–21].
Li1
Li2
Li3
Ni
2b
2c
4h
4g
4i
0.0000
0.0000
0.0000
0.0000
0.5000
0.0000
0.0000
0.5000
0.29
1.01
0.25
0.25
0.50
0.50
0.50
1.00
Intermediate compositions (obtained from precursors
with 1.25oLi/Nio2.05) also display additional low-
intensity peaks over the range of 22o2Yo281: For that
reason, the layered monoclinic ðC2=mÞ and the trigonal
space group (R-3m) are used for structural characteriza-
tion. A better fit is obtained using the R-3m space group:
RB ¼ 19:3 and 4.9, respectively. In order to achieve the
best possible characterization of the peaks at
22o2Yo281, a two-phase model comprising a main
phase with a layered structure and an impurity phase
with a monoclinic structure has also been investigated.
In this case again there is no good fitting. Moreover, the
positions of these peaks are changed with increasing the
Li-to-Ni ratio, thus indicating the formation of solid
solutions. This means that the diffraction lines in the 2Y
region 22o2Yo28 corresponds to super-lattice reflec-
tions rather than these peaks are due to the presence
of an impurity phase. Super-lattice reflections have
been detected for the complex ‘‘LiNiO2–Li2MnO3’’
system (or notation as a solid solution Li[Li1/3ꢀ2/3x
NixMn2/3ꢀx/3]O2) [14]. The X-ray patterns have been
interpreted by the formation of ‘‘LiNiO2–Li2MnO3’’
solid solutions, where long-range Li ordering on the 31/2
a ꢂ 31/2 a super-structure appears together with an
0.6868(5) 0.5000
0.1704(3) 0.0000
1.14
1.09
O1
O2
0.2397(9) 0.0000 0.2125(8) 0.32
0.2489(5) 0.3321(9) 0.2288(5) 0.47
8j
Cell parameters: a ¼ 4.8984 (8) A, b ¼ 8.4496 (13) A, c ¼ 4.9692 (5) A,
b ¼ 109.021 (2); V ¼ 194.60 (5) A3.
ðRB ¼ 28:3Þ: the main diffraction lines are fitted well,
while many additional reflections with appreciable
intensity have been calculated at 22o2Yo281 (Fig. 1).
In order to fit the low-intensity diffraction peaks
between 22o2Yo281, a monoclinic system with a
C2=m space group has also been checked ðRB ¼ 17:6Þ:
The refined cell parameters are given in Table 2. The
refined structure of Li2NiO3 with a C2=m space group
agrees well with the single crystal structure determina-
tion for Li2MnO3, where a monoclinic cell with a C2=m
space group was used [19]. The monoclinic system with a
C2=c or C2=m space group reflects a different arrange-
ment in the Li-layers, but the arrangement in the mixed
(LiM2)-layers is the same, where every Li ion is
surrounded by 6 Ni ions, and every Ni ion has 3 Li

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1665
uniformly mixing of Ni and Mn on the transition-metal
sites [14]. On the other hand, from NMR and TEM
analysis, formation of local Li2MnO3–rich regions has
been suggested [16].
(Fig. 2). The mean LixNi1ꢀx–O bond length decreases,
which is associated with partial oxidation of Ni3+ to
Ni4+ (Table 1). For the sake of comparison we shall
mention that the lengths of the Ni3+–O and Ni4+–O
bonds in lithium-transition metal oxides are 1.97 and
1.90 A, respectively [22]. The length of the Li1ꢀdNid–O
bond in the LiO2-layer also decreases with the change
in length of the LixNi1ꢀx–O bond in the mixed
[LixNi1ꢀx]O2 layers.
The morphology of the samples prepared under high
pressure is shown in Fig. 3. As one can see, there is no
difference in the morphology of samples prepared at low
and high Li-to-Ni ratios. The samples consist of plate-
like particles with an average diameter of 100–200 nm.
The plate-like particles are stacked into aggregates with
diameters larger than 1 mm.
