Characterisation of chemically lithiated heat-treated electrolytic manganese dioxide

Article abstract of DOI:10.1016/j.materresbull.2011.05.027

Heat treated manganese dioxide is partially lithiated using butyl-lithium to determine the changes in crystal structure, chemical composition and morphology upon reduction, as a means of simulating its discharge behaviour in a non-aqueous battery cathode. As reduction proceeds, and lithium ions are inserted into the heat treated electrolytic manganese dioxide (EMD) structure, the material undergoes a phase transition to LiMn₂O₄. This new phase is further reduced to Li₂Mn₂O₄. Reduction initially results in a 56% decrease in the surface area of the material; however, at higher degrees of reduction a slight increase in this value is observed, as a consequence of the strain placed on the lattice through continued lithium insertion.

Full text of DOI:10.1016/j.materresbull.2011.05.027

Materials Research Bulletin 47 (2012) 1827–1834
Contents lists available at ScienceDirect
Materials Research Bulletin
journal homepage: www.elsevier.com/locate/matresbu
Characterisation of chemically lithiated heat-treated electrolytic manganese
dioxide
*
Wesley M. Dose, Joshua Lehr, Scott W. Donne
Discipline of Chemistry, University of Newcastle, Callaghan, NSW 2308, Australia
A R T I C L E I N F O
A B S T R A C T
Article history:
Heat treated manganese dioxide is partially lithiated using butyl-lithium to determine the changes in
crystal structure, chemical composition and morphology upon reduction, as a means of simulating its
discharge behaviour in a non-aqueous battery cathode. As reduction proceeds, and lithium ions are
inserted into the heat treated electrolytic manganese dioxide (EMD) structure, the material undergoes a
phase transition to LiMn2O4. This new phase is further reduced to Li2Mn2O4. Reduction initially results in
a 56% decrease in the surface area of the material; however, at higher degrees of reduction a slight
increase in this value is observed, as a consequence of the strain placed on the lattice through continued
lithium insertion.
Received 18 March 2011
Received in revised form 9 May 2011
Accepted 18 May 2011
Available online 26 May 2011
ß 2011 Elsevier Ltd. All rights reserved.
1. Introduction
[6]. This is particularly noticeable for Mn(III) containing ions, since
it is a Jahn–Teller distorted ion. As a result, the basic structural unit
for most manganese oxide and hydroxide compounds is the
[Mn(O2ꢀ,OHꢀ,OH2)6]z+ octahedral unit [6]. Assembling these basic
1.1. Manganese dioxide as a battery material
The growing demand for battery systems to support high power
portable technologies prevalent in today’s society makes the study
of such systems vital. Systems using the manganese dioxide cathode
make up the majority of this market due primarily to the unique
combination of physio-chemical and electrochemical properties,
such as a relatively high density and purity, and the spectrum of
possible discharge rates maintainable for prolonged periods of time.
These characteristics, along with its low cost and low toxicity, make
it a difficult material to supersede [1]. While the aqueous Zn/MnO2
system is the most common battery system, since the 1980s non-
aqueous Li/MnO2 battery systems have found popular application in
high energy density and high power devices [2,3].
building blocks by corner and/or edge sharing arrangements gives
rise to the different structures [4,5].
The simplest and most thermodynamically stable form of
manganese dioxide is b-MnO2 (pyrolusite), which consists of corner
sharing octahedra in the a–b plane, and edge sharing octahedra in
the c direction. The result is a (1 ꢁ 1) tunnel structure progressing in
the c direction, as shown in Fig. 1a. For this tetragonal structure the
unit cell parameters are a0 = 0.439 nm and c0 = 0.287 nm [4,5].
Similarly, the ramsdellite structure can be visualised by using both
edge and corner sharing in the a–b plane to form a (2 ꢁ 1) tunnel
headed into the c direction. Ramsdellite has an orthorhombic unit
cell with a0 = 0.446 nm, b0 = 0.932 nm and c0 = 0.285 nm [4,5]. The
structure of this compound is shown in Fig. 1b.
1.2. Structure of manganese dioxide
g-MnO2 is the most commonly used manganese dioxide
structure in both the alkaline battery system [7], and as the
precursor that is heat treated prior to use in the non-aqueous Li/
Manganese dioxides form
a large and diverse group of
compounds comprising of many different crystal structures; over
30 different compounds have been identified to date [4,5]. These
structural differences affect significantly both the thermal and
electrochemical behaviour of these materials. Hence it is important
to describe, in some detail, the structures we will encounter here.
