Factors influencing the layered to spinel-like phase transition in layered oxide cathodes

Article abstract of DOI:10.1149/1.1497171

Li_0.5MO₂ [M = Mn, Co, Ni, Ni_1-yCo_y, Ni_1-yMn_y, and Mn_1-y(Cr, Al, Mg)_y] compositions obtained by chemically extracting lithium with Na₂S₂O₈

Full text of DOI:10.1149/1.1497171

Journal of The Electrochemical Society, 149 ͑9͒ A1157-A1163 ͑2002͒
A1157
0013-4651/2002/149͑9͒/A1157/7/$7.00 © The Electrochemical Society, Inc.
Factors Influencing the Layered to Spinel-like Phase Transition
in Layered Oxide Cathodes
*
**,z
S. Choi and A. Manthiram
Materials Science and Engineering Program, The University of Texas at Austin, Austin, Texas 78712, USA
Li0.5MO2 ͓M ϭ Mn, Co, Ni, Ni1ϪyCoy , Ni1ϪyMny , and Mn1Ϫy͑Cr,Al,Mg͒ ͔ compositions obtained by chemically extracting
y
lithium with Na2S2O8 from LiMO2 have been investigated by X-ray diffraction after heating at various temperatures. While
Li0.5MnO2 obtained from both the monoclinic ͑O3 stacked layer structure͒ and orthorhombic LiMnO2 transforms to the spinel-like
phases at ambient temperature during the lithium extraction process due to cation migration, Li0.5NiO2 needs mild heat ͑ϳ150°C͒
to transform to the spinel-like phase. Li0.5CoO2 does not transform even after heating at 200°C. The decrease in the structural
Ͼ
Ͼ
Li0.5MnO2 is explained on the basis of crystal field stabilization energies. Addi-
stability in the order Li0.5CoO2
Li0.5NiO2
tionally, the incorporation of foreign cations into the transition metal planes via cationic substitutions is found to increase the
structural stability by perturbing the cooperativity among the transition metal ions and thereby suppressing the cation migration.
© 2002 The Electrochemical Society. ͓DOI: 10.1149/1.1497171͔ All rights reserved.
Manuscript submitted September 7, 2001; revised manuscript received March 7, 2002. Available electronically July 25, 2002.
with Ni3ϩ and improve the electrochemical performance.20 In fact,
LiNi0.85Co0.15O2 shows much higher practical capacity of around
180 mAh/g compared to that of LiCoO2 ͑140 mAh/g͒ with good
cyclability. However, LiNi0.85Co0.15O2 too experiences a migration
of nickel ions under mild heat to give spinel-like phases, although to
a lesser extent than that found with unsubstituted LiNiO2 .15
The transformation of the layered Li1ϪxMO2 ͑charged cathode͒
phase to a spinel-like phase become facile as both of them have a
cubic close-packed oxygen array. For example, transformation of the
The rapid growth in portable electronic devices has intensified
the research activities in lithium-ion batteries as they provide higher
voltage and energy density compared to other rechargeable systems.
Among the various lithium insertion compounds investigated during
the past three decades,1 transition metal oxides have become attrac-
tive cathodes for lithium-ion cells as they offer higher cell voltage
than, for example, chalcogenides and pnictides. The higher voltage
of oxide cathodes is mainly due to the larger Madelung energy as-
sociated with the more ionic oxide lattice and a positioning of the
O:2p band at a lower energy compared to the 3p, 4p, or 5p energies
of the chalcogenides or pnictides.2 In the case of oxides, the LiMO2
͑Mϭ transition metal͒ layered structure and the LiM2O4 spinel
structure have become attractive as they provide reversible insertion/
extraction of the lithium ions into/from them with adequate elec-
tronic and lithium-ion conductivities.
layered Li0.5MO2 into the ideal cubic spinel phase ͑Li͒
M 16dO4
͔
2
͓
8a
requires a migration of 25% of the transition metal ion M3ϩ/4ϩ from
the octahedral sites ͑3b sites͒ of the M planes into the empty octa-
hedral sites ͑3a sites͒ of the lithium planes and a displacement of the
lithium ions from the 3a sites into the neighboring tetrahedral sites.
