Journal of New Materials for Electrochemical Systems 5, 129-133 (2002)
c J. New. Mat. Electrochem. Systems
Synthesis of α-, β-, and Low Defect γ-Manganese Dioxides
Using the Electrochemical-Hydrothermal Method
and Study of their Li Insertion Behavior
Laurie I. Hill∗, Alain Verbaere and Dominique Guyomard
Institut des Matériaux Jean Rouxel, Laboratoire de Chimie des Solides,
CNRS – Université de Nantes, Nantes, France
( Received June 6, 2001 ; received in revised form December 4, 2001 )
Abstract: Using a hydrothermal-electrochemical synthesis route, a number of MnO2 materials with α-, β-, or γ- structures have been synthesized.
The γ-MnO2 materials obtained are characterized by lower amounts of pyrolusite intergrowth and lower amounts of microtwinning defects
compared to those found in γ-MnO2 samples prepared by traditional electrochemical methods. Small amounts of lithium can be incorporated or
not into the structure of α-MnO2 materials during the one-step synthesis. The effect of variations in the synthesis conditions on the type of structure
obtained and also on the defect rates of the γ-MnO2 structures are presented. The lithium insertion properties of these materials are discussed in
relation to their composition and structure.
Key words : Electrochemical-hydrothermal synthesis, γ-MnO2 , α-MnO2 , Li-insertion behavior.
1.
INTRODUCTION
varying amounts of pyrolusite intergrowths, generally denoted
Pr (in percent) [7]. The structure also contains microtwinning
defects, the relative amount of which is denoted here as Mt (in
percent) [7, 8]. γ-MnO2 for industrial applications is typically
synthesized by electrochemical oxidation of acidic MnSO4 solutions. The resulting materials are generally characterized by a
Pr ≈ 50 and a high rate of microtwinning defects (≈ 80). Annealing of these samples for use in batteries generally leads to
materials with high pyrolusite contents and low microtwinning
rates. Previous studies have indicated that the lithium insertion
capacity of γ-MnO2 should increase with decreasing rates of
pyrolusite intergrowth and microtwinning defects. In this case,
it is anticipated that the best materials would be those approaching the Ramsdellite limit.
α- and γ-Manganese dioxides are of great interest for use as
3V electrodes of secondary lithium metal batteries due to their
low cost, low toxicity and high average voltage, as compared to
vanadium-based oxides.
α-MnO2 is usually prepared through chemical methods, with
the large 2x2 channels of the structure stabilized by large cations
such as K+ or NH4 + [1, 2]. However, for battery applications these large ions are thought to inhibit the mobility of Li+
through the tunnels. Thus to improve battery performance, these
large cations can be ion-exchanged for Li+ or H+ in the structure [3, 4]. Li+ or H+ containing α-MnO2 ’s can also be synthesized directly by chemical methods [5, 6].
With the goal of synthesizing new or modified manganese dioxides and γ-MnO2 compounds approaching the Ramsdellite limit,
we have employed the hydrothermal-electrochemical synthesis
method. This method has recently been used in the preparation
of various oxides [9, 10], phosphates [11] and vanadates [12].
The structure of γ-MnO2 materials is rather complicated, consisting of a Ramsdellite-like (2x1 channels) basic structure with
∗ To whom correspondence should be addressed: Fax: + 33240373930;
E-Mail: [email protected]
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L. I. Hill et al./ J. New Mat. Electrochem. Systems 5, 129-133 (2002)
This paper describes the synthesis of α-, β-, and low defect
γ-MnO2 obtained from this method as well as a preliminary
study of the lithium insertion properties of selected compounds.
2.
EXPERIMENTAL
2.1 Synthesis
pounds synthesized in this study demonstrate a larger degree of
crystallinity than is normally found for electrolytic manganese
dioxides (EMD). The relative amount of pyrolusite intergrowth
in γ-MnO2 samples can be qualitatively compared by noting
the position of the (110) reflection situated near 2-theta = 22◦ ,
a larger amount of pyrolusite intergrowth shifting the peak to
higher angle [7, 8]. This is illustrated and emphasized in the
figure by the two arrows.
The manganese dioxides in this study were prepared in a teflonlined autoclave, equipped with outlets for connection of the
working (Au, ~28 cm2 ), counter (C) and reference (Ag+ /Ag)
electrodes. A constant anodic current (J=0.35 mA/cm2 or
1.75 mA/cm2 ) was applied to acidic (measured [H+ ] = 10−1 ,
1 or 3 M) MnSO4 solutions, at a temperature of 80, 92, 113 or
180◦ C. The oxidation of Mn2+ forms an MnO2 deposit on the
electrode according to the following reaction:
M n2+ + 2H2 O → M nO2 + 4H + + 2e−
(1)
Solutions were also prepared by adding Li2 SO4 ([Li+ ] =
10[Mn2+ ]) in order to probe the effect of Li+ ions on the synthesis and the material obtained. The electrolysis was carried
out for the length of time corresponding to the deposition of
500-600 mg of MnO2 . After thoroughly rinsing and drying at
50-75◦ C for 1-2 hours, the product was removed from the electrode. The yield of the synthesis was always close to 100%,
except when the starting solutions had a pH < 0. In this case,
the solution was a deep rose color after electrolysis, consistent
with the formation of MnO4 − during the synthesis, and thus a
lower yield of MnO2 . This result is in accordance with Pourbaix
potential-pH diagrams [13].