Manganese in oxidation states II, III and IV prefers almost
exclusively an octahedral arrangement, albeit somewhat distorted
MnO2 system. The g-MnO2 structure is best described as a random
microscopic intergrowth between the pyrolusite and ramsdellite
forms of manganese dioxide [8], as shown in Fig. 1c. Other features
associated with the
g-MnO2 structure include cation vacancies,
lower valent manganese cations (Mn(III)), and structural water
present as protons associated with oxide anions which compen-
sate for the charge deficiency incurred by cation vacancies
and Mn(III) ions [9–12]. As a result, the chemical composition of
g
-MnO2 has been described as:
* Corresponding author. Tel.: +61 2 4921 5477; fax: +61 2 4921 5472.
4þ
2ꢀ
ðMn
Þ
ꢂð&ÞxꢂðMn3þÞyꢂðO
Þ
ꢂðOHꢀÞ4xþy
2ꢀ4xꢀy
(1)
1ꢀxꢀy
E-mail address: scott.donne@newcastle.edu.au (S.W. Donne).
0025-5408/$ – see front matter ß 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.materresbull.2011.05.027

1828
W.M. Dose et al. / Materials Research Bulletin 47 (2012) 1827–1834
(b) Ramsdellite
(a) Pyrolusite (β-MnO2)
(c) γ -MnO2
Fig. 1. Basic building blocks of the g-MnO2 structure in which (a) pyrolusite (b-MnO2) and (b) ramsdellite have been microscopically intergrown to produce (c) g-MnO2.
where x and y represent the mole fraction of cation vacancies (&)
and Mn(III) ions, respectively. In an unreduced -MnO2 sample,
typical ranges for x and y are 0.06–0.08 and 0.04–0.12, respectively
distribution. The continuing introduction of the larger Mn(III)
˚ ˚
g
species replacing Mn(IV) (ionic radii of 0.645 A and 0.530 A,
˚
respectively [17]), and the inclusion of the lithium ion (0.76 A [18])
[13]. Another characteristic which can explain some features in the
X-ray diffraction pattern of g-MnO2 is microtwinning of the (0 2 1)
and (0 6 1) growth planes [12]; however there is conflicting
physical evidence as to its actual existence [14].
into the structure of the material leads to an expansion of the
lattice. Structural rearrangement as a result of ion insertion, and
fewer available sites for lithium intercalation as depth of discharge
increases are factors impacting the discharge capacity of the cell.
As can be seen by Eq. (1), considerable structural variation can
be found between the multitude of materials that can be loosely
1.4. This work
classified as
g-MnO2. The structural features of a given g-MnO2
sample are dependant upon both the synthetic method, as well as
the experimental variables chosen for the material’s production
The overall focus of this study is to investigate the changes that
occur to heat treated manganese dioxide as it undergoes reduction
in a non-aqueous media. While in this work reduction is achieved
via chemical means, it is likely that similar changes take place
when this process is performed electrochemically. A deeper
understanding in this area provides insight into the changes that
take place in the Li/MnO2 cell and in turn will lead to
improvements in the use of manganese dioxide as a cathode
material in the non-aqueous Li/MnO2 system.
process.
g-MnO2 used as the cathode-active material is most
commonly prepared via electrolysis, and hence termed electrolytic
manganese dioxide (EMD) [7]. The original high resolution TEM
work on EMD, conducted by Turner and Buseck [15], showed
definitive evidence in support of De Wolff’s microscopic inter-
growth model for g-MnO2 [8]. More recent TEM work on EMD by
Heuer et al. [14] has indicated the material consists of needle-like
crystallites ꢃ10 nm ꢁ 10 nm ꢁ 100 nm in size. These needles were
found to be assembled in a somewhat ordered fashion into grains
(ꢃ200 nm in diameter) which are arranged randomly to form the
bulk structure. Also observed between the crystallites and grains
were pores, into which electrolyte is expected to permeate during
electrochemical discharge.
