Doping effects on structure and electrode performance of K-birnessite-type manganese dioxides for rechargeable lithium battery

Article abstract of DOI:10.1016/j.electacta.2007.11.038

The potassium birnessites doped with Al, Ni, and Co were prepared by calcination and aqueous treatment, which showed that single phase products were obtained with Ni and Al up to 5 at.% and Co up to 25 at.% addition to strating KMnO₄. The discharge-recharge capacities and capacity retentions in an aprotic Li cell were not improved by the Ni and Al dopings, but those of the cobalt doped birnessite were improved. The initial discharge capacities of the undoped and cobalt doped birnessites were 170 and 200 mAh g^-1 with capacity retentions of 56 and 80% during the initial 20 cycles, respectively. The reasons for the improvement of the battery performance by Co doping were considered as follows: (i) a change in the stacking structure, (ii) a decrease in the charge transfer resistance, and (iii) improved structural stability of the oxide. Their micro structures were evaluated by X-ray diffraction, photoelectron and Raman spectroscopies, and electron microscopy. Also, potassium birnessite synthesized by adding about 3 times excess potassium indicated that the stacking structure was similar to the 30 at.% cobalt doping sample, furthermore, the better capacity retention was achieved as cathode in a Li cell.

Full text of DOI:10.1016/j.electacta.2007.11.038

Available online at www.sciencedirect.com
Electrochimica Acta 53 (2008) 3084–3093
Doping effects on structure and electrode performance of K-birnessite-type
manganese dioxides for rechargeable lithium battery
A. Ogataa, S. Komabaa,∗, R. Baddour-Hadjeanb, J.-P. Pereira-Ramosb, N. Kumagaic
a Department of Applied Chemistry, Faculty of Science, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku, Tokyo 162-8601, Japan
b Institut de Chimie et des Mate´riaux Paris Est, CNRS-UMR 7182, GESMAT 2 rue Henri-Dunant, 94320 Thiais, France
c Department of Frontier Materials and Functional Engineering, Graduate School of Engineering, Iwate University, Iwate 020-8551, Japan
Received 25 September 2007; received in revised form 6 November 2007; accepted 11 November 2007
Available online 22 November 2007
Abstract
The potassium birnessites doped with Al, Ni, and Co were prepared by calcination and aqueous treatment, which showed that single phase
products were obtained with Ni and Al up to 5 at.% and Co up to 25 at.% addition to strating KMnO4. The discharge–recharge capacities and
capacity retentions in an aprotic Li cell were not improved by the Ni and Al dopings, but those of the cobalt doped birnessite were improved. The
initial discharge capacities of the undoped and cobalt doped birnessites were 170 and 200 mAh g−1 with capacity retentions of 56 and 80% during
the initial 20 cycles, respectively. The reasons for the improvement of the battery performance by Co doping were considered as follows: (i) a change
in the stacking structure, (ii) a decrease in the charge transfer resistance, and (iii) improved structural stability of the oxide. Their micro structures
were evaluated by X-ray diffraction, photoelectron and Raman spectroscopies, and electron microscopy. Also, potassium birnessite synthesized
by adding about 3 times excess potassium indicated that the stacking structure was similar to the 30 at.% cobalt doping sample, furthermore, the
better capacity retention was achieved as cathode in a Li cell.
© 2007 Elsevier Ltd. All rights reserved.
Keywords: Birnessite; Manganese dioxide; Lithium intercalation; Lithium battery; Positive electrode
1. Introduction
3 V vs. Li and high discharge capacities during initial cycling,
though there is the problem of structural collapse resulting in
discharge capacity fading [5,15]. In order to improve the battery
performance, Franger et al. [15] and Tsuda et al. [13] reported
the enhancement of the cycle performance by Co doping in
birnessite.
Previously, we reported the synthesis of a birnessite by
utilizing the thermal decomposition of KMnO4 and its electro-
chemical performance in Li cells [9,10]. In the present paper,
we report that potassium birnessites made by thermal decom-
position reaction of KMnO4 mixed with Co, Ni, Al, Mn, and K
nitrates, and we investigated the relation between the synthetic
conditions and electrochemical performance.
