Journal of Physics and Chemistry of Solids 67 (2006) 1347–1350
www.elsevier.com/locate/jpcs
Influence of sulphur substitution on structural and electrical
properties of lithium-manganese spinels
M. Molenda a, R. Dziembaj a,b,*, E. Podstawka b, W. Łasocha a, L.M. Proniewicz a,b
b
a
Faculty of Chemistry, Jagiellonian University, Ingardena 3 Str., 30-060 Cracow, Poland
Regional Laboratory of Physicochemical Analyses and Structure Research, Ingardena 3 Str., 30-060 Cracow, Poland
Abstract
The sulphur substitution of oxygen in the LiMn2O4KySy spinel phase is possible at least up to yZ0.25. The highest concentration of sulphur
causes after standard calcination new phase (Mn2O3 or Mn3O4) formation and segregation. The sulphur ions distorting the octahedral symmetry,
cause appearance of asymmetric A2u band in Raman spectra and suppress Jahn–Teller effect. The electrical conductivity diminishes with increase
of sulphur content but the polaron mechanism seems to be preserved.
q 2006 Elsevier Ltd. All rights reserved.
Keywords: C. Raman Spectroscopy; D. Electrical conductivity; D. Phase transitions
1. Introduction
Lithium manganese spinels are alternative cathode
materials for 4 V lithium-ion batteries. At present, LiCoO2
and LiNi0.5Co0.5O2 are used commercially, but they are toxic
and expensive [1,2]. The LiMn2O4 spinel is cheaper and
environmental friendly. Hence, in the last decade a large
number of papers was devoted to Li–Mn–O spinel and related
systems, reviewed in [1–3]. The capacity of lithium-manganese
spinel is similar to currently used materials, but stoichiometric
LiMn2O4 shows a reversible low temperature phase transition
from cubic (Fd3m) to orthorhombic (Fddd) structure near room
temperature [4]. This transition manifests itself in capacity
fading during cell cycling, retarding development of a new
electrode material based on LiMn2O4.
The phase transition is related to the Jahn–Teller distortion
of high-spin Mn3C ions located in high symmetry (O7h) sites
[5,6] and specific charge ordering in manganese sublattice [7].
Stabilisation of the high temperature structure may be
performed by partially substitution of Mn3C ions by 3d
metal ions (Cr, Fe, Ni, Co) [8,9], Al3C[10] ions or excess of
LiC ions [11,12]. But, these modifications result in decrease of
Mn3C/Mn4C ratio due to the substitution and/or to the charge
compensation in crystal lattice and in consequence causes a
remarkable decrease in 4 V capacity.
* Corresponding author. Tel./fax: C48 12 634 5579.
E-mail address: [email protected] (R. Dziembaj).
0022-3697/$ - see front matter q 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jpcs.2006.01.068
It seems that modification of oxygen sublattice by partially
izoelectronic substitution with sulphur should by sufficient to
avoid the Jahn–Teller effect without change in cathode
capacity. The sulphide ions replacing oxygen ions in the spinel
structure should reduce the symmetry of octahedra, and in
consequence suppress the phase transition.
The goal of this work is to check how much of sulphur can
be introduce to the spinel lattice by the sol–gel method, and to
characterise the obtained materials in relation to their structural
and electrical properties using XRD, DSC, Raman spectroscopy and electrical conductivity measurements.
2. Experimental
A set of sulphided lithium-manganese spinels LiMn2O4KySy
(0.09 %y%0.5) was prepared by the modified sol–gel method
[13] using standardised aqueous solutions of CH3COOLi,
(CH3COO)2Mn, (NH4)2S and concentrated ammonia as the
alkalizing agent. The synthesis was performed under constant
argon flow to prevent uncontrolled oxidation of Mn2C ions. Two
different ways of condensation were used. The condensation of
the first series (Q) of samples was performed in the air
atmosphere at 90 8C for 48 h, while the second one (B) was
condensed under argon (99.999%) atmosphere for the same time
and at the same temperature. Then, the obtained xerogels were
decomposed in air for 24 h at 300 8C with heating rate 1 8C/min.