2. Experimental
2.1. Material preparation
The samples of
g-MnO2 used in this work were prepared by
anodic electrodeposition, and hence given the designation
electrolytic manganese dioxide (EMD). The cell used for electroly-
sis was based on a temperature controlled 2 L glass beaker in which
two 144 cm2 (72 cm2 on either side) titanium sheets were used as
the anode substrate, and three similarly sized copper sheets were
used as the cathode substrate. The electrodes were arranged
alternately so that each anode was surrounded on both sides by a
cathode. The electrolyte was an aqueous mixture of 1 M MnSO4
(APS Chemicals, 98%) and 0.25 M H2SO4 (Lab-Scan, 98%) main-
tained at 97 8C. Electrodeposition of the manganese dioxide was
conducted with an anodic current density of 65 A/m2 according to
the reactions:
1.3. Electrochemical discharge of manganese dioxide
Although much work has been done in characterising the
discharge mechanism of the aqueous
g-MnO2 system, the
literature regarding the non-aqueous Li/MnO2 system for this
starting material is scarce and lacks detailed analysis. Ikeda et al.
[16] first proposed that lithium ions were inserted into the g-MnO2
lattice during discharge, in a manner analogous to that of aqueous
cells. They outlined the reaction mechanism for the cathode
material to be:
MnO2 þ Liþ þ e ! LiMnO2
(2)
Anode ðTiÞ: Mn2þ þ 2H2O ! MnO2 þ 4Hþ þ 2eꢀ
Cathode ðCuÞ: 2Hþ þ 2eꢀ ! H2
(3)
(4)
(5)
Similar to the aqueous system, it is thought that an electron is
inserted into the manganese dioxide, and to compensate for
charge, a lithium ion from the electrolyte solution is also inserted.
These ions diffuse through the lattice to achieve a uniform
Overall: Mn2þ þ2H2O ! MnO2 þ 2Hþ þ H2

W.M. Dose et al. / Materials Research Bulletin 47 (2012) 1827–1834
1829
The overall process was carried out for three days, during which
time the electrolyte Mn2+ concentration was depleted, and the H+
concentration increased. To counteract this, and hence maintain a
constant electrolyte composition over the duration of the
deposition, a concentrated (1.5 M) MnSO4 solution was added
continually at a suitable rate to replenish Mn2+ and dilute any
excess H2SO4 added. Control of the solution conditions was
typically maintained to within ꢄ2%.
representative 0.10 g sample of the manganese dioxide material
was degassed under vacuum at 110 8C for 2 h prior to analysis. An
adsorption isotherm was then determined over the partial
pressure range of 10ꢀ7 ꢀ 1 using N2 gas as the adsorbate at
77 K. The specific surface area was extracted from the gas
adsorption data using the BET isotherm (0.05 < P/P0 < 0.3, while
the pore size distribution was determined using a Density
Functional Theory-based approach (Micromeritics DFTPlus
V2.00).
A Philips XL30 scanning electron microscope (SEM) was used at
a range of magnifications to investigate the morphology of the
materials.
The compositional features of each material were determined
using a potentiometic titration technique as outlined in Vogel [21].
Using this method, 0.100 g of the manganese dioxide sample was
dissolved in 25 mL of 0.25 M acidified (0.1 M H2SO4) ferrous
ammonium sulfate (BDH Chemicals, 99%) solution, represented by
the equation:
After deposition was complete, the solid EMD deposit was
mechanically removed from the anode and broken into chunks
ꢃ0.5 cm in diameter, and then immersed in 500 mL of Milli-Q
water to assist in the removal of entrained plating electrolyte. The
pH of this chunk suspension was adjusted to pH 7 with the addition
of 0.1 M NaOH (Sigma–Aldrich, 98%). After ꢃ24 h at a pH of 7 the
suspension was filtered and the chunks dried at 110 8C. After
drying the chunks were milled to a ꢀ105
mm powder (mean
particle size ꢃ45 m) using an orbital zirconia mill. The powder
m
was then suspended in ꢃ500 mL of Milli-Q water and its pH again
adjusted to 7 with the further addition of 0.1 M NaOH. When the
pH had stabilised, the suspension was filtered and the collected
solids dried at 110 8C. The dry powdered EMD was removed from
the oven, allowed to cool to ambient temperature in a dessicator
and then transferred to an airtight container for storage.