Manganese oxides have been studied for electrochemical
energy storage due to being a low toxic and inexpensive material
which form lamellar, spinel and tunnel frameworks [1–3]. The
electrochemically active materials, which have a working poten-
tial for Li insertion/deinsertion around 3–4 V vs. Li, are strong
candidates for lithium (ion) batteries. As one of the manganese
oxides, the birnessite-type manganese oxide is a hydration-
layered manganese oxide, which forms MnO2 sheets of MnO6
octahedra assembled in layers and has cations of pillar (mainly
alkaline ion) and lattice water in the interlayer space. Up to now,
the syntheses of the birnessite are achieved by various methods
such as a calcination reaction [4–12], sol–gel synthesis [12–15],
hydrothermal synthesis [16,17], etc. The structural details of
potassium-containing birnessite were investigated as previously
reported [4–8]. These materials show a working potential around
2. Experimental
The layered manganese dioxides doped with Al, Ni, or Co
were obtained by the calcinations of mixtures (about 0.5 g) of
KMnO4 and Al, Ni, or Co nitrate, respectively, which were dis-
solved in several ml of distilled water, at 600 ◦C (50 ◦C h−1
)
∗
Corresponding author. Tel.: +81 3 5228 8749; fax: +81 3 5228 8749.
E-mail address: komaba@rs.kagu.tus.ac.jp (S. Komaba).
for 5 h in air. The calcined sample was ultrasonically treated
0013-4686/$ – see front matter © 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2007.11.038

A. Ogata et al. / Electrochimica Acta 53 (2008) 3084–3093
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for 5 min in a sufficient amount of water. The precipitate in the
ultrasonically treated solution was then separated by filtration,
washed with water until neutral, and dried at 80 ◦C for 24 h in
air. We synthesized the samples using metal nitrate addition with
the molar ratios Me/(Me +Mn) (Me = Al, Ni, and Co) of 0, 0.05,
0.15, 0.25 and 0.50 in strating materials, and these samples were
hereafter defined as undoped, Me-5, Me-15, Me-25 and Me-50,
respectively.
KMnO4 powder was mixed with manganese or potassium
nitrate at the molar ratios of K/Mn = 1/3, 1/2, 1 (pure KMnO4),
2, 3, 5, or 10, and we called the calcined samples as Mn-3, Mn-2,
undoped, K-2, K-3, K-5, and K-10, respectively. These calcined
samples were obtained under the same calcination conditions
and ultrasonically treated in water, filtered and dried.
The phase identification of the products was carried out
by powder X-ray diffractometry (XRD) using Cu K␣ radia-
tion. The composition of K, Mn and Co in the products was
determined by atomic absorption spectrometry (AAS) after dis-
solving the products in HCl and/or H2O2 aqueous solutions.
Water content was measured from weight loss by heating at
300 ◦C. The morphology of the products was observed by trans-
mission electron microscopy (TEM). The valences of the Mn
and Co in the products were evaluated by X-ray photoelec-
tron spectroscopy (XPS). The thermal behavior of the starting
mixture and obtained samples was examined by thermogravime-
try (TG) at 10 ◦C min−1. Raman scattering (RS) spectra were
taken between 140 and 1000 cm−1 at room temperature in
a backscattering configuration. A confocal Labram HR 800
(HORIBA JobinYvon) spectrometer combined with an open
microscope and a X,Y Piezoelectric stage to position the sam-
ple with the best accuracy and repeatability was used. The laser
light source was the 514.5 nm line radiation from an Argon
ion (514.5 nm) laser. The scattered light is collected by the
objective in a confocal geometry, and is dispersed onto an air
cooled CCD array by a 1800 lines/mm grating. The investi-
gated area is in the order of 1 ␮m2. The frequency stability
and the accuracy of the apparatus were checked recording the
Raman spectrum of silicon. To avoid sample photodecompo-
sition or damaging, RS spectra were recorded using a low
excitation power of 20 ␮W, an increase in lattice temperature
generally resulting in a shift of Raman peak wavenumbers up to
the formation of Mn3O4 oxide. A decomposition of the spec-
tra within the limits 300 and 800 cm−1 was performed with
the LABSPEC® automatic fitting software provided with the
spectrometer.
