Characterization of the bismuth-modified manganese dioxide cathodes in rechargeable alkaline cells

Article abstract of DOI:10.1149/1.1459713

Bismuth-modified manganese dioxide (BMD) cathodes are shown to exhibit good cycling characteristics with a theoretical two-electron capacity in rechargeable alkaline cells. With an aim to understand the discharge-charge mechanisms, the BMD cathodes are characterized by X-ray diffraction, scanning electron microscopy, and wet-chemical analysis at various levels of discharge and charge during the first two cycles and after various numbers of cycles. It is found that a well-ordered, crystalline birnessite MnO₂ is formed at the end of first charge, irrespective of the initial form of the manganese oxide. The discharge-charge mechanism involves a reversible conversion of birnessite MnO₂ to MnOOH to Mn(OH)₂ in the subsequent cycles. Wet-chemical analyses demonstrates for the first time that the discharge/charge process in rechargeable alkaline cells involves a reversible dissolution/incorporation of K⁺ ions from/into the cathode lattice into/from the electrolyte. The incorporation of the K⁺ ions into the lattice appears to stabilize a well-ordered birnessite structure during charge.

Full text of DOI:10.1149/1.1459713

Journal of The Electrochemical Society, 149 ͑4͒ A483-A492 ͑2002͒
A483
0013-4651/2002/149͑4͒/A483/10/$7.00 © The Electrochemical Society, Inc.
Characterization of the Bismuth-Modified Manganese Dioxide
Cathodes in Rechargeable Alkaline Cells
a
a
A. M. Kannan,a S. Bhavaraju,b, F. Prado, M. Manivel Raja,
*
,z
and A. Manthirama,
*
aTexas Materials Institute, The University of Texas at Austin, Austin, Texas 78712, USA
bRBC Technologies, College Station, Texas 77840, USA
Bismuth-modified manganese dioxide ͑BMD͒ cathodes are shown to exhibit good cycling characteristics with a theoretical
two-electron capacity in rechargeable alkaline cells. With an aim to understand the discharge-charge mechanisms, the BMD
cathodes are characterized by X-ray diffraction, scanning electron microscopy, and wet-chemical analysis at various levels of
discharge and charge during the first two cycles and after various numbers of cycles. It is found that a well-ordered, crystalline
birnessite MnO2 is formed at the end of first charge, irrespective of the initial form of the manganese oxide. The discharge-charge
mechanism involves a reversible conversion of birnessite MnO2 to MnOOH to Mn͑OH͒ in the subsequent cycles. Wet-chemical
2
analyses demonstrates for the first time that the discharge/charge process in rechargeable alkaline cells involves a reversible
dissolution/incorporation of Kϩ ions from/into the cathode lattice into/from the electrolyte. The incorporation of the Kϩ ions into
the lattice appears to stabilize a well-ordered birnessite structure during charge.
© 2002 The Electrochemical Society. ͓DOI: 10.1149/1.1459713͔ All rights reserved.
Manuscript submitted January 5, 2001; revised manuscript received November 20, 2001. Available electronically March 12, 2002.
The rapid growth in portable electronic devices and the desire to
develop electric vehicles have created enormous worldwide activity
in the development of advanced, high-energy-density battery sys-
tems. Cost, environmental issues, and safety characteristics of the
batteries are important considerations for large-volume consumer
applications such as electric vehicles. Although the nonaqueous
electrolyte-based lithium-ion cells offer higher energy density com-
pared to other commercially available rechargeable systems, the
high cost and toxicity of the currently used cobalt oxide cathodes
and the safety issues are of some concern in developing these cells
for electric vehicles. In this regard, aqueous systems based on inex-
pensive electrode materials and offering energy densities compa-
rable to lithium-ion cells would be attractive. However, the aqueous-
based systems are limited in their cell voltage and therefore the high
energy density needs to be realized with electrodes having high
specific capacity. For example, electrode materials involving revers-
ible redox reactions with two electrons per transition metal ion can
offer energy densities comparable to the currently available lithium-
ion cells. For comparison, the lithium cobalt oxide used in commer-
cial lithium-ion cells exhibits a practical capacity corresponding to
0.5 electrons per Co.
Aqueous alkaline primary cells based on manganese dioxide
cathodes are widely used for consumer applications, as manganese
is inexpensive and environmentally benign. However, the use of
manganese oxide cathodes for alkaline secondary batteries is still
challenging because of the poor reversibility of the reduction/
oxidation process involving two electrons per Mn. Reduction be-
yond the one-electron level leads to the formation of oxides such as
Mn2O3 and Mn3O4 that have poor rechargeability. Wroblowa
et al.1-6 at Ford Motor Company demonstrated more than a decade
ago that the addition of bismuth by chemical modification changes
the mechanism of the MnO2 reduction/oxidation process and
thereby makes it possible to reversibly discharge/charge MnO2 cath-
odes over two-electron capacity. Prior to the work at Ford, Kordesch
et al.7-10 had shown that the ␥-MnO2 is rechargeable up to a con-
siderable number of cycles only if the discharge depth is limited to
less than 35% of one-electron capacity.
takes place in two steps as proposed by Kozawa et al.16-21 The first-
electron reduction called the ‘‘electron-proton insertion process’’ in-
volves the insertion of protons from the electrolyte into the ␥-MnO2
structure and the reduction reaction can be represented as
MnO2 ϩ H2O ϩ eϪ → MnOOH ϩ OHϪ
͓1͔
During this reduction process, the potential decreases continuously
with composition, indicating a single-phase reaction mechanism.