MnOn þ ð2n ꢀ 2ÞFe2þ þ 2nHþ ! Mn2þ þ ð2n ꢀ 2ÞFe3þ þ nH2O (6)
This solution was titrated against
a standardised 0.2 M
potassium permanganate (KMnO4; Ajax Finechem, 99%) solution
(oxalate method [21]) and the volume of permanganate added
denoted V1. This reaction can be represented by:
2.2. Heat treatment of EMD
MnO4ꢀ þ 8Hþ þ 5Fe2þ ! Mn2þ þ 4H2O þ 5Fe3þ
(7)
The prepared EMD was heat treated in
a temperature
A blank titration of this type was also performed, without any
sample added, and recorded as V0. Approximately 20 g of tetra-
sodium pyrophosphate (Na4P2O7ꢂ10H2O; Ajax Finechem, 99%) was
added to the resulting solution from the first titration to stabilise
the formation of the Mn(III) complex formed in the subsequent
titration, ensuring the formation of a saturated solution. The pH of
this solution was then adjusted to the range 6–7 by the addition of
ꢃ0.20 M sulphuric acid. A second potentiometic titration was
performed using the same KMnO4 solution, and recording the
volume to reach the end point as V2; i.e.,
controlled furnace at 350 8C for 1.52 h. The duration of the heat
treatment was specifically calculated to be the time required to
remove all surface and structural water from the EMD material.
This kinetic analysis was performed using the iso-conversional
method and has been described in detail in our earlier works
[19,20].
2.3. Chemical lithiation of HEMD
The entire chemical lithiation process was carried out in an
argon atmosphere to avoid protonation of the heat treated EMD
(HEMD) by atmospheric water, and reaction of the highly toxic
reagents used with air. A sample of HEMD was chemically lithiated
by first adding ꢃ10 mL of hexane (Sigma–Aldrich, 95%) to a 100 mL
conical flask. The desired volume of 1.6 M n-butyl lithium in
hexane (Sigma–Aldrich) corresponding to the required mole
fraction of lithium, was added to the flask using an Epindorph
pipette and the resulting solution made up to ꢃ50 mL with hexane.
Two grams of the HEMD (recently dried at 110 8C) was slowly
added to the flask which was then stoppered and taped with Teflon
to seal the system. The reaction was left to proceed for one week,
but stirred daily. After the elapsed time, the lithiated HEMD was
filtered, rinsed with hexane, and subsequently dried under vacuum
at 110 8C. Under these conditions the HEMD is expected to be
reduced relatively slowly and thus avoid complications potentially
due to poor reduction kinetics which may polarise the cathode
material surface.
4Mn2þ þ MnO4ꢀ þ 8Hþ þ 15H2P2O7 ! 5MnðH2P2O7Þ33ꢀ þ 4H2O
2ꢀ
(8)
The value for n in LinMnO2 was then calculated using:
5ðV0 ꢀ V1Þ
n ¼ 2 ꢀ
(9)
V2 ꢀ V1
The total manganese content in the sample (%Mn) can be found
from the second titration (Eq. (8)) by taking into account the
amount of manganese added through the addition of permanga-
nate in the first titration (Eq. (7)). Using this result (mass of
manganese; mMn), and the dry mass of the manganese dioxide
sample, found by subtracting the mass of surface water lost from
the sample after heating at 110 8C from the original mass
ðmMnO ðdryÞÞ, the total manganese content can be found using:
2
mMn
%Mn ¼
ꢁ 100%
(10)
m
MnO2 ðdryÞ
2.4. Material characterization
To calculate the relative proportion of manganese (III) and (IV)
species (%Mn(III) and %Mn(IV) respectively), we have:
The structural features of each material were examined by X-
ray diffraction using a Phillips 1710 diffractometer equipped with a
˚
a radiation source (l = 1.5418 A) and operated at 40 kV and
Cu K
%MnðIIIÞ ¼ n ꢁ %Mn
(11)
(12)
30 mA. The scan range was from 108 to 808 2u with a step size of
0.058 and a count time of 2.5 s. Phase identification in the XRD
patterns collected was carried out by comparison with data from
the Inorganic Crystal Structure Database (ICSD).
%MnðIVÞ ¼ ð1 ꢀ nÞ ꢁ %Mn
Finally, the cation vacancy fraction (CVF) can be found by taking
into account the percentage structural water (%H2Ostructural), found
from the difference in mass after heating the sample at 400 8C for
Morphology was examined by gas adsorption using a Micro-
meritics ASAP 2020 Surface Area and Porosity Analyser.