Fig. 1. TG curve of thermal decomposition reaction of KMnO4 between room
temperature to 600 ◦C at 10 ◦C min−1
.
3. Results and discussion
Fig. 1 shows the TG curve of the KMnO4 powder. Below
230 ◦C, the weight of KMnO4 is constant. At temperatures
higher than 230 ◦C, the weight quickly decreased to 85.5%, and
then the weight did not change up to 600 ◦C. The weight loss
around 250 ◦C is due to the thermal decomposition of KMnO4
as previously reported [11]. The weight loss is expressed by the
following equation.
5KMnO4 → 3MnO2 + K2MnO4 + K3MnO4 + 3O2
Based on this thermal decomposition, the weight loss due to the
O2 evolution is estimated to be about 12 wt% which corresponds
to the experimental weight loss around 250 ◦C. Therefore, it
indicated the formation of an MnO2 product and potassium
manganates at >300 ◦C. Since these potassium manganates were
removedfromthesampleafter ultrasonically treatingwithwater,
we obtained the single phase of a layered MnO2, at that time,
potassium ions and water molecules were incorporated in an
MnO2 product, resulting in KxMnO2·nH2O. Previously, we
reported that the K-birnessite-type manganese oxide obtained
by calcination at 600 ◦C and washing with water indicated better
battery performance [9,10].
3.1. Doping with Al, Ni, and Co
The XRD patterns of the synthesized products from mixtures
of KMnO4 and the Al, Ni, or Co nitrates at 600 ◦C are shown
in Fig. 2. Although the single phase of birnessite was obtained
by the thermal decomposition of pure KMnO4, the products
depended on the addition of the nitrates.
Fig. 2 confirmed that a single phase was produced when Co
nitratewasaddedupto25%. TheCo-50, whichcontainedCo3O4
as an impurity, was formed. The Al-5 and Ni-5 samples were
a single phase of birnessite. For the Al-15 and Al-25 samples,
there appeared diffraction peaks of birnessite with several unin-
dexed peaks, indicating that a single phase was not obtained.
From the diffraction patterns of Ni-15 and Ni-25, the birnes-
site appeared together with NiO as an impurity. These results
For the electrochemical measurements, the positive electrode
mixtures consisted of manganese oxide, graphite, acetylene
black as the conductive agent, and poly(vinylidene fluoride)
as the binder in the weight ratio of 7:1:1:1. Lithium foil was
used for the counter electrodes. The electrolyte was 1 mol dm−3
LiClO4-propylene carbonate (PC). The discharge–recharge tests
were carried out between 2.0 and 4.3 V vs. Li/Li+ at 20 mA g−1
.
The alternating current (AC) impedance measurement before
and after the initial discharge was carried out in the frequency
region between 1 mHz and 100 kHz, and the AC amplitude was
10 mV. Cells were stored for 24 h at open circuit prior to these
measurements.

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A. Ogata et al. / Electrochimica Acta 53 (2008) 3084–3093
capacity at 4 V gradually increases during successive cycling.
It is likely that the appearance of 4 V plateau was due to the
phase transition from birnessite to spinel during Li intercala-
tion/deintercalation, as observed for the other MnO2 compounds
[18].
Fig. 4 shows the relation between the discharge capacity of
these birnessites and cycle number. The maximum discharge
capacities were reached at the 2nd cycle, and then the capacities
gradually decreased during the following cycles for all samples.
After 20 cycles, the discharge capacities of the undoped, Al-
5, and Ni-5 decreased from 172, 161, and 155 to 97, 86, and
96 mAh g−1, respectively. Their capacity retention ratios were
about 56–62%. The Co-doping improved not only the discharge
capacity, but also the capacity retention, i.e., the second capacity
of 193 mAh g−1 decreased to 134 mAh g−1 at the 20th cycle. Its
capacity retention increased from 56 to 70% due to the Co-
doping. We will discuss the details of the Co doped birnessite in
the next section.
3.2. Improvement by Co doping
Fig. 5(I) shows the XRD patterns of the undoped and the
Co-5, −15, and −25 products. All the diffraction peaks can
be indexed as birnessite-type manganese oxides without any
secondary phase peaks, that is, no impurity phase, and the sin-
gle phase of birnessite was prepared by calcining mixtures of
KMnO4 and Co nitrate. Their d values of 0 0 l around 2θ = ca.