Therefore, the first-electron reduction is also called a homogeneous
reduction process.
At the end of the first-electron reduction, the Mn3ϩ ions begin to
dissolve from the cathode into the electrolyte. During the second-
electron reduction, the Mn3ϩ ions are reduced to Mn2ϩ ions by
charge transfer on the surface of the conducting graphite present in
the cathode mix. Since the solubility of Mn2ϩ ions in alkaline me-
dium is relatively low compared to that of Mn3ϩ ions, the Mn2ϩ
ions precipitate as Mn͑OH͒ phase on the surface of the graphite
2
particles. Therefore, this second-electron reduction is called ‘‘disso-
lution and precipitation’’ process. The potential during this process
remains constant with composition, and therefore this is also called
a heterogeneous reduction process.
The reduction products of ␥-MnO2 cathodes in alkaline cells
have been investigated by McBreen25,26 using X-ray dif-
fraction ͑XRD͒ in conjunction with slow scan cyclic voltammetry
and his results agree well with the mechanism proposed by Kozawa
and Yeager.16 McBreen25 has shown that the initial reduction in-
volves the incorporation of protons into the ␥-MnO2 lattice, leading
to the formation of an amorphous ␣-MnOOH phase. Further reduc-
tion of the amorphous phase yields Mn͑OH͒2 . On reversing the
redox process, Mn͑OH͒ is oxidized initially to ␤-MnOOH,
2
␥-MnOOH, and ␥-Mn2O3 phases and further oxidation results in
the formation of birnessite ␦-MnO2 phase.
However, McBreen’s work25,26 has focused mainly on the redox
process of ␥-MnO2 . Following the report of Wroblowa and Gupta2
on bismuth-modified manganese dioxide ͑BMD͒ materials, a con-
siderable amount of work has been carried out by several groups27-35
in order to understand the mechanism of the redox process and the
role of Bi3ϩ ions on the rechargeability. Conway et al.27-29 have
studied BMD samples obtained by coprecipitation and electrodepo-
sition methods. They have shown that the dissolution and precipita-
tion is an important part of the reduction process and suggested that
bismuth may help to enhance the nucleation and growth of solid
species from the soluble intermediates and thereby avoid the forma-
tion of Mn3O4 phase. Various techniques such as cyclic voltamme-
With an aim to understand the mechanism of the redox process,
several groups have investigated the manganese dioxide cathodes
in secondary alkaline cells.11-24 There is general agreement that
the two-electron reduction of manganese dioxide in alkaline medium
* Electrochemical Society Active Member.
z E-mail: rmanth@mail.utexas.edu
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Journal of The Electrochemical Society, 149 ͑4͒ A483-A492 ͑2002͒
Table I. Chemical compositions of HSA and LSA samples.
Composition ͑wt %͒
͑Mn/Bi͒
mole ratio
Mn
Bi
K
Sample
Chemical formula
H2O
HSA
LSA
46
54
10
11.5
7.5
0
9
K4BiMn16Ox•10H2O
MnOx ϩ Bi2O3
ϳ16
ϳ16
Ͻ0.5
try, species detection using sensing electrodes, and UV-visible spec-
troscopy have been used to determine the intermediate species and
to study the reaction mechanism of two-electron cycling of BMD
materials. Extended X-ray absorption fine structure ͑EXAFS͒ spec-
troscopy and X-ray absorption near edge spectroscopy ͑XANES͒
have also been used to identify the states of Bi and Mn.23 Donne
et al.30-32 have studied the redox process in various forms of BMD
cathodes. They concluded that birnessite-type MnO2 results in the
formation of electrochemically inactive Mn3O4 at deeper discharge
and the presence of Bi3ϩ ions suppresses the formation of Mn3O4
phase. Thus, it is generally accepted that the presence of bismuth
avoids the formation of the electrochemically inactive Mn3O4 phase
and thereby makes the MnO2 materials rechargeable.
However, it is commonly believed in the literature that the re-
chargeable alkaline manganese dioxide cells require a high amount
of carbon ͑approx. 90 wt %͒ as a conducting substrate in the cathode
to achieve good cyclability. Cells made with such a high amount of
carbon with 10% active material will have very low specific capac-
ity and are unsuitable for commercial applications. Interestingly,
RBC Technologies has been successful recently in achieving good
electrochemical performance for the two-electron process with
much higher loading ͑50-75 wt %͒ of active materials and thus
rechargeable alkaline cells based on manganese oxides are
promising.