A

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W.M. Dose et al. / Materials Research Bulletin 47 (2012) 1827–1834
2 h, and using:
retained in the structure. Kinetic limitations of the oxidation of
Mn(III) to Mn(IV) during heat treatment and consequently the
removal of associated protons from the structure, may explain this
phenomenon.
m
CVF ¼
(13)
(14)
m þ 2
where
3.2. Compositional changes upon lithiation
n
MMn ꢁ %H2Ostructural
m ¼
þ
2
MH O ꢁ %Mn
2
Describing the chemical composition of the lithiated HEMD
samples serves primarily to confirm that reduction of the material
has taken place, supported by an increase in Mn(III) and a
corresponding decrease in Mn(IV) (Table 1). A comparison of the
experimentally determined n in LinMnO2 (n in LinMnO2 (expt)) and
the corresponding theoretical value based on preparation (n in
LinMnO2 (prep)) demonstrates that the correlation between these
values is relatively good, but becomes progressively worse with
higher amounts of reduction. This arises as a result of (i) kinetic
limitations of the reduction process for the samples in which a
higher degree of lithiation was required, and/or (ii) the formation
of an equilibrium between the reactants and products in the
reaction vessel, hence limiting the extent to which this process was
able to progress.
and MMn and MH are the molar masses of manganese and water,
respectively.
O
2
3. Results and discussion
3.1. Starting material properties
The compositional, structural and morphological properties of
the precursor EMD and HEMD are listed in the first two rows of
Tables 1–3, respectively. From the data it is evident that the heat
treatment has resulted in near complete conversion of the g-MnO2
phase to pyrolusite. This is further supported by the XRD patterns
(Fig. 2) for these materials, which show peaks characteristic of
pyrolusite, particularly the merging of the (1 1 0) and (1 3 0) peaks
in the original EMD material (based on an orthorhombic unit cell)
3.3. Structural changes
to a single peak at ꢃ298 2
u for the HEMD. Significantly, all cation
vacancies have been removed from the material; however, a
substantial percentage of Mn(III) and associated structural water is
X-ray diffraction is the most popular tool for determining the
crystal structure of manganese dioxide. However, the finely
Table 1
Changes in composition for lithiated HEMD materials.
n in LinMnO2 (pred)
n in LinMnO2 (expt)
Mn(T) (%)
Mn(IV) (%)
Mn(III) (%)
%H2O (+110 8C)
EMD
HEMD
0.15
0.30
0.45
0.60
0.75
0.90
0.10
0.10
0.20
0.30
0.40
0.56
0.64
0.72
58.4
60.2
60.6
59.8
56.4
56.6
54.6
48.1
52.6
54.0
48.8
41.6
34.2
24.4
20.2
13.3
5.8
6.22
11.8
18.2
22.1
32.2
34.4
34.8
4.1
0.84
2.18
2.67
3.83
1.94
1.90
1.25
Table 2
Unit cell parameters and volume for lithiated HEMD samples.
Unit cell volume (nm3)
˚
˚
˚
n in LinMnO2
-MnO2 orthorhombic unit cell
Pr
a0 (A)
b0 (A)
c0 (A)
g
EMD (0.10)
HEMD (0.10)
0.20
0.34
1.00
–
4.47
4.42
4.23
4.11
4.05
9.55
9.11
2.83
2.87
2.89
2.82
2.80
0.121
0.115
0.116
0.122
0.122
9.47
0.30
–
10.53
10.79
0.40
–
LiMn2O4 cubic unit cell
0.56
0.64
0.72
–
–
–
8.24
8.27
8.22
–
–
–
–
–
–
0.559
0.565
0.556
Table 3
Morphological changes of HEMD with respect to reduction.
n in LinMnO2
BET surface area
(m2/g)
Micro-pore volume
(cm3/g)
Meso-pore volume
(cm3/g)
EMD (0.10)
HEMD (0.10)
0.20
37.2
24.9
17.9
14.1
11.9
10.9
11.3
13.4
0.0074
0.0021
0.0013
0.0011
0.0000
0.0009
0.0009
0.0015
0.029
0.038
0.024
0.020
0.211
0.017
0.022
0.022
0.30
0.40
0.56
0.64
0.72

W.M. Dose et al. / Materials Research Bulletin 47 (2012) 1827–1834
1831
Li2Mn2O4
LiMn2O4 / Li2Mn2O4
n=0.72
HEMD
n=0.64
n=0.56
n=0.40
n=0.30
LiMn2O4
EMD
10
20
30
40
50
60
70
80
2θ
n=0.20
Fig. 2. X-ray diffraction pattern for the starting EMD and heat-treated material.