12◦ ofundoped, Co-5, Co-15, andCo-25are7.22, 7.18, 7.18, and
Fig. 2. XRD patterns of synthesized samples, (a) undoped, (b) Co-5, (c) Co-15,
(d) Co-25, (e) Co-50, (f) Al-5, (g) Al-15, (h) Al-25, (i) Ni-5, (j) Ni-15, and (k)
Ni-25. The symbols of (↓), (ꢀ) and (ꢀ) indicate unindexed, NiO and Co3O4
diffraction peaks, respectively.
˚
7.17 A, respectively, revealing that the d values were gradually
decreased by Co doping. The decrease of the d value might be
due to the change of stacking structure between 2H and 3R [6–8]
and/or the substitution of the smaller ion of Co3+ (0.63 A) for
˚
3+
˚
reveal that the single phase products of birnessite were obtained
for undoped, Co-5, Co-15, Co-25, Al-5, and Ni-5, and their d
values of 0 0 l at 2θ = ca. 12◦ are 7.22, 7.18, 7.18, 7.17, 7.19,
larger ion of Mn (0.68 A) as described below in Fig. 7. These
XRD patterns of the samples showed that the diffraction peaks
between 35◦ and 70◦ are different. In previous reports [6–9],
several stacking phases are known to be formed as the birnessite-
type manganese dioxides, such as the 2H phase (hexagonal unit
cell and two layer periodicity, space group: P63/mmc) and 3R
phase (rhombohedral unit cell and three layer periodicity, space
group: R-3m), etc. The different stacking model of the 2H and 3R
phases is schematically shown in Fig. 6. The stacking structures
of these samples were able to be assigned to the mixtures of the
2H and 3R stacking phases from the XRD patterns in Fig. 5(I).
Fig. 5(II) shows the variation of the 2H and 3R phases by the bar
graph, which represented the fluctuations of the stacking phases.
Based on the relative intensity of the XRD profile, the ratios of
the 3R stacking phase to the 2H stacking phase are changed by
Co doping, which were observed by the increased 3R stacking
structure and decreased 2H stacking structure due to the increase
in the Co doping.
The chemical compositions of the undoped and Co doped
birnessites are shown in Table 1 by assuming no oxygen defi-
ciency. From the table, when the starting Co mole ratio increased
up to 0.25, the final Co mole ratio in the resultant birnessite
increased to 0.30. And, the amount of the potassium constituent
in birnessite did not significantly change between 0.2 and 0.3,
which is not very different among the four birnessites. Based
˚
and 7.08 A, respectively. We believe that these birnessites were
doped with definite amounts of Co, Al, and Ni.
Fig. 3 shows the discharge–recharge curves of the single
phase birnessite products of the undoped and doped with Ni, Al,
and Co. All the birnessite electrodes indicated S-shaped curve
characteristics of the layer structure around 3 V. The undoped
product showed the typical discharge–recharge behavior of Li
de-/intercalation from/into the birnessite-type MnO2 [4,5,9–17].
The shapes of the discharge–recharge curves hardly depended
upon the doping with Al, Ni, and Co, suggesting that the Li
intercalation into birnessite was not remarkably affected by
the doping. On the other hand, when we compared the maxi-
mum discharge capacities, the capacity of the undoped is about
170 mAh g−1 whereas lower capacities of Al and Ni and a higher
capacity of Co were obtained. For Ni-5 and Al-5, the obtained
capacity was about 160 mAh g−1. However, the higher capac-
ity of ca. 190 mAh g−1 was achieved for Co-5 when compared
to that of the undoped one. During the successive cycles, the
capacity retention depends on the doping as seen in Fig. 3.
Also, the additional small plateau was observed around 4 V in
discharge–recharge curves. The plateau would indicate the for-
mation of the spinel Li–Mn–O phase, and the corresponding

A. Ogata et al. / Electrochimica Acta 53 (2008) 3084–3093
3087
Fig. 3. Discharge–recharge curves of undoped and Me-doped products, (a) undoped, (b) Al-5, (c) Ni-5, and (d) Co-5. Current density is 20 mA g−1
.
on these results, we confirmed single phase birnessite doped
with Co, when the mixture of KMnO4 and Co nitrate was heat-
treated at 600 ◦C. Because of the monotonous increase of y in
KxMn1–yCoyO2·nH2O from 0 to 0.30, furthermore, the increase
in the amount of added Co nitrate produced a higher level of Co
doping in birnessite, accompanied by a change in the stacking
sequence from 2H to 3R.