The migration of the transition metal ions can be achieved via the
neighboring tetrahedral sites ͑designated as T1 and T2͒, as illus-
trated in Fig. 1. Such a migration of the transition metal ions can
impede the lithium-ion diffusion in the lithium layer and degrade the
electrochemical performance of the layered phases. Moreover, re-
cent theoretical calculations based on first principles modeling have
shown that the spinel structure is energetically more stable com-
pared to the layer structure for the Li0.5MO2 compositions with M
ϭTi, Cr, Mn, Fe, Co, and Ni.21
Among the layered LiMO2 oxides, LiCoO2 is widely used in
commercial lithium-ion cells as it shows good performance and
structural stability during the charge-discharge process.3 However,
cobalt is relatively expensive and toxic and only 50% of the theo-
retical capacity of LiCoO2 could be practically utilized. In this re-
gard, much attention has been focused toward other layered oxides
with M ϭ Ni, Mn, Fe, V, and Cr. Unfortunately, the performance of
these oxides is inferior to that of LiCoO2 . While LiCrO2 is difficult
4
5
to charge, both layered LiVO2 and LiFeO2 suffer from poor cy-
clability arising from cationic migrations. Similarly, both the mono-
6-9
clinic ͑O3 layer structure͒ LiMnO2 that is generally obtained by
an ion exchange of NaMnO2 and the orthorhombic LiMnO210 trans-
form to a spinel-like structure during the charge-discharge process.
LiNiO2 is difficult to synthesize as an ordered layered material11,12
and Ni3ϩ undergoes Jahn-Teller distortion.13,14 Moreover, charged
Li1ϪxNiO2 has been found to experience a migration of nickel ions
under mild heat to give spinel-like phases at moderate
temperatures.15
However, cationic substitutions have been found to help to im-
prove the electrochemical properties in the case of layered manga-
nese and nickel oxides. For example, the substitution of Cr3ϩ and
16-18
Al3ϩ for Mn3ϩ in LiMn1ϪyMyO2
͑M ϭ Cr and Al͒ has been
found not only to access the monoclinic LiMnO2 by conventional
ceramic procedures, but also to improve the stability of the layered
phase and the electrochemical performance during cycling. Simi-
larly, the substitution of Co3ϩ for Ni3ϩ in LiNi1ϪyCoyO219 has been
found not only to improve the ordering of the cations in the layer
lattice, but also to suppress the Jahn-Teller distortion13,14 associated
Figure 1. Illustration of the paths for the migration of the transition metal
ion Mnϩ from the octahedral sites in the transition metal layer to the octa-
hedral sites in the lithium layer via the neighboring tetrahedral sites ͑T1 and
T2͒.
* Electrochemical Society Student Member.
** Electrochemical Society Active Member.
z E-mail: rmanth@mail.utexas.edu
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A1158
Journal of The Electrochemical Society, 149 ͑9͒ A1157-A1163 ͑2002͒
thesized by solid-state reactions of required amounts of LiOH,
Mn2O3 , Cr2O3 , Al2O3 , and Mg͑OH͒ at 1050°C under inert atmo-
2
sphere ͑nitrogen or argon͒ with one intermittent grinding.
Chemical extraction of lithium from the various LiMO2 phases
to obtain Li0.5MO2 was accomplished by stirring for 2 days the
LiMO2 powders in an aqueous solution containing appropriate quan-
tities of the oxidizer, sodium perdisulfate (Na2S2O8). The following
chemical reaction occurs during this process
2LiMO2 ϩ 0.5Na2S2O8 → 2Li0.5MO2 ϩ 0.5Na2SO4
ϩ 0.5Li2SO4
͓1͔
After the chemical extraction reaction, the product Li0.5MO2 formed
was washed with deionized water several times to remove the
soluble Na2SO4 , Li2SO4 , and any unreacted Na2S2O8 . The
Li0.5MO2 samples thus obtained were then heated in air for 3 days at
various temperatures to assess the ease of formation of spinel-like
phases.