Table 1 shows that the appearance of Li+ ions (x) in
the NiO2-layers starts at an initial Li/NiX1.25. The
incorporation of Li in the lattice is accompanied by a
change in the structural parameters as compared to the
well-known LiNiO2. The unit cell parameters a and c
decrease while the trigonal distortion (expressed by c=a)
increases. Irrespective of the manner of the Li-to-Ni
ratio determination, the observed changes in the unit cell
parameters do not obey the Vegard’s law, which can be
related with non-random Li–Ni distribution. By increas-
ing the Li/(Li+Ni) ratio, there is a volume contraction
The Li/Ni distribution in [LixNi1ꢀx]O2 layers can be
monitored by infrared spectroscopy. Fig. 4 presents
comparison of IR spectra of oxides with 1.05pLi/
Nip2.05 in the 400–800 cmꢀ1 region where vibrations of
the transition metal layers are established.
102
101
100
99
The almost stoichiometric Li1ꢀdNi1+dO2 sample
(do0.02) exhibits two intense peaks at 500 and
565 cmꢀ1 due to NiO2-vibrations. The IR spectrum of
the end composition with Li/Ni ¼ 2 shows 5 compo-
nents at 425, 505, 562, 615 and 648 cmꢀ1, respectively
(Fig. 4). The increase in number of the IR bands for
Li[Li1/3Ni2/3]O2 is consistent with a [Li1/3Ni2/3]-arrange-
ment in the mixed [Li1/3Ni2/3]O2 layers as was observed
from XRD analysis. Samples with intermediate compo-
98
Li1+xNi1-xO2
97
0.45
0.50
0.55
0.60
0.65
Li/(Li+Ni) in Li1+xNi1-xO2 (chemical analysis)
102
101
100
99
1
sitions ð0oxo3Þ display splitting of the two main IR
bands, their number increasing with the Li amount in
the layer. This result provides evidence for additional
short range ordering of lithium and nickel at these
compositions. The similar IR profiles suggest that
short-range Li/Ni ordering in intermediate compo-
sitions mimic the cationic distribution of the end
compositions.
98
Li1+xNi1-xO2
97
0.45
0.50
0.55
0.60
0.65
Summarizing the XRD and IR data, layered Li[Lix
Ni1ꢀx]O2 oxides, over the whole concentration range,
can be prepared by solid state reaction between Li2O2
and NiO under high-pressure conditions in oxygen-rich
atmospheres. The incorporation of Li into NiO2-layers
causes contraction of the unit cell parameters, with
Li/(Li+Ni) in Li1+xNi1-xO2 (Rietveld analysis)
Fig. 2. Unit cell volume vs. Li-to-Ni ratio in Li1+xNi1ꢀxO2
determined from chemical and Rietveld analysis. For the sake of
comparison, Li1+xNi1ꢀxO2 prepared from precursor with Li/
Ni ¼ 2.05 is indexed also in R-3/m.
Fig. 3. SEM images of Li1+xNi1ꢀxO2 prepared from precursors with Li/Ni ¼ 1.2 (left) and 1.5 (right).

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Ni 3+ isolated
103 K
Li/Ni=2.05
Li/Ni=1.50
Li/Ni=2.05
Li/Ni=1.25
Li/Ni=1.20
Li/Ni=1.5
Li/Ni=1.10
Li/Ni=1.05
Li/Ni=1.25
100
200
300
400
500
600
Li/Ni=1.2
Li/Ni=1.1
B, mT
Fig. 5. EPR spectra in X-band of Li1+xNi1ꢀxO2 prepared at 3 GPa
from precursors with Li/Ni ratio varied between 1.05 and 2.05.
400
500
600
700
800
1.0
(c)
Wave number, cm-1
0.9
0.8
0.7
Fig. 4. IR spectra of Li1+xNi1ꢀxO2 prepared at 3 GPa from precursors
with Li/Ni ratios between 1.1 and 2.05.
Vagard’s law not being obeyed. There is a strong
tendency for Li and Ni to order in the NiO2-layers.