In order to confirm the manganese and cobalt valences, XPS
analyses were carried out. Fig. 7 shows the Mn 2p and Co 2p
spectra of the products obtained for the undoped and Co doped
birnessites. The Mn 2p3/2 peaks of the Mn(III) and (IV) chem-
ical states appear around 641.4 and 642.4 eV [19]. Note that
the ratio of the peak intensity of manganese 3+ decreased and
the manganese 4+ increased with the increasing cobalt quantity.
Therefore, the manganese average valences were increased by
the cobalt ion doping in the structure. In general, the Co ion
exists in the 2+ or 3+ oxidation states in the oxide formed at
600 ◦C in air. The binding energy of Co 2p3/2 at 780.3 eV prob-
ably indicates the presence of Co(III) [19]. As a result, the Mn
valence states of the undoped and Co doped samples are 3 and
4+ coexist. Consequently, the Co(III) in birnessite was partially
substituted for the Mn(III), and the cobalt ion partially occupied
the Mn site.
Fig. 4. Variation in discharge capacity of (a) undoped, (b) Al-5, (c) Ni-5, and
(d) Co-5.
Because it is sensitive to amorphous components and those
with short-range order, Raman spectroscopy is very useful to
studythestructureofMnOx materials, wherecrystallinedisorder
may be expected. Because Mn atoms in manganese dioxides are
Table 1
Chemical compositions of synthesized birnessites from mixtures of KMnO4 and
cobalt nitrate
Samples
Composition of products
Fig. 5. (I) XRD patterns of (a) undoped (K0.31MnO2·0.37H2O), (b) Co-5
(K0.21Mn0.88Co0.12O2·0.41H2O), (c) Co-15 (K0.21Mn0.79Co0.21O2·0.37H2O),
and (d) Co-25 (K0.27Mn0.70Co0.30O2·0.39H2O) products. The h k lH and h l kR
of Millar Indexes indicate 2H stacking structure and 3R stacking structure,
respectively. (II) The bar graph shows the ratio of the 2H and 3R phases.
Undoped
Co-5
Co-15
Co-25
K0.31MnO2·0.37H2O
K
0.21Mn0.88Co0.12O2·0.41H2O
K0.21Mn0.79Co0.21O2·0.37H2O
K0.27Mn0.70Co0.30O2·0.39H2O

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A. Ogata et al. / Electrochimica Acta 53 (2008) 3084–3093
Fig. 6. The different stacking models of (a) 2H and (b) 3R phases.
always 6-fold coordinated, the Raman bands of MnO2 materials
depends on the MnO6 octahedral environments (e.g. edge/corner
sharing, Jahn–Teller distorsion, interactions with water/cations
in the tunnels or layers of MnO2 materials). Their Raman fre-
quencies in the 500–700 and 200–500 cm−1 range have been
assigned to the Mn–O stretching of MnO6 octahedra and the
Mn–O–Mn bending vibrations in the MnO2 octahedral lattice,
respectively [20].
birnessite-type MnO2 compounds is their low Raman activ-
ity. Three major features have been previously reported at
500–510, 575–585 and 625–650 cm−1 [14]. The two high-
wavenumber bands are dominating all spectra, while bands
in the low-frequency region appear with a rather weak inten-
sity. According to the group theoretical calculation, manganese
dioxide materials with monoclinic C2/m space group (Z = 4),
i.e. C23h spectroscopic symmetry, are predicted to show nine
Raman-active modes with 3Ag + 6Bg species. Experimental
results show that four components are always detected here,
located at 409, 498, 575, and 638 cm−1 for the undoped sam-
Fig. 8 shows the Raman spectra of the undoped and Co-25
manganese oxides samples having the birnessite-type struc-
ture. The general peculiarity of the vibrational features of
Fig. 7. Mn 2p and Co 2p XPS spectra of (a) undoped, (b) Co-5, (c) Co-15, and (d) Co-25 products.