Lithium contents in the products were determined by atomic ad-
sorption spectroscopy ͑AAS͒. Structural characterizations were car-
ried out by fitting the XRD data with the Rietveld program. Elec-
trochemical properties were evaluated with coin-type cells using
circular cathodes of 0.64 cm2 area, metallic Li anodes, and LiPF6 in
ethylene carbonate ͑EC͒/diethyl carbonate ͑DEC͒ electrolyte. The
cathodes were fabricated by hand-mixing the samples with 25 wt %
fine carbon for about 30 min and mixing the composite with 5 wt %
polytetrafluoroethylene ͑PTFE͒.
Figure 2. XRD patterns of ͑a͒ LiCoO2 , ͑b͒ Li0.5CoO2 obtained by extract-
ing lithium from LiCoO2 , ͑c͒ Li0.5CoO2 after heating at 150°C, ͑d͒
Li0.5CoO2 after heating at 250°C, and ͑e͒ Li0.5CoO2 after heating at 400°C
for 3 days in air.
Results and Discussion
Comparison of the structural stability of Li0.5MO2 (MϭMn, Co,
and Ni).—Figure 2 gives the XRD patterns of LiCoO2 and
Li0.5CoO2 that was obtained by chemical lithium extraction and the
evolution of the phases on subjecting Li0.5CoO2 to mild heat. The
as-prepared Li0.5CoO2 maintains the rhombohedral layer structure of
the parent LiCoO2 as indicated by the separation between the ͑018͒
and ͑110͒ reflections centered around 2␪ ϭ 66° ͑Fig. 2a and b͒. The
Li0.5CoO2 sample also maintains the layer structure even after heat-
ing at moderate temperatures ͑Fig. 2c and d͒, although the ͑018͒ and
͑110͒ reflections have moved slightly toward each other. At high
enough temperatures, Li0.5CoO2 disproportionates to give Co3O4
impurity ͑Fig. 2e͒. These observations reveal the good structural
stability of LiCoO2 and attest to its excellent cyclability in the
lithium-ion cells. Although the formation of a spinel phase has been
claimed from transmission electron microscopy ͑TEM͒ studies in
cycled LiCoO2 cathodes by Wang et al.,22 our experiments with
chemically prepared bulk Li0.5CoO2 reveal that no spinel-like phase
is formed even after heating to 250°C ͑Fig. 2͒.
Figure 3 shows the evolution of the phases on subjecting the
chemically prepared Li0.5NiO2 to mild heat. The as-prepared
Li0.5NiO2 ͑Fig. 3b͒ maintains the rhombohedral layer structure like
the parent LiNiO2 ͑Fig. 3a͒, as indicated by the separation between
the ͑018͒ and ͑110͒ reflections centered around 2␪ ϭ 65°. How-
ever, on heating to 200°C, the rhombohedral Li0.5NiO2 transforms to
a cubic phase ͑Fig. 3c͒, as indicated by the merger of the ͑018͒ and
͑110͒ reflections into a single reflection of ͑440͒. On further heating
Despite the observation of the formation of spinel-like phases
during the electrochemical cycling of certain layered oxides, no sys-
tematic study focusing on the factors driving such transformations is
available in the literature. Identification of the various factors influ-
encing the layered to spinel-like phase transition can help to design
structurally stable cathode hosts with high energy density. We
present here a systematic investigation of the layered to spinel-like
phase transition by evaluating the structural stability of bulk
Li0.5MO2 ͓M ϭ Mn, Co, Ni, Ni1ϪyCoy , Ni1ϪyMny , and
Mn1Ϫy͑Cr,Al,Mg͒ ͔ compositions obtained by chemically extracting
y
lithium from the corresponding LiMO2 phase. The structural stabil-
ity is assessed by comparing the X-ray diffraction ͑XRD͒ patterns
recorded before and after heating the bulk Li0.5MO2 samples at vari-
ous temperatures. From the results obtained, the factors that drive
the transformation and help to suppress the transformation are iden-
tified. Additionally, the understanding gained with cationic substitu-
tions is applied to design structurally stable layered lithium manga-
nese oxides.