Li1-δNi1+δ O2
Li1+xNi1-xO2
0.6
1.0
0.8
0.6
0.4
(b)
3.2. Oxidation state of Ni in Li[LixNi1ꢀx]O2
The oxidation state of Ni ions in the Li[LixNi1ꢀx]O2
oxides has been studied by EPR. Nearly stoichiometric
LiNiO2 has an EPR spectrum consisting of a single line
with Lorentzian shape and g ¼ 2:137 due to exchange-
coupled Ni3+ ions [23]. A signal with the same shape and
g value, but with a larger line width, corresponds to
Li[LixNi1ꢀx]O2 (Fig. 5). The EPR line width increases
with the number of lithium ions incorporated in the
nickel layer (Fig. 6). The intensity of the EPR
signal decreases in the same sequence. With the Li[Li1/3
Ni2/3]O2 end composition the broad signal disappears and
a narrow low-intensity signal with g ¼ 2:14 appears
instead. According to the EPR study of Ni3+ in
LiNixCo1ꢀxO2 solid solutions [24], these parameters
suggest that the broad EPR signal observed may be
attributed to exchange-coupled Ni3+ ions, while the
narrow low-intensity signal comes from magnetically
isolated Ni3+ ions. To rationalize the dependence of the
EPR line width and EPR intensity on the Li-content, Fig.
6 also gives changes in the number of Ni3+ ions. As
expected, changes in EPR line width and EPR intensity
correlate with the amount of Ni3+. The increase in line
width with the amount of lithium in the LixNi1ꢀxO2
layers can be explained to be the results of magnetic
dilution of paramagnetic Ni3+ ions by diamagnetic Ni4+
90
80
70
60
50
40
30
(a)
Li1-δ Ni 1+δ O2
Li1+xNi 1-xO2
0.48
0.50
0.52
0.54
0.56
Li / (Ni+Li)
Fig. 6. EPR line width (a) and EPR relative intensity (b) determined at
103 K in X-band versus Li-content, Li/(Li+Ni), in Li1+xNi1ꢀxO2. The
Ni3+/Nitotal amount (c) determined according to following cation
distribution Li1ꢀdNi2dþ½LixNi13þꢀ3xþdNi
ꢁO2 as a function of the Li-
4þ
2xꢀd
content.
ions. On the other hand, this indicates the formation of
solid solutions between LiNiO2 and Li[Li1/3Ni2/3]O2. For
the end Li[Li1/3Ni2/3]O2 composition there is an EPR

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1667
signal of isolated Ni3+ ions only, whose surroundings
include diamagnetic ions (Li+ and/or Ni+4 ions).
Additional differentiation between the samples con-
taining lithium in the nickel layers can be achieved by
EPR spectroscopy at high frequencies (HF-EPR). HF-
EPR has been shown to provide better resolution as
compared to conventional X-band EPR [25]. The
advantage of using high-frequency EPR has been
demonstrated for Li1ꢀdNi1+dO2 with 0.02pdp0.12:
while in the X-band the symmetric EPR line gives rise
to the EPR profile, the high-frequency measurements
reveals an asymmetry of the signal [26]. Figs. 8 and 9
show the EPR spectra at 115 GHz of nearly stoichio-
metric Li1ꢀdNi1+dO2 and Li1ꢀdNid[LixNi1ꢀx]O2 samples
obtained from precursors with Li:Ni ratios of 1.2 and
1.5, respectively. Above 30 K the EPR spectrum of
paramagnetic Li1ꢀdNi1+dO2 consists of an asymmetric
line (Fig. 8). Below 30 K the appearance of resonance
absorption in high-and low field indicates the presence
of magnetically correlated spins (Fig. 8). However, long-
range magnetic ordering is not achieved with this
sample. In contrast to nearly stoichiometric Li1ꢀd
Ni1+dO2, the sample Li1ꢀdNid[LixNi1ꢀx]O2 containing
Li in the nickel layers displays only one broad symmetric
Lorentzian line above 13 K even in the high-frequency