The objective of the present investigation is to have a further
understanding of the reduction/oxidation mechanism of the BMD
materials at higher loading and develop optimum cathodes for re-
chargeable alkaline cells. Toward this task, we have investigated two
different types of BMD cathodes developed at the RBC Technolo-
gies: ͑i͒ high surface area ͑HSA͒ Bi-birnessite type material, and ͑ii͒
low surface area ͑LSA͒ MnO2 ͑with small amounts of Mn2O3
phase͒ ϩ Bi2O3 mixture made by thermal decomposition of corre-
sponding nitrates. This paper focuses on the phase evolution, com-
positional changes, and microstructural changes of the BMD cath-
odes during the discharge-charge process and cycling.
Experimental
The HSA samples were synthesized by a chemical route
described earlier.36,37 It involves the addition of excess KOH
solution to a solution of Mn(NO3)2 and Bi(NO3)3 to obtain a hy-
droxide gel, followed by a conversion of the gel into the oxide by
passing gaseous oxygen. This sample was found to have a surface
area of approximately 125 m2/g. The chemical composition of the
sample is given in Table I. The LSA samples were synthesized by a
thermal decomposition of an aqueous mixture consisting of
Bi(NO3)3 and Mn(NO3)2 . 1 kg Mn(NO3)2 was dissolved in 1600
mL distilled water, and 85.4 g Bi(NO3)3 was dissolved in a mixture
consisting of 372 mL distilled water and 128 mL nitric acid. The two
solutions were then mixed together and heated first at 125°C over-
night and then at 325°C for 5 h. The resulting material was then
ground with a mortar and pestle and sieved using a 200 mesh screen.
The dry material thus obtained had a surface area of approximately
5 m2/g. The chemical composition is given in Table I.
Crystal structures and the degree of crystallinity of the samples
were characterized by XRD. The X-ray patterns were recorded with
a very slow scan rate involving a counting time of 10 s per 0.02°.
The slow scan rate was necessary to have an accurate analysis of the
phases present. Chemical compositions were determined by atomic
Figure 1. Discharge and charge profiles of BMD cathodes at 60% loading
and C/2 rate: ͑a͒ first charge and second discharge curves of HSA sample, ͑b͒
second charge and third discharge curves of LSA sample, ͑c͒ fiftieth
discharge/charge curves of HSA sample, and ͑d͒ fiftieth discharge/charge
curves of LSA sample.
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Journal of The Electrochemical Society, 149 ͑4͒ A483-A492 ͑2002͒
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Figure 2. Electrochemical cycling performance ͑C/2 rate͒ of the HSA and
LSA cathodes at 60% loading.
absorption spectroscopy ͑AAS͒. The solutions for AAS were pre-
pared by digesting the discharged/charged cathode with dilute hy-
drochloric acid and removing the insoluble graphite and binder by
filtration. Microstructural analysis and compositional mapping were
carried out with a LEO 1530 scanning electron microscope ͑SEM͒
equipped with energy-dispersive spectroscopic ͑EDS͒ analysis.
Electrochemical cycling experiments were conducted with both
the HSA and LSA materials at 50-65% loading, the balance being
carbon and binder. Half-cells were made with BMD cathode as the
working electrode, sintered Ni͑OH͒ as the counter electrode, Hg/
2
HgO as the reference electrode, and 31% aqueous KOH solution
as the electrolyte. The Ni͑OH͒ counter electrode involves the
2
Ni͑OH͒2 /NiOOH couple similar to that in the rechargeable nickel-
cadmium and nickel-metal hydride batteries
Ni͑OH͒2 ϩ OHϪ → NiOOH ϩ H2O ϩ eϪ
͓2͔
The counter electrode contained excess Ni͑OH͒ and thus the cell
2
capacity is not limited by it. The cells were cycled at C/2 rate. The
HSA and LSA cathodes were discharged/charged to various depths
͑25, 50, 75, and 100% of the capacities shown in Fig. 1͒ during the
first two cycles, and the products were then structurally and compo-
sitionally characterized using X-ray powder diffraction and AAS,
respectively. In addition to this, the cathodes were also discharged/
Figure 4. XRD patterns of HSA cathodes at various levels of discharge
during the first cycle: ͑a͒ 0, ͑b͒ 25, ͑c͒ 50, ͑d͒ 75, and ͑e͒ 100% discharge;
*
͑᭺͒ birnessite, ͑᭹͒ Mn͑OH͒ or MnOOH, ͑ ͒ Mn3O4 , and ͑ࡗ͒ graphite.
2
charged to various numbers of cycles and the cycled cathodes were
characterized by X-ray powder diffraction and AAS.
Results and Discussion
Electrochemical characterization.—Figure
1
shows the
discharge-charge curves of half-cells made with HSA and LSA cath-
odes at 60% loading and cycled at C/2 rate ͑based on 445 mAh/g for
HSA and 475 mAh/g for LSA cathodes͒ between the potentials of
0.35 and Ϫ1.1 V with reference to the Hg/HgO electrode. The data
in Fig. 1a corresponds to the first charge and second discharge cycle
of the HSA sample and that in Fig. 1b corresponds to the second
charge and third discharge cycle of the LSA sample. These cycles
were chosen as they show the highest discharge capacity that is
evident from the cyclability data given in Fig. 2. Additionally, the
first discharge profile, particularly for the LSA sample, was different
from the subsequent discharge profiles as the initial phases are dif-
ferent from those formed after the first charge and is not included in
Fig. 1 for clarity ͑see later͒. Figure 1c and d shows the charge/
discharge curves for the fiftieth cycle of the HSA and LSA samples.