Experimental
LiCoO2 was synthesized by solid-state reaction between required
amounts of Li2CO3 and Co3O4 at 800°C in air. LiNi1ϪyCoyO2
samples with 0 р y р 0.75 were synthesized by a sol-gel proce-
dure described elsewhere,16 in which the gel obtained with an acetic
acid solution containing required amounts of Li2CO3, cobalt acetate,
and nickel acetate was fired at 800°C under flowing oxygen for 24 h.
LiNi1ϪyMnyO2 samples with 0 р y р 0.5 were synthesized by fir-
ing coprecipitated hydroxides of nickel and manganese with lithium
hydroxide at 800°C under flowing oxygen. The coprecipitation of
the nickel and manganese hydroxides were achieved by adding
KOH to the corresponding acetate solution followed by filtering,
washing, and drying of the product. Orthorhombic LiMnO2 was
synthesized by solid-state reaction between Li2CO3 and Mn2O3 at
1000°C under flowing nitrogen. Monoclinic LiMnO2 was obtained
by ion-exchanging ␣-NaMnO2 with LiBr in hexanol at 150°C.
Monoclinic LiMn1ϪyMyO2 ͑M ϭ Cr, Al, or Mg͒ samples were syn-
Ͼ
to T
200°C, the cubic phase begins to disproportionate with a
loss of oxygen to give LiNiO2 and NiO ͑Fig. 3e͒.
Rietveld analysis of the cubic phase obtained at 200°C with
¯
Fd3m space group indicates the formation of a spinel-like phase,
but with the nickel ions present in both the 16d and 16c sites ͑Fig.
4͒. As pointed out in the introduction, transformation of the layered
Li0.5NiO2 into the ideal spinel phase ͑Li͒ Ni 16dO4 requires the
͓
8a
͔
2
migration of 25% of the nickel ions from the nickel planes ͑3b sites͒
into the lithium planes ͑3a sites͒ of the initial rhombohedral layer
structure. The presence of nickel ions in both the 16d and 16c sites
indicates that the migration is incomplete at the moderate tempera-
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Journal of The Electrochemical Society, 149 ͑9͒ A1157-A1163 ͑2002͒
A1159
Figure 5. XRD patterns of ͑a͒ orthorhombic LiMnO2 , ͑b͒ Li0.5MnO2 ob-
tained from orthorhombic LiMnO2 , ͑c͒ monoclinic LiMnO2 , ͑d͒ Li0.5MnO2
obtained from monoclinic LiMnO2 , and ͑e͒ sample d after heating at 150°C.
Figure 3. XRD patterns of ͑a͒ LiNiO2 , ͑b͒ Li0.5NiO2 obtained by extracting
lithium from LiNiO2 , ͑c͒ Li0.5NiO2 after heating at 150°C, ͑d͒ Li0.5NiO2
after heating at 200°C, and ͑e͒ Li0.5NiO2 after heating at 300°C for 3 days.
tures of 200°C. Further increase in temperature to complete the mi-
difficulties in the long-term cyclability of LiNiO2 cathodes at el-
evated temperatures, unlike in the case of LiCoO2 cathodes.15
Figure 5 gives the XRD patterns of monoclinic and orthorhombic
LiMnO2 and the Li0.5MnO2 samples obtained from them by chemi-
cal lithium extraction. The data reveal that in both cases, a nearly
spinel-like phase is already formed during the chemical delithiation
process at ambient temperature, which is consistent with that found
by several groups during electrochemical cycling.6-10 Between the
two systems, the orthorhombic LiMnO2 appears to form the spinel-
like phase more readily compared to the monoclinic LiMnO2 , as
indicated by the formation of a single nearly spinel-like phase at
ambient temperature in the former case. In the case of monoclinic
LiMnO2 , a spinel-like phase with some additional unidentified re-
flections is found after lithium extraction at room temperature, and a
single spinel-like phase is formed on heating at 150°C.