divided and disordered nature of manganese dioxide powder
results in broad peaks in the XRD pattern, somewhat limiting the
usefulness of the technique. The modelling work conducted by
Chabre and Pannetier [12] has been used in this work to extract
data pertaining to the material properties from the XRD patterns.
XRD patterns for the partially lithiated manganese dioxide
samples (LinMnO2) are shown in Fig. 3. A number of important
changes occur in the patterns with increasing degree of lithiation.
The patterns can be divided into two groups, the first relating to
reduction in the range n < 0.5, and the second to reduction in the
range n > 0.5. This suggests two (likely overlapping) reduction
processes are operative. Key changes in the XRD patterns for the
first half of reduction, i.e., n < 0.5, are: (i) a gradual disappearance
HEMD (n=0.10)
10
20
30
40
50
60
70
80
2θ
Fig. 3. X-ray diffraction data for the partially lithiated HEMD samples.
applied in accommodating the larger lithium ion. Increasing
lithiation simply leads to a greater fraction of LiMn2O4 regions
found dispersed in the parent
b-MnO2 structure.
Changes evident in the XRD patterns in the reduction range
n > 0.5 can be summarised as (i) further decrease in relative
of the peak at ꢃ298 2
, (iii) shifting of peaks at ꢃ378 and ꢃ578 2
and (iv) early and strengthening emergence of a number of peaks
not present in the starting HEMD, most noticeably at ꢃ198 2
The first two changes noted highlight a decrease in the relative
intensity for peaks characteristic of -MnO2 (pyrolusite). The unit
cell of this structural phase is also found to expand as a
consequence of reduction, indicated by the shift in -MnO2 peaks
to lower diffraction angles. The expansion likely relates to the
lattice strain induced as lithium ions are inserted into the structure
and/or Jahn–Teller distortion resulting from the presence of
Mn(III). The final point indicates the formation of a different
structural phase not found in the starting HEMD, but present even
in samples with low degrees of reduction. These emerging peaks
are in positions characteristic of the spinel LiMn2O4 structure,
indicating a structural transition to this phase. Notably, the
identification of this phase in the discharge of HEMD is a new
finding previously unreported in literature.
u
, (ii) decreasing intensity of the peaks at ꢃ578
intensity of
strengthening relative intensity and slight shift of the peak at
ꢃ578 2 to lower diffraction angles, and (iii) emergence of
numerous peaks by n = 0.72 in two broad regions in the diffraction
pattern; namely, 32–478 2 and 57–688 2 . The first point suggests
that further structural transition of pockets of the reduced
MnO2 structure to LiMn2O4 occurs as a consequence of the
continuing lithium insertion. Additionally, the formation of the
LiMn2O4 structure in the early stages of reduction makes viable
the further reduction of this phase to Li2Mn2O4. When the XRD
pattern for Li2Mn2O4 is compared with the patterns of the highly
reduced materials, peaks consistent with this material are found.
Admittedly, by this stage the peaks in the XRD pattern have
become increasingly difficult to interpret since at least three
structural phases have been utilised in explaining the XRD data,
and also because of the highly disordered structure formed.
b
-MnO2 peaks (e.g., peaks at ꢃ578
2u), (ii)
2u
u
to lower 2 values,
u
u
u.
u
u
b
b-
b
However, the movement of the peak at ꢃ198 2
u to lower
diffraction angles and the emergence of peaks in the regions
defined above both support the notion of the reduction of LiMn2O4
to Li2Mn2O4.
The relative intensity of the peaks associated with this phase
increase with greater amounts of reduction. This suggests that the
reduction of manganese dioxide (in which lithium ions are inserted
into the structure) causes regions of the reduced LinMnO2 structure
to convert to the spinel LiMn2O4 phase. This is most probably
driven by the need to stabilise the material as a result of the strain
The calculation used to determine the unit cell parameters for a
material assumes constant unit cell geometry. In the current
system this assumption decreases in validity with the formation of
the new LiMn2O4 phase and its subsequent reduction to Li2Mn2O4.