The data show that the discharge capacity during the fiftieth cycle
͑Fig. 1c and d͒ occurs at a slightly lower average voltage compared
to that found during the initial cycles ͑Fig. 1a and b͒. The decrease
in the average working voltage with cycle number is due to the
increase in cell impedance. The reduction of the Mn3ϩ species oc-
curs on the surface of the conductive carbon,28,34 resulting in a coat-
ing of the carbon surface by manganese species and a loss of con-
ductive pathways with cycling. Additionally, the large volume
changes accompanying the reduction of Mn4ϩ to Mn2ϩ may cause a
Figure 3. Electrochemical cycling performance ͑C/2 rate͒ of the ͑a͒ HSA
cathodes at 50% loading and ͑b͒ LSA cathodes at 65% loading.
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A486
Journal of The Electrochemical Society, 149 ͑4͒ A483-A492 ͑2002͒
Figure 5. XRD patterns of HSA cathodes at various levels of charge during
the first cycle: ͑a͒ 0, ͑b͒ 25, ͑c͒ 50, ͑d͒ 75, and ͑e͒ 100% charge; ͑᭺͒
Figure 7. XRD patterns of a mechanical mixture of Mn3O4 ϩ Bi2O3 ͑a͒
before discharge ͑as-prepared͒ and ͑b͒ after fifth charge illustrating the for-
*
birnessite, ͑᭹͒ Mn͑OH͒ or MnOOH, ͑ ͒ Mn3O4 , and ͑ࡗ͒ graphite.
2
*
mation of birnessite MnO2 as the mixture is cycled: ͑᭺͒ birnessite, ͑ ͒
Mn3O4 , ͑᭿͒ Bi2O3 , and ͑ࡗ͒ graphite.
breakdown of the contact between the carbon particles with cycling.
These factors lead to the development of impedance with cycling.
Furthermore, the capacity utilization ͑number of cyclable electrons͒
and the average working voltage also depend on the amount of
carbon present. As the active material loading increases, the internal
resistance increases and the capacity utilization and working voltage
decrease. In this regard, the data shown in Fig. 1 ͑60% active ma-
terial loading͒ are quite different from that of Wroblowa et al.,1-6
who reported a cycling of 1.9 electrons at 10% active material load-
ing.
achieved at high loading ͑50-65%͒ of active materials. The cathodes
retain more than 80% of their initial capacity over 300 cycles at 50%
loading ͑Fig. 3a͒ and over 200 cycles at 65% loading ͑Fig. 3b͒.
These levels of loading and cycle life are attractive for developing
practical cells with high energy density. The good cycling character-
istics achieved with these cathodes at higher loading motivated us to
further investigate the reduction and oxidation mechanism of these
cathode materials. The characterizations of the cathodes with 60%
loading are presented in the following sections. However, it should
be noted that these BMD cathodes ͑HSA and LSA cathodes͒ could
not be cycled for more than 20 or 30 cycles against zinc anodes as
the dissolved zincate ions combine with the manganese ions to give
inactive zinc-manganese species during the second-electron dis-
charge process of the BMD cathode.
Figures 2 and 3 show the cyclability data of HSA and LSA cath-
odes. The data demonstrate that good cycling performance can be
Structural characterization during the first two cycles.—With an
objective to understand the discharge-charge mechanisms, the HSA
and LSA cathodes ͑60% loading͒ were subjected to various levels
͑25, 50, 75, and 100% of the capacities shown in Fig. 1a and b͒ of
discharge and charge and characterized. Figures 4 and 5 show the
XRD patterns of the HSA cathodes during the first discharge-charge
cycle at various levels of discharge and charge. An analysis of the
X-ray data shows that the fresh ͑0% discharge͒ HSA cathode con-
sists of a poorly crystalline birnessite MnO2 phase (␦-MnO2) in
addition to a few reflections corresponding to graphite. While no
noticeable change occurs in the XRD pattern at 25% discharge ͑Fig.
4b͒, the reflections corresponding to the birnessite MnO2 phase van-
Figure 6. Electrochemical cyclability data of Mn3O4 and a mixture of
Mn3O4 and Bi2O3 : ͑ࡗ͒ Mn3O4 and ͑᭺͒ Mn3O4 ϩ Bi2O3 .
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Journal of The Electrochemical Society, 149 ͑4͒ A483-A492 ͑2002͒
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Figure 9. XRD patterns of HSA cathodes at various levels of charge during
the second cycle: ͑a͒ 0, ͑b͒ 25, ͑c͒ 50, ͑d͒ 75, and ͑e͒ 100% charge; ͑᭺͒
Figure 8. XRD patterns of HSA cathodes at various levels of discharge
during the second cycle: ͑a͒ 0, ͑b͒ 25, ͑c͒ 50, ͑d͒ 75, and ͑e͒ 100% discharge;
*
birnessite, ͑᭹͒ Mn͑OH͒ or MnOOH, ͑ ͒ Mn3O4 , ͑᭿͒ Bi, and ͑ࡗ͒ graphite.