gration and obtain the ideal spinel ͑Li͒ Ni 16dO4 , however, re-
͓
8a
͔
2
sults in a disproportionation of the phase to give NiO impurity. The
results thus clearly reveal that it is difficult to access the ideal spinel
͑Li͒ Ni 16dO4 , although it has been suggested11,23 to be formed
͓
8a
͔
2
before by simply looking at the merger of the ͑018͒ and ͑110͒ re-
flections in the X-ray pattern. Nevertheless, the tendency of nickel
ions to migrate to the lithium planes under mild heat may pose some
A comparison of the results obtained with the three systems with
M ϭ Co, Ni, and Mn indicates that the tendency of the layered
Li0.5MO2 to transform to cubic spinel-like phase decreases in the
Ͼ
Ͼ
Li0.5CoO2 . While a spinel-like
order Li0.5MnO2
Li0.5NiO2
phase is readily formed during lithium extraction for M ϭ Mn, a
mild temperature is needed for the case of M ϭ Ni. It is difficult to
obtain a spinel-like phase for M ϭ Co even after heating at 200°C.
As pointed out earlier, the transformation of the layered phase to a
spinel-like phase involves the migration of the Mnϩ ions from the
octahedral sites of transition metal layer to the octahedral sites of the
lithium layer via the nearby tetrahedral sites. Therefore, the relative
stability of the Mnϩ ions in the octahedral vs. tetrahedral sites may
play a critical role in the relative ease of formation of the spinel-like
phases.
Figure 4. Rietveld refinement results of Li0.5NiO2 after heating at 200°C for
3 days in air. The observed and calculated X-ray profiles, peak positions, and
the difference between the observed and calculated profiles are shown. Also,
the cation distribution, R factors, and lattice parameter are given.
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A1160
Journal of The Electrochemical Society, 149 ͑9͒ A1157-A1163 ͑2002͒
Table I. CFSEs and OSSEs of nickel, cobalt, and manganese ions.
Octahedral
coordination
Tetrahedral
coordination
Configurationa
Configurationa
CFSE
e
CFSEb
Ion
t2g
eg
t2
OSSE
Ni3ϩ:3d7
Ni4ϩ:3d6
Co3ϩ:3d6
Co4ϩ:3d5
Mn3ϩ:3d4
Mn4ϩ:3d3
Ϫ18 Dq
Ϫ24 Dq
Ϫ24 Dq
Ϫ20 Dq
Ϫ6 Dq
Ϫ5.33 Dq
Ϫ2.67 Dq
Ϫ2.67 Dq
0.00
Ϫ1.78 Dq
Ϫ3.56 Dq
Ϫ12.67 Dq
Ϫ21.33 Dq
Ϫ21.33 Dq
Ϫ20.00 Dq
Ϫ4.22 Dq
Ϫ8.44 Dq
6
6
6
5
3
3
1
0
0
0
1
0
͑LS͒
͑LS͒
͑LS͒
͑LS͒
͑HS͒
͑HS͒
4
3
3
2
2
2
3
3
3
3
2
1
͑HS͒
͑HS͒
͑HS͒
͑HS͒
͑HS͒
͑HS͒
Ϫ12 Dq
a LS and HS refer, respectively, to low spin and high spin configurations.
b Obtained by assuming ⌬t ϭ 4/9⌬o ; ⌬t and ⌬o refer, respectively, to tetrahedral and octahedral splittings.
Table I gives the crystal field stabilization energies ͑CFSEs͒ for
the octahedral and tetrahedral coordination along with the octahedral
site stabilization energies ͑OSSEs͒, which is the difference between
the CFSE values of octahedral and tetrahedral coordination, for vari-
ous ions. For the M3ϩ ions ͓low-spin ͑LS͒ configurations for Ni3ϩ
and Co3ϩ and high-spin ͑HS͒ configuration for Mn3ϩ in octahedral
cation-substituted Li0.5Ni0.5Mn0.5O2 does not, even after heating at
150°C. The increased stability achieved by the substitution of Mn
for Ni suggests that the presence of multiple cations ͑mixed cations͒
in the transition metal plane suppresses cation migration and the
formation of spinel-like phases, since the structural stability of
Li0.5MnO2 is poorer than that of Li0.5NiO2 . Further increase in tem-
perature leads to a segregation of the layered Li0.5Ni0.5Mn0.5O2 into
a layered and a spinel phase, as indicated by the Rietveld refinement
of the data for the sample heated at 200°C. We have also carried out
a Rietveld refinement of the X-ray data of other Li0.5Ni1ϪyMnyO2
compositions with y ϭ 0.1 and 0.3 after heating at 200°C. A com-
parison of the phase fractions obtained after heating the
Li0.5Ni1ϪyMnyO2 compositions at 200°C indicates that the fraction
of the spinel phase decreases and that of the layered phase increases
3ϩ
geometry͔, the OSSE values increase in the order Mn3ϩ
Ni
Ͻ
Co3ϩ. A larger OSSE value for the Co3ϩ ions makes the migra-
tion via the tetrahedral site difficult, and therefore, the formation of
a spinel-like phase becomes difficult for M ϭ Co. A smaller OSSE
value for Mn3ϩ makes the formation of spinel-like phase easier.