region (Fig. 9). The line width decreases with increasing
temperature from 13 to 80 K, the g-value remaining
constant: 2.155 (Fig. 10). In this temperature range the
signal intensity varies with the registration temperature
following the Curie–Weiss law with a Weiss constant
YEPR ¼ þ4 K: The constant thus determined is lower
than the Weiss constant determined for Li1ꢀdNi1+dO2
One can differentiate between the two kinds of
compositions, Li1ꢀdNid[LixNi1ꢀx]O2 and Li1ꢀdNi1+dO2,
on the basis of the EPR line width and its temperature
dependence (Fig. 7). With Li1ꢀdNi1+dO2 oxide, the
signal shows a linear decrease of the line width with
decreasing registration temperature within the range
400–100 K. The slope of the linear dependence,
dDHpp=dT; increases with the amount of Ni2+ in the
lithium layers: from 0.492 to 0.421 mT/K for samples
prepared from Li/Ni ¼ 1.05 and 1.10, respectively. This
result is in agreement with previous EPR investigations
on Li1ꢀdNi1+dO2 oxides [6] where it has been shown
that the dDHpp=dT slope depends on the strength of the
1801 and 901 Ni–O–Ni exchange interaction, the
coordination number of exchange coupled particles
and the distance between them. An unexpected result
is obtained for the sample prepared from precursor with
Li/Ni ¼ 1.2, where there is a marked deviation from the
linear dependence of the EPR line width. This indicates
peculiarities in the cationic distribution, which can be
associated with the presence of impurity lithium ions in
the nickel layer. For this sample, the number of lithium
ions in the nickel layer is low enough to prevent them
from being detected by analysis of the X-ray diffraction
lines. This result may explain the sharp change in
composition of the oxides obtained from precursors
with a Li/Ni ratio of 1.2 and 1.25: while the X-ray
structure analysis shows the formation of Li1ꢀdNi1+dO2
oxides from precursors with Li/Ni ¼ 1.2, EPR of Ni3+
ions provides evidence for the presence of small amounts
of Li in the nickel layers. It should be noted that on the
basis of temperature dependence analysis of the EPR
signal, one cannot determine whether the lithium ion
impurities are distributed regularly or is segregated in
the mixed LixNi1ꢀxO2-layer.
prepared from precursor with Li/Ni ¼ 1.2: YEPR
¼
þ43 K; X-band. At temperatures below 13 K the EPR
line shape for Li1ꢀdNid[LixNi1ꢀx]O2 undergoes a change,
most possibly due to short-range magnetic correlations.
Similarly to the case of Li1ꢀdNi1+dO2, a long-range
magnetic ordering down to 5 K cannot be achieved with
the Li1ꢀdNid[LixNi1ꢀx]O2 sample. This result shows
10 K
16 K
Li/Ni=1.05
300
Li/Ni=1.50
22 K
250
Li/Ni=1.10
Li/Ni=1.25
30 K
200
Li/Ni=1.20
60 K
115 GHz
150
100
200
300
400
1
2
3
4
5
6
7
T, K
B, T
Fig. 7. Temperature variation of the EPR line width of Li[LixNi1ꢀx]O2
prepared at 3 GPa from precursors with Li/Ni ratios between 1.05 and
1.50.
Fig. 8. EPR spectra at 115 GHz for nearly stoichiometric
Li1ꢀdNi1+dO2 prepared at 3 GPa from precursor with Li/Ni ¼ 1.20.

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E. Shinova et al. / Journal of Solid State Chemistry 178 (2005) 1661–1669
1668
moments (mexp). The decrease in the magnetic moment of
5 K
the nickel ions with the lithium amount in the nickel-rich
layers reveals the appearance of diamagnetic Ni4+ ions
(mNi(4+) ¼ 0).