2
*
͑᭺͒ birnessite, ͑᭹͒ Mn͑OH͒ or MnOOH, ͑ ͒ Mn3O4 , ͑᭿͒ Bi, and ͑ࡗ͒
2
graphite.
apart from graphite. Thus, at the end of the first discharge-charge
cycle ͑Fig. 5e͒, a well crystalline and ordered birnessite MnO2 phase
is formed, even though the initial cathode consisted of a poorly
crystalline birnessite MnO2 phase ͑Fig. 4a͒. More importantly, the
Mn3O4 that is present at the end of first discharge ͑Fig. 4e or 5a͒
vanishes completely at the end of first charge ͑Fig. 5e͒. This obser-
vation reveals that the presence of bismuth that is in intimate contact
with the Mn3O4 helps to recharge the Mn3O4 to birnessite MnO2 .
To understand this point further, we have carried out discharge/
charge cycling experiments with cells fabricated with conventional
Mn3O4 and a physical mixture consisting of conventional Mn3O4
and 10 wt % Bi2O3; the cycling data are given in Fig. 6. While
Mn3O4 shows little reversible capacity in the absence of bismuth, it
shows a gradual increase in capacity in the presence of bismuth. As
the Mn3O4 ϩ Bi2O3 mixture is cycled, more and more manganese
oxide becomes in intimate contact with bismuth and thereby partici-
pates in the reversible discharge/charge process. Additionally, the
XRD data shown in Fig. 7 illustrate that Mn3O4 is transforming to
birnessite MnO2 during cycling in the presence of bismuth. The data
in Fig. 6 and 7 clearly demonstrate that while pure Mn3O4 is diffi-
cult to recharge, the presence of bismuth in intimate contact results
in good rechargeability.
ishes and new reflections corresponding to Mn3O4 phase occur at
50% discharge ͑Fig. 4c͒. At 50% discharge, reflections correspond-
ing to MnOOH are not apparent possibly due to the poor crystallin-
ity or amorphous nature. On further discharge to 75% ͑Fig. 4d͒,
reflections corresponding to Mn͑OH͒ begin to appear and the fully
2
͑100%͒ discharged cathode ͑Fig. 4e͒ consists of both Mn͑OH͒ and
2
Mn3O4 . However, the amount of Mn3O4 in the 100% discharged
sample ͑Fig. 4e͒ appears to be lower than that found in the 50%
discharged sample ͑Fig. 4c͒, as indicated by a slight decrease in the
intensity of the reflections corresponding to Mn3O4 . Thus, on going
from 50 to 100% discharge, a small fraction of the Mn3O4 may
possibly undergo reduction.
On charging the discharged cathodes ͑Fig. 5͒, Mn͑OH͒ vanishes
2
and birnessite MnO2 phase is formed. At 25% charge ͑Fig. 5b͒, the
pattern shows reflections corresponding to only Mn͑OH͒ and
2
Mn3O4 phases in addition to graphite. At 50% charge ͑Fig. 5c͒, the
birnessite MnO2 phase begins to appear and then continues to grow
while the amount of Mn͑OH͒ or MnOOH decreases with further
2
charge ͑Fig. 5d͒. It is possible that a small amount of crystalline
␤-MnOOH, whose strongest reflection overlaps with the strongest
reflection of Mn͑OH͒ at around 2␪ ϭ 20°, may be present in the
2
Figures 8 and 9 show the XRD patterns of the HSA cathodes
during the second cycle at various levels of discharge and charge.
During the second discharge, the birnessite MnO2 vanishes and
25-75% charged samples. This may be particularly true in the 75%
charged sample as indicated by the presence of a weak reflection at
around 2␪ ϭ 20° ͑Fig. 5d͒, since no Mn͑OH͒ is expected to be
2
present at this depth of charge. The fully charged cathode ͑100%͒
Mn͑OH͒ is formed. At 25% discharge ͑Fig. 8a͒, a reflection begins
2
consists of mostly a well-ordered crystalline birnessite MnO2 phase
to occur at around 2␪ ϭ 20°, which could be due to the formation
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Journal of The Electrochemical Society, 149 ͑4͒ A483-A492 ͑2002͒
Figure 10. XRD patterns of LSA cathodes at various levels of discharge
during the first cycle: ͑a͒ 0, ͑b͒ 25, ͑c͒ 50, ͑d͒ 75, and ͑e͒ 100% discharge;
Figure 11. XRD patterns of LSA cathodes at various levels of charge during
the first cycle: ͑᭺͒ birnessite, ͑᭹͒ Mn͑OH͒ or MnOOH, ͑␪͒ Mn2O3 , ͑᭿͒ Bi
2
*
͑ᮀ͒ ␥-MnO2 , ͑᭹͒ Mn͑OH͒ or MnOOH, ͑␪͒ Mn2O3 , ͑᭿͒ Bi or Bi2O3 , ͑ ͒
2
*
or Bi2O3 , ͑ ͒ Mn3O4 , and ͑ࡗ͒ graphite.