Additionally, the ability of the M3ϩ ions to disproportionate to
M2ϩ and M4ϩ may also play a role in assisting the formation of
Ͻ
spinel-like phases. Mn3ϩ is well known to disproportionate easily to
24
Mn4ϩ and Mn2ϩ
.
The Mn2ϩ ions formed by disproportionation
can readily migrate to the lithium planes via the empty tetrahedral
sites as the HS Mn2ϩ with a d5 electronic configuration has no
OSSE. Compared to Mn3ϩ, the disproportionation of both Co3ϩ and
Ni3ϩ is energetically less favorable.
The observed trend in cation migration and structural stability
correlates with the electrochemical performance of the layered oxide
cathodes. LiCoO2 with a good structural stability provides the best
electrochemical performance, while LiMnO2 with a poor structural
stability provides the worst electrochemical performance among the
three layered LiMO2 systems compared here. One of the ways that
has been found to improve the electrochemical performance is cat-
ionic substitutions. For example, as pointed out previously, the sub-
20
stitutions of cobalt for nickel in LiNi1ϪyCoyO2 and chromium for
16,17
manganese in LiMn1ϪyCryO2
have been found to improve the
electrochemical performance. In the next section, we examine how
the cationic substitutions influence the cationic migration and struc-
tural stability.
Structural stability of cation-substituted Li0.5Ni1ϪyCoyO2 and
Li0.5Ni1ϪyMnyO2.—With an objective to understand the influence of
cationic substitutions on structural stability, we investigated the
Li0.5Ni1ϪyCoyO2 (0 р y р 1) and Li0.5Ni1ϪyMnyO2 (0 р y
р 0.5) systems. Figure
6 compares the XRD patterns of
Li0.5Ni1ϪyCoyO2 (0 р y р 1) after heating at 200°C. The data re-
veal that the structural stability increases with increasing cobalt con-
tent, as indicated by the separation between the ͑018͒ and ͑110͒
reflections. It is clear that the substitution of cobalt for nickel sup-
presses the formation of spinel-like phase.
Figure 7 shows the XRD patterns of Li0.5Ni0.5Mn0.5O2 (y
ϭ 0.5) before and after heating at various temperatures. The ͑018͒
and ͑110͒ reflections centered around 2␪ ϭ 65° do not merge even
after heating at 150°C, suggesting the difficulty of formation of the
spinel-like phase. It is interesting to note that while both Li0.5NiO2
͑Fig. 3͒ and Li0.5MnO2 ͑Fig. 5͒ transform to cubic spinel-like
phases, respectively, at 150-200°C and room temperature, the
Figure 6. Comparison of the XRD patterns of the 200°C heated ͑a͒
Li0.5NiO2 ,
͑b͒
Li0.5Ni0.75Co0.25O2 ,
͑c͒
Li0.5Ni0.5Co0.5O2 ,
͑d͒
Li0.5Ni0.25Co0.75O2 , and ͑e͒ Li0.5CoO2 .
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Journal of The Electrochemical Society, 149 ͑9͒ A1157-A1163 ͑2002͒
A1161
Table II. List of cation-substituted LiMn1Ày„Cr,Al,Mg…yO2
phases.