1832
W.M. Dose et al. / Materials Research Bulletin 47 (2012) 1827–1834
However, approximate values of the unit cell parameters can be
determined by simply assuming an orthorhombic unit cell (i.e.,
that for g-MnO2) for materials with n < 0.5, and using a cubic unit
0.5 cm3/g
cell (LiMn2O4) for those more reduced than this level. This
assumption, based on the structure expected to be accommodating
the majority of the charge in the given region, is no doubt an over-
n=0.72
simplification of the system, although it provides
approximation.
a useful
n=0.64
n=0.56
The unit cell parameters and volume determined for the
lithiated samples are listed in Table 1. From this data, it is
evident that lithiation of HEMD up to n = 0.40 results in a 5.9%
increase in the unit cell volume. The a0 and c0 parameters both
show a decrease in value with higher degrees of lithiation, while
there is
a significantly more drastic expansion of the b0
parameter. Reduction for materials above n = 0.5, in which a
cubic unit cell is assumed, demonstrates a relatively constant
unit cell volume, with variation attributed to errors in the peak
fitting procedure.
n=0.40
n=0.30
In summary, these structural findings suggest that reduction of
b
-MnO2 (i.e., HEMD) does not occur by the simple one-step
mechanism as previously proposed in the literature, but rather by a
multi-step mechanism which can be summarised as:
n=0.20
MnO2 þ nLiþ þ neꢀ ! LinMnO2
Electrochemical
(15)
LinMnO2! nLiMn2O4þð1ꢀ 2nÞMnO2 Disproportionation ðn < 0:5Þ
HEMD (n=0.10)
(16)
LiMn2O4 þ Liþ þ eꢀ ! Li2Mn2O4
Electrochemical ðn > 0:5Þ
(17)
EMD (n=0.10)
3.4. Morphological changes
1
10
100
1000
Pore Width (Å)
The BET surface area data for the series of lithiated HEMD
samples is displayed in Table 2. While some of the initial porosity
of the starting EMD has been lost due to heat treatment, the
starting HEMD used in this study had a BET surface area of 24.9 m2/
g. As lithiation progressed to higher depths, the surface area
decreased by 56% to a minimum value of 10.9 m2/g at n = 0.56.
Beyond this level of lithium insertion, an increase in the BET
surface area was evident. Consideration of pore volume with
respect to pore width (Fig. 4) further elucidates the nature of
Fig. 4. Normalised pore area distribution over the range of pore widths for partially
lithiated HEMD samples.
3.5. Comparisons to aqueous chemical reduction of EMD
In our previous work [24], non-heat treated EMD was
chemically reduced and the resultant properties of the materials
studied. Those results may be compared to those presented in the
current work to draw comparisons between the behaviour of
manganese dioxide to proton versus lithium intercalation. Firstly,
it is important to note that a key difference between these studies
lies in the starting material used. Specifically, the manganese
dioxide used in this work was heat treated prior to chemical
reduction in order to mimic the preparatory conditions of the
cathode used in non-aqueous cells. Conversely, the EMD used in
the study by Malloy and Donne [24] received no such thermal
treatment.
˚
porosity change as a function of reduction. Pores ꢃ7 A wide are
removed from the sample as lithiation increases, becoming
completely absent by n = 0.64. Additionally, pores with widths
˚
˚
between 20 A and 200 A at first decrease in volume up until
n = 0.40, after which they increase. Also contributing to the
increase in porosity beyond n = 0.56 is the widening of pores found
˚
˚
between 14.8 A and 17.1 A. The initial decrease in porosity is most
likely due to the collapse of pores caused by the swelling of the
lattice as reduction occurs [22], while the strain on the structure of
the material as lithium intercalation proceeds beyond n = 0.56 can
increase in porosity for these materials.