A. Ogata et al. / Electrochimica Acta 53 (2008) 3084–3093
3089
Fig. 8. Raman spectra of (a) undoped and (b) Co-25 samples.
ple, at 409, 498, 572 and 647 cm−1 for the doped Co-25 sample
(Fig. 8).
sizes does not affect the discharge–recharge capacity and dis-
charge capacity retention of the Li battery cathode materials. In
order to examine the dependence of the amount of Co doping,
we carried out galvanostatic discharge–recharge tests.
These Raman features are very similar to those previously
reported for several birnessite-type compounds [14]. The dis-
crepancy between group factor analysis and Raman data is
attributedtothedefectchemistryandthelocaldisorderofthebir-
nessitestructure. Thepresenceofvacanciesinthe[MnO6]sheets
of MnO2·nH2O compounds breakdowns the crystal symmetry
and lowers the spectroscopic symmetry. Therefore, Raman spec-
tra of synthetic birnessites show variation in band position and,
to a larger extent, relative band intensity.
The RS spectra of the Co-free sample support its proposed
layered structure. The Raman band at 638 cm−1 can be viewed as
the symmetric stretching vibration ν2(Mn–O) of MnO6 groups.
ItisassignedtotheA1g symmetricmodeintheC23h spectroscopic
space group. The band located at 575 cm−1 usually attributed to
the ν3(Mn–O) stretching vibration in the basal plane of [MnO6]
sheets, is particularly strong in birnessite compounds, which can
be related to the high rate of Mn(IV) in the birnessite family. The
RS spectrum of the Co-25 sample, where cobalt partly substi-
tutes for manganese, displays similar features than the Co-free
sample. The most important Raman change is observed on the
ν2(Mn–O) stretching frequency, which presents a shift of 9 cm−1
towards the high-wavenumber side for the Co-doped sample.
This result indicates a strengthening of the Mn O bond along the
The discharge–recharge experiments of the undoped and Co-
doped birnessite were performed at room temperature in the
voltage range of 2.0–4.3 V by applying a current density of
20 mA g−1. In Fig. 10, the discharge capacities of the undoped
and Co-25 samples at the 2nd cycle indicated about 170 and
200 mAh g−1, and the discharge reactions of these electrodes
mainly progressed at 2.7–3.5 V. Though the redox capacity of
Mn3+/4+ coupleinthebirnessiteusuallydecreasedbycobaltdop-
ing as described in [15], surprisingly, the discharge capacities
increased with the increasing Co quantity. The reasons for the
increase of capacities should be due to the change in the 3R/2H
stacking structure (Fig. 5) and higher electronic conductivity
[21].
Fig. 11 shows the variation in the discharge capacities of the
undoped and Co doped birnessites up to 20 cycles. The discharge
capacities from the 2nd cycle to the 20th cycle of the undoped
and Co-25 samples are changed from 170 to 95 mAh g−1 and
from 200 to 160 mAh g−1, respectively. Moreover, the discharge
capacity retentions of the undoped and Co-25 birnessite are
about 56 and 80% between 2nd and 20th cycles. The increases in
thedischargecapacityandtheimprovementofcapacityretention
by Co doping are probably due to improvement of the electro-
conductivity [21] and the change of the ratio of stacking phase by
the increase of 3R phase and decrease of 2H phase. It seems that
the3Rstackingfavorsthereversiblelithiationcomparedwiththe
2H one in structural points of view. Furthermore, the structure
should be more stable during lithiation cycling since the shape
of the discharge curves of the Co-25 (3R stacking phase) was
almost preserved during 20 cycles whereas that of the undoped
sample (2H stacking phase) was changed as seen in Fig. 10a and
d. Additionally, the Co(III) valence state is more appropriate to
stabilize the whole oxide against reversible lithiation than the
Mn(III) valence state [13].
interlayer direction in the Co-doped sample, as a consequence
3+
˚
of the difference between ionic radii, i.e. r(Mn ) = 0.68 A and
3+
˚
r(Co ) = 0.63 A. This result gives evidence for the effective par-
tial replacement of Mn3+ by Co3+, which induces a stronger
network energy of the Co-doped material. We can recognize that
the Mn3+ was partly replaced by Co3+. Therefore, the results of
Raman measurement correspond to the results of the XRD, XPS,
and AAS measurements.