Sample
Composition
1
2
3
4
5
Li0.5Cr0.1Mn0.9O2
Li0.5Cr0.07Al0.03Mn0.9O2
Li0.5Cr0.15Mn0.85O2
Li0.5Cr0.07Al0.06Mg0.02Mn0.85O2
Li0.5Cr0.15Al0.05Mn0.80O2
Cation-substituted
Li0.5Mn1Ϫy(Cr,Al,Mg)yO2,
structural
stability.—Recognizing that the presence of mixed cations helps to
suppress the cation migration and the spinel phase formation, we
then applied this strategy to improve the structural stability of lay-
ered Li1ϪxMnO2 , which tends to form spinel-like phases at room
temperature during electrochemical cycling.6-9 As discussed in the
introductory remarks, the substitution of a small amount of
16-18
Cr3ϩ and Al3ϩ for Mn3ϩ in LiMn1ϪyMyO2
͑MϭCr and Al͒ is
known to improve the electrochemical performance. Such substitu-
tions also help to access the monoclinic LiMnO2 by conventional
high-temperature synthesis procedures without requiring the ion ex-
change of NaMn1ϪyMyO2 . We investigated the substitution of sev-
eral cations such as Mg2ϩ, Al3ϩ, Ti4ϩ, Cr3ϩ, and Co3ϩ for manga-
nese. While the substitution of Ti4ϩ and Co3ϩ resulted in the
formation of impurity phases, single-phase samples could be ob-
tained with the substitution of Cr3ϩ, Mg2ϩ, and Al3ϩ as given in
Table II, and illustrated by the XRD patterns in Fig. 9.
The samples in Table II were subjected to lithium extraction to
obtain Li0.5Mn1Ϫy͑Cr,Al,Mg͒yO2 and then heated at various tem-
peratures: T ϭ 100, 120, 150, and 200°C. The XRD patterns of the
as-prepared Li0.5Mn1Ϫy͑Cr,Al,Mg͒yO2 are shown in Fig. 10. The
extent of layered to spinel-like transformation was assessed by
monitoring the variation of the c/a ratio with heating temperature.
Figure 7. XRD patterns of ͑a͒ Li0.5Mn0.5Ni0.5O2 , ͑b͒ Li0.5Mn0.5Ni0.5O2 after
heating at 150°C, ͑c͒ Li0.5Mn0.5Ni0.5O2 after heating at 200°C, ͑d͒
Li0.5Mn0.5Ni0.5O2 after heating at 300°C, and ͑e͒ Li0.5Mn0.5Ni0.5O2 after heat-
ing at 500°C for 3 days in air.
with increasing Mn content y ͑Fig. 8͒. It is possible that the layered
phase may be richer in nickel and the spinel phase may be richer in
manganese. This observation of a decrease in the spinel fraction
with increasing manganese substitution again suggests that the pres-
ence of mixed cations in the transition metal plane suppresses cation
migration.
Figure 8. Variations with manganese content y of the molar percent of the
layered and spinel-like phases present in the 200°C heated
Li0.5Ni1ϪyMnyO2 .
Figure 9. XRD patterns of LiMn1Ϫy͑Cr,Al,Mg͒yO2 : ͑a͒ sample 1, ͑b͒
sample 2, ͑c͒ sample 3, ͑d͒ sample 4, and ͑e͒ sample 5 in Table II.
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A1162
Journal of The Electrochemical Society, 149 ͑9͒ A1157-A1163 ͑2002͒
ing at 200°C. The data clearly demonstrate that the cationic substi-
tutions help to suppress the migration of cations and the formation
of spinel-like phases.