Mn3O4 , and ͑ࡗ͒ graphite.
of ␤-MnOOH, since Mn͑OH͒ is not expected to be formed at this
is present as found at the end of first charge ͑Fig. 5e͒. The Mn3O4
that is present at the end of second discharge ͑Fig. 8e or 9a͒ vanishes
completely at the end of second charge ͑Fig. 9e͒, similar to that
found during the first charge ͑Fig. 5e͒. The data confirm further that
the presence of bismuth helps to recharge the Mn3O4 to birnessite
MnO2 .
Figures 10 and 11 show the XRD patterns of the LSA cathodes at
various levels of discharge and charge, respectively, during the first
cycle. The X-ray data show that the fresh ͑0% discharge͒ LSA cath-
ode ͑Fig. 10a͒ consists of mainly ␥-MnO2 and ␣-Mn2O3 phases
apart from graphite. Although the material consists of a nominal 10
wt % Bi2O3 , X-ray data do not indicate its presence, possibly due to
the poor crystallinity. The ␥-MnO2 phase has an intergrowth struc-
ture consisting of ramsdellite and pyrolusite units. The appearance
of a reflection at around 2␪ ϭ 20° in the 25 and 50% discharged
cathodes ͑Fig. 10b and c͒ could be due to the formation of
2
discharge level. However, a similar reflection was not observed at
25% discharge during the first discharge cycle ͑Fig. 4b͒, possibly
due to a poor crystallinity of the ␤-MnOOH or the absence of the
formation of crystalline ␤-MnOOH during the first discharge cycle.
It is possible that the formation of a well-ordered crystalline birnes-
site MnO2 phase at the end of first charge ͑Fig. 5e͒ may help to form
the crystalline ␤-MnOOH phase or improve the crystallinity of the
␤-MnOOH phase formed during the second discharge. It suggests
that the crystallinity of the reduction/oxidation products depends on
the crystallinity of the material in the preceding stage. At 50% dis-
charge ͑Fig. 8c͒, reflections corresponding to Mn3O4 appear and the
cathode consists of birnessite MnO2, Mn3O4 , and ␤-MnOOH ͓or
Mn͑OH͒ ͔ phases. On further discharge, the amount of birnessite
2
MnO2 decreases and the fully discharged cathode ͑Fig. 8e͒ consists
of Mn͑OH͒ and Mn3O4 phases and a small amount of bismuth
2
metal apart from graphite. More importantly, the Mn3O4 phase that
began to appear at 50% discharge ͑Fig. 8c͒ decreases slightly at
100% discharge ͑Fig. 8e͒. Thus, part of the Mn3O4 undergoes re-
duction on going from 50-75% discharge to 100% discharge in Fig.
8, similar to that found during the first discharge in Fig. 4.
␤-MnOOH rather than Mn͑OH͒2 , since Mn͑OH͒ is not expected at
2
this level of discharge. The formation of crystalline ␤-MnOOH dur-
ing the first discharge cycle, unlike in the case of HSA cathodes,
could be due to the better crystallinity of and a different form of
manganese oxide (␥-MnO2) in the initial LSA sample that was
obtained by a thermal decomposition procedure; in the case of HSA
cathodes, the formation of crystalline ␤-MnOOH becomes apparent
only during the first charge ͑Fig. 5͒ or later discharge ͑Fig. 8͒.
Above 50% discharge ͑Fig. 10d and e͒, the LSA cathodes also yield
On charging back ͑Fig. 9͒, Mn͑OH͒ is oxidized to a well-
2
ordered birnessite MnO2 phase and the reflections corresponding to
the bismuth metal vanish due to the oxidation of metallic Bi to Bi3ϩ
.
This indicates that bismuth also undergoes redox reactions during
the discharge-charge process. At 100% charge ͑Fig. 9e͒, mostly a
well-ordered crystalline birnessite MnO2 phase apart from graphite
Mn3O4 along with the Mn͑OH͒ phase. Additionally, the strongest
2
reflections of metallic bismuth and possibly Bi2O3 , which overlap at
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Journal of The Electrochemical Society, 149 ͑4͒ A483-A492 ͑2002͒
A489
Figure 12. XRD patterns of discharged and charged HSA cathodes before
and after subjecting them to various numbers of cycles: ͑᭺͒ birnessite, ͑᭹͒
Figure 13. XRD patterns of discharged and charged LSA cathodes before
and after subjecting them to various numbers of cycles: ͑ᮀ͒ ␥-MnO2 , ͑᭺͒
*
Mn͑OH͒ or MnOOH, ͑ ͒ Mn3O4 , ͑ࡗ͒ graphite, ͑DCh͒ discharge, and ͑Ch͒
*
birnessite, ͑᭹͒ Mn͑OH͒ or MnOOH, ͑␪͒ Mn2O3 , ͑᭿͒ Bi or Bi2O3 , ͑ ͒
2
2
charge.