The TEM images of the undoped and Co-doped birnessites
are displayed in Fig. 9. In this figure, regardless of the undoped
and doped with Co in the birnessite, the crystallite sizes of these
products are not very different. The particles of all the sam-
ples are plate-like crystals and the crystallite sizes are about
50–200 nm. It is considered that the difference in the particle
The Cole–Cole plots of the undoped and Co-25 products
between before and after the first discharge reaction were shown

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A. Ogata et al. / Electrochimica Acta 53 (2008) 3084–3093
Fig. 9. TEM images of (a) undoped, (b) Co-5, (c) Co-15, and (d) Co-25 products.
Fig. 10. Discharge–recharge curves of undoped and Co-doped birnessites, (a) undoped, (b) Co-5, (c) Co-15, and (d) Co-25. Current density is 20 mA g−1
.

A. Ogata et al. / Electrochimica Acta 53 (2008) 3084–3093
3091
Fig. 13. (I) XRD patterns of synthesized samples with various mole ratios of
K/Mn, (a) Mn-3, (b) Mn-2, (c) undoped, (d) K-2, (e) K-3, (f) K-5, and (g) K-10.
(II) The bar graph shows the phase type of the prepared samples. The 2H, 3R,
and Mono indicate 2-layer hexagonal, 3-layer rhombohedral, and monoclinic
stacking phases, respectively.
Fig. 11. Variation in discharge capacities of undoped and Co-doped birnessites,
(a) undoped, (b) Co-5, (c) Co-15, and (d) Co-25.
in Fig. 12. In the initial state (before the first discharge), the semi-
circles in the high-medium frequency range exhibit a charge
transfer resistance [15], and the charge transfer resistance of
Co-25 is apparently lower than the undoped product because the
semi-circle is smaller than that of the undoped product. After the
first discharge reaction, the resistance of the Co-25 product is
lower than the undoped one, and the charge transfer resistances
of both the undoped and Co-25 increased when comparing the
Cole–Cole plots before and after. The resistance of the Co-25
product is always lower than that of the undoped one. Conse-
quently, the K-birnessite doped with Co in its structure had a
decreased charge transfer resistance which contributed to the
increase in the discharge capacity.
material). Fig. 13(II) indicates the phases of the prepared sam-
ples using a bar graph. The synthesized products of the Mn-3
and Mn-2 samples consisted of the coexisting birnessite and hol-
landite phase from these diffraction peaks. For the mole ratio of
K/Mn = 1 or higher, all the diffraction peaks can be indexed as
the birnessite phase in the XRD patterns, indicating birnessite
without any secondary phases. And the d values of 0 0 l around
2θ = ca. 12◦ of undoped, K-2, K-3, K-5, and K-10 products are
˚
7.22, 7.15, 7.17, 7.12, and 7.10 A, respectively. The decrease
of the d value might be suggestive of the change of stacking
structure between 2H, 3R, and monoclinic phase [6–8]. From
the AAS, the atomic ratios of K and Mn corresponded to around
0.25 in these birnessites (undoped and K-2, −3, −5, and −10),
independent of the starting K/Mn values, though the XRD pat-
terns of the undoped and K-2 to −10 are different between 35◦
and 70◦. The ratios of the 2H and 3R stacking phases in these
peaks depend on the amount of the potassium additives. Sim-
ilar to those in Fig. 5, the decrease in the 2H stacking phase
and increase in the 3R stacking phase were supported by these
diffraction peaks. For K-10, the diffraction peaks, which are
based on the 2H and 3R stacking phases, disappeared, and a
3.3. Synthesis of K-birnessite by addition of excess K or Mn
As mentioned above, the addition of the cobalt nitrate led
to the partial replacement of Mn site with cobalt. In the same
manner, the dependence of K/Mn atomic ratio was investi-
gated by adding potassium and manganese nitrates into KMnO4.