The obtained residue was grinded in an agate mortar providing a
brown–black powder. The powder was pelletised (5000 kg/cm2)
and the pellets were calcined at 800 8C in the air for 24 h. Then,
the samples were quenched to freeze the high-temperature
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M. Molenda et al. / Journal of Physics and Chemistry of Solids 67 (2006) 1347–1350
Table 1
Chemical composition, preparation conditions and phase analysis of the sulphided spinel samples
Sample
Apparent formula
Preparation
Phase analysis
Q9
Q25
Q50
B20
B30
B40
B50
LiMn2O3.91S0.09
LiMn2O3.75S0.25
LiMn2O3.5S0.5
LiMn2O3.8S0.2
LiMn2O3.7S0.3
LiMn2O3.6S0.4
LiMn2O3.5S0.5
Sol–gel, condensation in the air (90 8C/24h), calcination
in the air at 300 8C and the 800 8C, quenched
100% spinel (aZ0.82630 nm)
100% spinel (aZ0.82801 nm)
51% spinel (aZ0.82438 nm) 49% bixbyite Mn2O3
71% spinel (aZ0.82401 nm) 29% hausmanite Mn3O4
75% spinel (aZ0.82413 nm) 25% bixbyite Mn2O3
42% spinel (aZ0.8241 nm) 58% bixbyite Mn2O3
35% spinel (aZ0.82443 nm) 65% bixbyite Mn2O3
Sol–gel, condensation in Ar (90 8C/24 h), calcination in
the air at 300 8C and the 800 8C, quenched
structure. In our recent work we proved that sulphided spinels
are stable in air up to 900 8C, above this temperature they
decompose with evolution of sulphur oxide [14]. In Table 1 the
chemical composition, conditions of the syntheses and phase
analysis are presented.
The crystal structure of the obtained samples was examined
by X-Ray powder diffraction technique on PW3710 Philips
X’Pert apparatus using Cu Ka1 (lZ0.154178 nm) radiation
with a graphite monochromator. The phase analysis of the
XRD patterns was performed by the Rietveld method using
PDF-2 standards and DBWS-9411 software package [15].
Thin pellets containing 10 mg of the obtained material
and 200 mg of KBr were used for Raman measurement. The
spectra were recorded at room temperature using a triple
grating spectrometer (Jobin Yvon, T 64000). A liquid
nitrogen cooled CCD detector (Jobin Yvon, Model
CCD3000) was used in these measurements. The spectral
resolution of 2 cmK1 was set. An excitation wavelength at
514.5 nm was provided by an Ar-ion laser (Spectra-Physics,
Model 2025). The laser power at the sample position was
about 20 mW (32 W/cm2). Such a low power of the laser
radiation was required to prevent any decomposition of the
samples. Raman scattered light was collected with a 1358
geometry, and 5000 scans were accumulated to ensure an
acceptable signal-to-noise ratio.
DSC experiments were performed in Mettler-Toledo 821e
microcalorimeter equipped with intracooler Haake in 40 ml
closed aluminium crucibles with fZ0.5 mm hole in the lid
under flow of argon (80 ml/min) within temperatures range
K40OC60 8C.
Electrical conductivity was measured on rectangular shape
pellets using a four probe AC method at 33 Hz within the range
of temperatures K20 8COC50 8C. To improve the electrical
contact between the sample and electrodes a silver paste with
acrylic resin was used.
The other samples showed additional 1 or even 2 phases
(Table 1). The best fit of XRD patterns for the B20 sample
was achieved using two phase system: spinel and hausmanite—Mn3O4 (I4/amd). In case of B30 to B50 and Q50
samples the additional phase (besides spinel) giving the best
fit was bixbyite—Mn2O3 (Ia3). Such a phase segregation
caused increase in Li/Mn ratio in the remaining spinel phase,
leading to lowering of the lattice constant. The effect of
sulphur substitution should be just opposite. It may explain
why the lattice constant is stabilised at about 0.824 nm. The
concentration of the spinel phase in calcined materials
decreases with introduction higher and higher amount of
sulphur, showing the limited sulphur substitution up to yZ
0.25. Moreover for the samples Q50, B30, B40 and B50 a
small amount (less than 4%) of Li2SO4$H2O was observed
(peaks around 21.5 and 258).
The Raman spectra of the sulphided lithium-manganese
spinels are presented in Fig. 2, together with the spectrum for
the stoichiometric LiMn2O4. The Raman spectra of the
sulphided spinels are dominated by two bands at 630 and
660 cmK1. The first one is related to the A1g symmetric Mn–O
stretching of the manganese octahedra. It is the same to those
observed for LiMn2O4 spinel. According to Ammundsen et al.
[16] calculations the band at 650 cmK1, related to the A2u
asymmetric Mn–O stretching, should be inactive in the
Raman spectra in case of fully symmetric MnO6 octahedra.
3. Results and discussion
Powder diffraction patterns of the sulphided lithiummanganese spinel are presented in Fig. 1. The set of
diffraction peaks observed for the samples Q9 and Q25 are
related to the regular Fd3m spinel phase. These samples Q9
and Q25 reveal higher lattice constant 0.82630 and
0.82801 nm, respectively—therefore, higher than observed
for stoichiometric LiMn2O4 (aZ0.82517 nm) [11]. It confirms
sulphur substitution of oxygen in the spinel structure.