8 K
10 K
4. Conclusions
13 K
16 K
High-pressure synthesis in an oxygen rich atmosphere
yields solid solutions between LiNiO2 and Li2NiO3 over
the whole concentration range. The crystal structure of
22 K
30 K
1
Li[LixNi1ꢀx]O2, 0pxp3; is changed from trigonal R-3 m
1
to monoclinic C2/m at Li-to-Ni ratio of 2 ðor x ¼ 3Þ:
115 GHz
The incorporation of Li into NiO2-layers causes a
decrease in the mean Li–O and Ni1ꢀxLix–O bond
distance. Li and Ni ions in the mixed Ni1ꢀxLixO2-layers
display a tendency to order at a small-scale range in such
a way that mimics the Li1/3Ni2/3-arrangment in the end
Li[Li1/3Ni2/3]O2 composition.
1
2
3
4
5
6
7
B, T
Fig. 9. EPR spectra at 115 GHz for Li[LixNi1ꢀx]O2 prepared at 3 GPa
from precursor with Li/Ni ¼ 1.50.
At a first glance, the formation of solid solutions
between trigonal LiNiO2 and monoclinic Li2NiO3
bears a resemblance with the formation of complex
‘‘LiNiO2–Li2MnO3’’ electrode materials [14]. However,
the most important difference comes from the charge
distribution in these oxides. Both Ni3+ and Ni4+ ions are
stabilized in ‘‘LiNiO2–Li2NiO3’’ solid solutions, while
charge distribution in complex ‘‘LiNiO2–Li2MnO3’’
proceeds via stabilization of Ni2+ and Mn4+ [15]. In
addition, HF-EPR spectroscopy indicates that mono-
clinic Li2NiO3 is integrated in layered LiNiO2 without the
formation of domains enriched in Ni3+ or Ni4+ ions.
2.16
2.12
2.08
2.04
115 GHz
0.8
0.6
0.4
0.2
0
10 20 30 40 50 60 70 80 90 100 110
T, K
Acknowledgments
Fig. 10. Temperature variation of the EPR line width and of the g-
value (determined at 115 GHz) for Li[LixNi1ꢀx]O2 prepared at 3 GPa
from precursor with Li/Ni ¼ 1.50. For the sake of comparison, DHpp
and g determined at X-band are given also.
E.Zh., R.S. and E.Sh. acknowledge the National
Science Fund of Bulgaria (Contract no. Ch1304/2003)
for financial support. E.Sh. is grateful to the EC for a
grant of a Marie Curie Host Fellowship Programme
(Contract No. HPMT – CT – 2001 – 00231 to D.C.
Rubie) in order to perform high-pressure synthesis
experiments at the Bayrisches Geoinstitute. The high-
frequency EPR measurements carried out at High
Magnetic Field Laboratory in Grenoble, France, were
supported by European Community ‘‘Access to Re-
search Infrastructure action of the Improving Human
Potential Programme’’. The authors are very grateful to
Dr. T. Boffa-Ballaran, from Bayerisches Geoinstitut—
Universitat Bayreuth, and Dr. A.-L. Barra, from High
Magnetic Field Laboratory in Grenoble, for their help.
clearly that the EPR spectrum of Li1ꢀdNid[LixNi1ꢀx]O2
is due to low-spin Ni3+ ions. However, in contrast to
nearly stoichiometric Li1ꢀdNi1+dO2, the low-spin Ni3+
ions in Li1ꢀdNid[LixNi1ꢀx]O2 are magnetically diluted.
The lack of any additional EPR signals in high-
frequency EPR spectrum further supports that Li1ꢀd
Nid[LixNi1ꢀx]O2 represents a solid solution rather than
complex intergrowth system with local inhomogeneous
regions enriched in Ni3+ or Ni4+ ions.
The preparation of Li[LixNi1ꢀx]O2 oxides under high
oxygen pressure is also proved by magnetic susceptibility
measurements. Using the spin-only values of the magnetic
moments of the Ni2+ and Ni3+ ions (2.83 and 1.73 BM,
respectively), Table 1 shows the values of the average
magnetic moments of the nickel ions calculated for a
cation distribution Li1ꢀdNi2d+[LixNi13ꢀ+3x+dNi24+xꢀd]O2
(mcalc) as well as the experimental values of the magnetic
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