Several additional points could also be recognized by having a
close examination of the data in Fig. 11. First, the structural stability
increases with increasing degree of substitution. For example,
sample 5 in Table II that has the highest degree ͑15% Cr and 5% Al͒
of substitution shows the best structural stability by maintaining the
highest c/a ratio of 4.94, even after heating at 200°C. The structural
stability of sample 5 is also evident by the clear separation between
the ͑018͒ and ͑110͒ reflections in Fig. 10. Second, at a given degree
of substitution, the substitution of two different types of cations
instead of a single type of cation gives better structural stability. For
example, sample 2 with 7% Cr and 3% Al substitutions exhibits
slightly better stability than sample 1 with 10% Cr substitution on
heating. However, between samples 3 and 4, sample 4 with three
different types of substituents ͑7% Cr, 6% Al, and 2% Mg͒ tends to
show a faster decrease in the c/a ratio on heating compared to that
of sample 3 that has a single substituent ͑15% Cr͒. Although sample
4 has a higher c/a ratio in the as-prepared state compared to sample
3, the latter has a higher c/a ratio than the former after heating at
Figure 10. XRD patterns of the Li0.5Mn1Ϫy͑Cr,Al,Mg͒yO2 samples obtained
by extracting 50% of lithium from ͑a͒ LiMnO2 , ͑b͒ sample 1, ͑c͒ sample 2,
͑d͒ sample 3, ͑e͒ sample 4, and ͑f͒ sample 5 in Table II. The magnification of
the pattern over a small 2␪ range is shown on the right.
Ͼ
T
100°C. Thus, the effectiveness of suppressing the cation mi-
gration also depends on the nature of the substituent cation. The data
reveal that Cr3ϩ with strong preference for octahedral sites ͑larger
OSSE value͒ is more effective than the other substituents indicated
in Table II in suppressing the cation migration.
It appears that the migration of the transition metal ions requires
cooperation of the neighboring cations, which is perturbed and sup-
pressed by the presence of other foreign cations ͑mixed cations͒ in
the transition metal plane. Additionally, as stated previously, the
migration of manganese ions may involve first a disproportionation
As the transformation of the layered phase to the spinel-like phase
progresses, the c/a ratio decreases and reaches the ideal value of
4.90 at the cubic symmetry.
Figure 11 shows the variations of c/a ratio with heating tempera-
tures for the samples listed in Table II along with that for
Li0.5MnO2 . In the case of as-prepared samples, the cation-
substituted Li0.5Mn1Ϫy͑Cr,Al,Mg͒yO2 compositions have a higher
c/a ratio compared to that of Li0.5MnO2 . The c/a ratios of all the
samples decrease with increasing temperature, but for a given heat-
ing temperature, the cation-substituted compositions have a higher
c/a ratio than Li0.5MnO2 . The Li0.5MnO2 sample becomes cubic
with a c/a ratio of 4.90 at 120°C, while most of the cation-
substituted samples maintain a c/a ratio of Ͼ4.90, even after heat-
Figure 12. Charge/discharge curves indicating the cyclability of ͑a͒ ortho-
rhombic LiMn2 , ͑b͒ monoclinic layered LiMnO2 , ͑c͒ sample 3 in Table II,
and ͑d͒ sample 5 in Table II in the voltage range 2.5-4.0 V at a rate of
0.03 mA/cm2.
Figure 11. Variations of the c/a ratios of the Li0.5Mn1Ϫy͑Cr,Al,Mg͒yO2
samples with heating temperature.
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Journal of The Electrochemical Society, 149 ͑9͒ A1157-A1163 ͑2002͒
Acknowledgment
A1163
of Mn3ϩ ions into Mn2ϩ and Mn4ϩ and then a migration of the
Mn2ϩ ions from the octahedral sites to the tetrahedral sites. If so, the
substitution of other cations can perturb such a disproportionation
and the cation migration.
This work was supported by the Welch Foundation grant F-1254
and Texas Advanced Technology Program grant 003658-0488-1999.
The University of Texas at Austin assisted in meeting the publication
costs of this article.
With an objective to correlate the structural stability to the elec-
trochemical properties, we have also investigated the electrode per-
formance of some of the compositions listed in Table II. The elec-
trochemical cycling data of samples 3 and 5 in Table II that show the
best structural stability are compared with those of monoclinic
LiMnO2 and orthorhombic LiMnO2 in Fig. 12. Both the monoclinic
and orthorhombic LiMnO2 show a clear development of the 3 V
plateau characteristic of the formation of spinel-like phases within
ten cycles ͑Fig. 12a and b͒. Both samples 3 and 5 do not show the
development of a clear 3 V plateau at least within the first ten cycles.
However, the cation-substituted samples also show capacity fading,
since the layered to spinel-like transformation is not fully elimi-
nated, as evidenced from the X-ray patterns and the change in the
c/a ratio.
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