Fig. 1. XRD patterns of sulphided lithium-manganese spinels. The vertical lines
indicate spinel phase, (*) Mn2O3 bixbyite phase, (!) Mn3O4 hausmanite phase.
M. Molenda et al. / Journal of Physics and Chemistry of Solids 67 (2006) 1347–1350
Fig. 2. Raman spectra of sulphided lithium-manganese spinels and LiMn2O4.
1349
The ratio of integral intensities of the A1g to A2u bands for
the single phase materials (samples Q9 and Q25) is around
1.65. For the other, multiphase samples (Q50, B20, B30, B40
and B50) this ratio is around 0.9, in accordance with higher
distortion of MnO6 due to sulphur substitution of oxygen. An
additional distortion may be evolved by changes in Li/Mn
stoichiometry. Similarly, the observed decrease in the amount
of the spinel phase results in diminishing of the overall
intensity of the Raman spectra.
DSC measurements of the sulphided spinels recorded near
room temperature (K30OC50 8C) are presented in Fig. 3.
The thermal effects observed during cooling and heating on
DSC curves are related to the phase transition from cubic to
orthorhombic structure, similarly to the LiMn2O4 spinel. The
enthalpies (DH) of this transformation are collected in Table 2.
The values are lower than that for the stoichiometric LiMn2O4,
and they decrease with increase of sulphur concentration. This
may suggest structural stabilisation due to sulphur substitution
and that the Jahn–Teller distortion of the high-spin Mn3C ions
is suppressed.
Measurements of the electrical conductivity of sulphided
lithium-manganese spinels are presented in Fig. 4. The
observed nonlinearity of the electrical characteristics is related
to the phase transition. The effect is diminished with increase
of sulphur content. In Table 2 the values of electrical
conductivity at 25 8C and its activation energy in the range
C40OC25 8C are presented. For all the samples decrease of
the electrical conductivity is observed with increase of the
sulphur content. The diminishing of electrical conductivity
may be related to lowering of the spinel phase concentration
(the best conductor in the system). The similarity of the
electrical conductivity characteristics suggests preserving of
the small polaron mechanism.
4. Conclusions
Fig. 3. DSC measurements of sulphided lithium-manganese spinels.
It may be assumed that any octahedral deformation can
activate the A2u band. In accordance to that, all the sulphided
spinels having deformed octahedra show this asymmetric
vibration.
The successful substitution of sulphur for oxygen in the
spinel structure was achieved using the sol–gel method
following by condensation in the air atmosphere. By this
method a single phase sulphided lithium-manganese spinels
(LiMn2 O4KyS y) may be obtained up to yZ0.25. The
substitution of sulphur for oxygen introduced distortion of
the local structure in the spinel crystal lattice, which is
Table 2
Enthalpies (DH) of the phase transition and electrical properties of the sulphided lithium-manganese spinels
Sample
Apparent
formula
Transition around
10O25 8C DH (JgK1)
Electrical conductivity
at 25 8C s (ScmK1)
Activation energy of electrical
conductivity (C40OC25 8C) Ea
(eV)
LiMn2O4
Q9
Q25
Q50
B20
B30
B40
B50
LiMn2O4
LiMn2O3.91S0.09
LiMn2O3.75S0.25
LiMn2O3.5S0.5
LiMn2O3.8S0.2
LiMn2O3.7S0.3
LiMn2O3.6S0.4
LiMn2O3.5S0.5
8.6
6.5
6.2
2.9
4.8
3.9
1.9
1.7
4.37!10K4
1.86!10K4
1.86!10K5
2.75!10K6
5.01!10K5
2.88!10K5
7.41!10K6
–
0.32
0.29
0.32
0.36
0.28
0.24
0.30
0.17
1350
M. Molenda et al. / Journal of Physics and Chemistry of Solids 67 (2006) 1347–1350
authors (M.M.) would like to acknowledge the Foundation for
Polish Science for the support in the form of The Annual
Stipend for Young Scientists.
References
Fig. 4. Electrical conductivity of sulphided lithium-manganese spinels.
manifested by appearance of the asymmetric A2u vibration in
Raman spectra. Diminishing of the octahedral symmetry
suppresses the Jahn–Teller distortion of Mn3C ions. In
consequence phase transition at around room temperature is
weakened with increase of sulphur content as well as the
electrical conductivity. One may assume preservation of the
small polaron mechanism of conductivity. This mechanism
seems not be changed by sulphur substitution in the oxygen
lattice.
Acknowledgements
The work is supported by the Polish Committee for
Scientific Research under grant 3 T08D 010 28. One of the
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