Fig. 13(I) shows the XRD patterns of the synthesized samples
with the various mole ratios of K/Mn = 1/3, 1/2, 1 (undoped), 2,
3, 5, and 10 (in case of K/Mn = 1, pure KMnO4 as the starting
Fig. 12. Cole–Cole plots of (a) undoped and (b) Co-25 products before first discharge and (c) undoped and (d) Co-25 products after first discharge.

3092
A. Ogata et al. / Electrochimica Acta 53 (2008) 3084–3093
Fig. 14. Discharge–recharge curves of synthesized samples with various K/Mn ratios; (a) Mn-3, (b) undoped, (c) K-2, and (d) K-3. Current density is 20 mA g−1
.
monoclinic phase (JCPDS: 42-1317) was observed [8]. There-
fore, it was found that the stacking phases were modified by the
addition of potassium nitrate as the starting materials.
at the 2nd cycle. However, these capacity retentions are differ-
ent during 20 cycles. The discharge capacities of the K-3, K-2,
undoped, and Mn-3 products at the 20th cycle are 145, 133,
96, and 41 mAh g−1, respectively. The capacity retentions, the
discharge ratio between the 2nd and the 20th cycles, of K-3,
−2, and undoped samples are about 85, 80, and 56%, respec-
tively. The synthesized products with excess potassium additives
exhibited an improvement in the capacity retention. However,
the discharge capacities of the K-5 and K-10 (not shown here)
were similar to that of undoped sample during cycling, that is,
the electrode performances of the monoclinic stacking phase
are not improved. The Mn-3 has a lower discharge capacity with
capacity fading. Therefore, we recognized that the enhancement
of the capacity retention depends on the stacking structure, i.e.,
increaseofthe3Rstackingphaseanddecreaseofthe2Hstacking
phase. It is believed that the effect of cobalt doping and 3R phase
plays an important role on the cycling behavior. The lamellar
manganese dioxides for a lithium secondary battery application
would provide an improvement in the high rechargeable capacity
and satisfactory cycle performance by designing nano structures
including the stacking sequence resulted from the modification
of calcination conditions.
Fig. 14 shows the discharge–recharge curves of the synthe-
sized products of Mn-3, undoped, K-2, and K-3 samples. All
the birnessite samples in Figs. 14(b–d) that underwent galvano-
static cycling had S-shaped curves and the discharge reaction
occurred around 3 V. The discharge capacity of Mn-3 is much
lower than that of the undoped sample. The decrease in the dis-
charge capacity is due to the formation of the high crystalline
hollandite phase [22] because the K ion in the tunnel prevents
diffusion of the Li ion. The initial discharge capacities of K-3
and K-2 are similar to that of the undoped sample.
Fig. 15 shows the plots of the discharge capacities vs. cycle
number for Mn-3, undoped, K-2, and K-3 samples. In this figure,
the synthesized products of the undoped, K-2, and K-3 indicate
analogous maximum discharge capacities of about 170 mAh g−1
4. Conclusion
The potassium birnessites doped with Co, Ni, or Al were
successfully obtained from KMnO4 mixed with these nitrates
by a calcination synthesis. Among the doped compounds, Co-
25 birnessite has the highest discharge capacity of 200 mAh g−1
and capacity retention of about 80% for 20 cycles. It was found
that the electrochemical performance of birnessite doped with
appropriate amount of cobalt was improved due to the change
in the stacking structure and the decrease in the charge trans-
fer resistance. Furthermore, the initial discharge capacity of the
synthesized birnessite by excess K additives was comparable to
Fig. 15. Variation in discharge capacities of synthesized samples with various
mole ratios of K/Mn; (a) Mn-3, (b) undoped, (c) K-2, and (d) K-3.

A. Ogata et al. / Electrochimica Acta 53 (2008) 3084–3093
3093
the undoped birnessite, and the capacity retention of the syn-
thesized birnessite with excess K additives is higher than the
undoped one. The difference in the electrochemical performance
was ascribed to the structure change, as confirmed by XRD,
AAS, XPS, and Raman measurements.
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Acknowledgements
The authors thank Dr. T. Sasaki and Mr. K. Saka for their
helpful assistance in the experimental work. This study was par-
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Development Organization (NEDO), Japan, Yazaki Memorial
Foundation for Science and Technology, and Iwatani Naoji
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