The structural response of the manganese dioxide to the
insertion of protons and lithium ions was found to be noticeably
different. In the aqueous system, reducing the EMD resulted in a
swelling of the lattice, as indicated by the shift of certain peaks in
Images of the material surface taken using scanning electron
microscopy (SEM) further confirm the changes in morphology
already discussed. Surface images taken at 10,000ꢁ magnification
for the starting HEMD, Li0.40MnO2, Li0.56MnO2 and Li0.72MnO2 are
shown in Fig. 5. What is immediately evident from these images is
the development of cracks on the surface of the material. These
fissures, not observed in the HEMD, become more pronounced
with increased lithiation. These can be accounted for by either the
strain on HEMD as a consequence of the insertion of lithium ions,
and/or conversion of the outside regions of the particles to the
different density LiMn2O4 phase (4.28 g/cm3 [23], compared with
the XRD pattern to lower 2
discharge, i.e., n = 0.84 for HnMnO2, did a change to a distinct
Mn(III) phase occur. In essence, the -MnO2 structure, although
u angles. Only at the higher depths of
g
strained, was retained as reduction proceeded. Conversely, lithium
ion intercalation leads to reduction, structural transition, and
further reduction of this newly formed phase, as already described
in Eqs. (15)–(17).
Comparisons in the unit cell parameters and volume can only be
made for the first half of reduction where the unit cell type
5.19 g/cm3 for
b-MnO2 [12]).

W.M. Dose et al. / Materials Research Bulletin 47 (2012) 1827–1834
1833
Fig. 5. SEM images for starting HEMD and selected partially lithiated samples at 10,000ꢁ magnification.
assumed is the same. Fig. 6 shows the relative increase in unit cell
volume for the non-aqueous and aqueous systems. In the aqueous
system this increase is determined almost exclusively by the
reduction of Mn(IV) to the larger (and more distorted) Mn(III) ion,
since the atomic radius of O2ꢀ is very close to that of OHꢀ [24].
Conversely, in the non-aqueous system the inclusion of the larger
lithium ion also contributes to lattice expansion during reduction.
Interestingly, this additional factor leads to an expansion over
twice the value observed for the aqueous system in this
compositional range.
Morphological changes to the BET surface area show some
similarities, but also significant differences. The percentage
decrease in BET surface area is similar for the two systems: a 54%
decrease for aqueous reduction and 56% for the non-aqueous
system. However, the slight increase in surface area for high
depths of discharge in the non-aqueous system is not apparent
from proton intercalation. The larger size of the lithium
ion compared to the proton, and the additional strain its
intercalation causes, is the most probable explanation for this
difference.
0.008
Non-aqueous
4. Summary and conclusions
0.006
This work describes the preparation and characterization of a
series of partially lithiated HEMD samples using a collection of
physical methods to determine the changes in structure, chemical
composition and morphology upon lithium insertion into the
material. The key conclusions that can be made from this work are:
0.004
Aqueous
(i) Regions within the structure of the HEMD undergo a phase
transition from the b-MnO2 to LiMn2O4 phase, and subsequent
reduction of this phase to Li2Mn2O4, in response to lithium
intercalation.
0.002
(ii) Lithiation of HEMD up until a depth of Li0.60MnO2 resulted in a
decrease in the BET surface area. However, reduction beyond
this point resulted in a slight increase in surface area as a result
of lattice strain in response to the accommodation of the larger
lithium ion.
(iii) Comparisons between lithium insertion and proton insertion
into HEMD and EMD, respectively, highlighted numerous
differences in structural and morphological changes. These
contrasting findings make a further study of the mechanism of
0
0
0.1
0.2
0.3
0.4
0.5
n in LinMnO2
Fig. 6. Relative change in unit cell volume for non-aqueous and aqueous reduction.

1834
W.M. Dose et al. / Materials Research Bulletin 47 (2012) 1827–1834
[8] P.M. De Wolff, Acta Cryst. 12 (1959) 341.
the non-aqueous Li/MnO2 system paramount and shed some
degree of doubt on broad statements assuming equivalent
behaviour of aqueous and non-aqueous manganese dioxide
systems.
[9] P. Ruetschi, J. Electrochem. Soc. 131 (1984) 2737.
[10] P. Ruetschi, J. Electrochem. Soc. 135 (1988) 2657.
[11] P. Ruetschi, R. Giovanoli, J. Electrochem. Soc. 135 (1988) 2663.
[12] Y. Chabre, J. Pannetier, Prog. Solid State Chem. 23 (1995) 1.
[13] R.P. Williams, PhD Thesis, University of Newcastle, Australia, 1995.
[14] C. Kim, Z. Akase, L. Zhang, A.H. Heuer, A.E. Newman, P.J. Hughes, J. Solid State
Chem. 179 (2006) 753.
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