Mn3O4 , ͑ࡗ͒ graphite, ͑DCh͒ discharge, and ͑Ch͒ charge.
reversible redox process during the subsequent cycles as in the case
of HSA samples, even though the initial LSA sample is structurally
different from the initial HSA sample.
around 2␪ ϭ 28°, are seen clearly in the 25-100% discharged cath-
odes, unlike in the case of the HSA cathode where metallic bismuth
reflections are seen only at the end of the second discharge ͑Fig. 8e͒.
This difference could be due to the better crystallinity of and a
different form of manganese oxide ͑␥-MnO2 and ␣-Mn2O3͒ in the
initial LSA sample. It is possible that the bismuth could be distrib-
uted differently in the initial LSA and HSA samples. The amount of
␥-MnO2 phase that is present in the initial LSA sample decreases
during first discharge and it vanishes completely at 75-100% dis-
charge. On the other hand, the amount of ␣-Mn2O3 appears to de-
crease only slightly at the end of the first discharge cycle.
The data of HSA and LSA cathodes clearly reveal that the pres-
ence of bismuth in the BMD cathodes renders good rechargeability.
As discussed earlier, this assertion is supported by the electrochemi-
cal data of a mixture consisting of conventional Mn3O4 and Bi2O3 .
This mixture was found to have good rechargeability ͑Fig. 6͒ and a
well-ordered, crystalline birnessite MnO2 was formed at the end of
first charge. An explanation for the formation of well crystalline
birnessite MnO2 at the end of first charge irrespective of the initial
form of manganese oxide is provided in a later section.
On charging back the discharged cathodes ͑Fig. 11͒, the
Mn͑OH͒ phase vanishes while the birnessite MnO2 phase is
Structural characterization of cycled cathodes.—Both the HSA
and LSA cathodes were analyzed by XRD at the discharged and
charged states after subjecting them to various numbers of cycles.
The XRD patterns of the HSA and LSA cathodes before and after
cycling are shown in Fig. 12 and 13. As discussed in the previous
section, a well-ordered, crystalline birnessite MnO2 phase is formed
at the end of first charge both in the case of HSA and LSA, which
then undergoes a reversible discharge-charge process during subse-
quent cycling. An examination of the X-ray patterns in Fig. 12 and
13 reveals that the crystallinity and amount of the birnessite MnO2
2
formed, as in the case of HSA cathodes ͑Fig. 5͒. Additionally, the
Mn3O4 that is present at the end of the first discharge ͑Fig. 10e or
11a͒ vanishes completely at the end of the first charge ͑Fig. 11e͒,
indicating the rechargeability of Mn3O4 in the presence of bismuth,
as we found with the HSA sample. At the end of first charge, a
well-ordered crystalline birnessite phase is formed as in the case of
HSA cathodes, even though the initial LSA sample consists of a
different form of MnO2 (␥-MnO2) and Mn2O3 . During the second
discharge, the birnessite MnO2 phase is reduced to Mn͑OH͒ and
2
and Mn͑OH͒ phases decreases with increasing number of cycles
during the second charge, the Mn͑OH͒ phase is oxidized back to
2
2
the birnessite phase as in the case of the HSA cathode. These ex-
periments demonstrate that a well-ordered and crystalline birnessite
MnO2 is formed at the end of first charge, which then undergoes a
due to an increase in the amount of Mn3O4 ͑see the following͒ and
a possible increase in the amorphous components. Also, the amount
of Mn2O3 phase in the LSA cathodes ͑Fig. 13͒ decreases with in-
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A490
Journal of The Electrochemical Society, 149 ͑4͒ A483-A492 ͑2002͒
Figure 14. SEM photographs of the HSA cathode ͑a͒ after second charge
and ͑b͒ after 40th charge. The region of bismuth clustering in the cycled
electrode is indicated by a rectangle.
creasing number of cycles, indicating a participation of Mn2O3 in
the redox process in the presence of bismuth. Although little or no
Mn3O4 is found at the end of charge during the initial number of
cycles, some amount of Mn3O4 is observed at the end of charge
after extended cycling. However, for a given cycle, the amount of
Mn3O4 found at the end of discharge is larger than that found at the
end of charge, even after extended cycling. This observation again
confirms the participation of Mn3O4 in the redox process even after
extended cycling. The increase in the amount of Mn3O4 at the end
of charge with cycling is due to the clustering of the bismuth metal
and an isolation of the manganese oxide away from bismuth, as
indicated by EDS analysis of the cycled cathodes in SEM. With a
decreasing intimate contact with bismuth, the Mn3O4 particles be-
come increasingly inactive during extended cycling. The increase in
the amount of isolated Mn3O4 phase with cycling could be one of
the reasons for the capacity fade found during cycling.
Figure 15. Variation of potassium content in HSA cathodes with level of
discharge/charge: ͑a͒ during first cycle and ͑b͒ during second cycle.
the second charge exhibits a uniform distribution of bismuth ͑Fig.
14a͒, that after the 40th charge shows a clustering of bismuth as
indicated in Fig. 14b. More important, the EDS compositional map-
ping for Mn, O, K, Bi, and C showed some important differences
between the discharged and charged cathodes. While no significant
change was observed in the distribution of other elements, the
amount of potassium was found to be higher in the charged state
compared to that in the discharged state.
In order to have a further understanding, the potassium contents
in the cathodes at various levels of discharge and charge were ana-
lyzed by AAS. The variations of potassium content with the depth of
discharge and charge are shown in Fig. 15 and 16 for the HSA and
LSA cathodes, respectively, during the first and second cycles. The
Microstructural and compositional characterizations.—The mi-
crostructures of both the HSA and LSA cathodes were examined by
SEM equipped with EDS analysis during the first two discharge-
charge cycles and at the discharged and charged states after subject-
ing them to various numbers of cycles. SEM data indicate the pres-
ence of hexagonal shape Mn͑OH͒ particles in the discharged
2
cathodes. In addition, EDS analysis indicates a clustering of bismuth
after extended cycling, as shown in Fig. 14. While the cathode after
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Journal of The Electrochemical Society, 149 ͑4͒ A483-A492 ͑2002͒
A491
change in K content is nearly reversible as in the case of the HSA
cathode ͑Fig. 15b͒.
The chemical analysis data clearly establish, for the first time, a
change in the K content during the discharge-charge process. The
data reveal dissolution of potassium from the crystal lattice into the
electrolyte during discharge and an incorporation of potassium from
the electrolyte into the crystal lattice during charge. We believe that
the incorporation of the Kϩ ions from the electrolyte into the man-
ganese oxide cathode is to stabilize the layered birnessite structure.
It appears that the stabilization of the layered birnessite structure
requires significant amount of positively charged cations, such as the
Kϩ ions between the layers. The presence of potassium between the
layers leads to a well-ordered, crystalline birnessite phase, as evi-
dent in the XRD patterns discussed in an earlier section. In the case
of an LSA cathode that does not contain potassium initially, the
incorporation of potassium from the electrolyte into the LSA cath-
ode during the first charge leads to the formation of birnessite, as
illustrated by the X-ray data in Fig. 11. The chemical analysis ex-
periments thus establish that the discharge/charge mechanism of the
manganese oxide cathodes in rechargeable alkaline cells involves a
dissolution/incorporation of Kϩ ions into/from the electrolyte from/
into the cathode lattice.
Conclusions
Bismuth-modified manganese dioxide cathodes have been shown
to exhibit good cyclability at high ͑50-65%͒ loadings. The cathodes
retain more than 80% of initial capacity over 300 cycles at 50%
loading. Layered birnessite-type MnO2 is formed after the first
discharge-charge cycle irrespective of the initial form of the BMD
materials. The cathodes have higher potassium content in the
charged state compared to the discharged state due to an incorpora-
tion of the Kϩ ions from the electrolyte into the cathode lattice
during charge. The higher potassium content in the charged state is
to stabilize the layered birnessite structure. The rechargeable alka-
line cells thus involve an incorporation/dissolution of potassium ions
into/from the cathodes during the charge/discharge process. The
presence of bismuth renders good cyclability to the manganese di-
oxide cathodes as well as other manganese oxides that cannot oth-
erwise be cycled. The mechanism of BMD cycling is similar to that
of EMD in the sense that it involves dissolution of Mn3ϩ interme-
diate and the second electron reduction is a dissolution/precipitation
process. However, most of the first electron reduction in BMD oc-
curs in a heterogeneous manner, unlike in EMD. The species that are
involved during the cycling of BMD have also been identified in the
literature.22,23,27,28,33,34 Nevertheless, the exact mechanism of bis-
muth participation in rendering good rechargeability to the manga-
nese oxides remains to be established concretely.
Acknowledgments
This work was supported by the RBC Technologies at College
Station, Texas, through a NIST-Advanced Technology Program. The
authors thank Dongmin Im for his assistance during the course of
this work, and Dr. Frank Jamerson, Dr. Brendan Coffey, and Dr.
Ramesh Kainthla for valuable discussions.
Figure 16. Variation of potassium content in LSA cathodes with level of
discharge/charge: ͑a͒ during first cycle and ͑b͒ during second cycle.
The University of Texas at Austin assisted in meeting the publication
costs of this article.
initial HSA sample has about 7.5 wt % K and the K content de-
creases continuously up to 50% discharge ͑Fig. 15a͒. The sample
with 50-100% discharge has around 1 wt % K. Upon charging back
the discharged cathodes, the K content remains around 1 wt % up to
50% charge and thereafter increases. The fully charged cathode has
around 10 wt % K, which is slightly higher than that in the initial
HSA cathode. During the second cycle ͑Fig. 15b͒, the change in K
content is fully reversible. In the case of LSA cathodes ͑Fig. 16͒, the
K content remains around 0 wt % throughout the first discharge,
since the initial LSA cathodes did not contain any K. During the first
charge, the K content increases and the fully charged sample has
around 8 wt % K ͑Fig. 16a͒. During the second cycle ͑Fig. 16b͒, the
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