2.2
Characterization
The products were analyzed by X-Ray powder diffraction,
TGA/DSC, AAS/ICP, and redox titration. Lithium insertion
studies were performed in Swagelok cells consisting of a composite cathode (70 wt% active material, 20 wt% carbon black,
10% PVDF binder), a separator soaked in electrolyte (1 M LiPF6
in 2:1 ethylene carbonate (EC):dimethyl carbonate (DMC)), and
a lithium metal anode. The cells were cycled using a MacPile
system (Biologic, Claix, France).
3.
3.1
Figure 1: X-ray diffraction patterns of some products obtained
in this study. The arrows accent the shift in the peak located at
2θ ≈ 22◦ with varying values of Pr in γ-MnO2 .
Figure 2 shows phase diagrams as a function of pH and temperature for (a) MnSO4 and (b) MnSO4 /Li2 SO4 solutions. From
these diagrams, it can be seen that at pH≥0 and T≥92◦ C, the
addition of Li+ to the solution has no effect on the type of
structure obtained. At temperatures >120◦ C and pH=1, pure
β-MnO2 is formed. Lowering the temperature leads to formation of mixtures of γ- and β-MnO2 (113◦ C) or pure γ-MnO2
(92◦ C). In further increasing the acidity of the solution (pH≤0)
or lowering the temperature (80◦ C), a clear effect of Li+ emerges
in terms of the structure obtained. In the absence of Li+ , mixtures of α- and γ-MnO2 are formed, while pure α-MnO2 is
formed in the presence of Li+ . However, it was found that
α-MnO2 can also be synthesized in the absence of Li+ if the
applied current density is increased five-fold (not shown in the
figure).
RESULTS AND DISCUSSION
X-ray diffraction and characterization
Using this synthesis route, a number of MnO2 materials with α-,
β-, or γ- structures have been synthesized in one step [14, 15].
X-ray powder diagrams representative of the different structures
obtained are shown in Figure 1. It is noted that in addition to
the pure phases, it is possible to synthesize intimate mixtures of
α- and low Pr γ-MnO2 . It is also noted that the γ-MnO2 com-
The X-ray powder diagrams for the various γ-MnO2 -containing
materials have been examined to quantitatively determine the
relative levels of pyrolusite intergrowth, Pr , and microtwinning,
Mt, using a method [8] developed in our group based on that
proposed by Chabre and Pannetier [7]. Figure 3 shows the
position in the (Pr ,Mt) plane of the γ-containing compounds
synthesized here. Also shown on the diagram are regions containing γ-MnO2 compounds prepared by traditional methods.
As illustrated, the γ-MnO2 materials prepared here are charac-
Synthesis of α-, β-, and Low Defect γ-Manganese Dioxides . / J. New Mat. Electrochem. Systems 5, 129-133 (2002)
Figure 2: Phase diagrams as a function of temperature and
pH for the samples presented here. (a) Samples synthesized
from acidic MnSO4 solutions, and (b) samples synthesized from
acidic MnSO4 /Li2 SO4 solutions.
terized by lower amounts of pyrolusite intergrowth and lower
rates of microtwinning defects compared to those γ-MnO2 materials prepared by traditional electrochemical methods. The
amount of pyrolusite intergrowth can be controlled by changing the temperature or the acidity of the solution, with lower
amounts of pyrolusite intergrowth favored by lower temperatures and more acidic solutions. The rate of microtwinning defects remains nearly constant (~20) for all γ-MnO2 materials
synthesized here, with slightly lower amounts for samples consisting of a mixture of γ-MnO2 and β-MnO2 .
In preparation for lithium insertion studies, the samples were
annealed at 250◦ C in air for 16 hours, in order to remove adsorbed water and to decrease the amount of water contained in
the structure. The structural parameters of the γ-MnO2 containing compounds after heat-treatment are also shown in
Figure 3. In contrast to the behavior of previously reported
EMD’s on heating (a large increase in pyrolusite content accompanied by a large decrease in the occurrence of microtwinning,
i.e. a shift toward the pyrolusite edge of the (Pr ,Mt) plane), the
γ-MnO2 materials synthesized in this study exhibit the unusual
behavior of a nearly constant pyrolusite content, in combination
with the expected decrease in microtwinning.
3.2 Physico-chemical characterization
TGA/DSC analysis was performed on the materials prepared
here to determine the structural water content, n, in MnO2 ·nH2 O.
For γ-MnO2 , the value of n was found to range from 0.05 to 0.1,
while this value was slightly higher for α-MnO2 (0.1 to 0.15),
owing to the larger channels found in α-MnO2 . Redox titration revealed the Mn oxidation state in all materials to be 4+.
AAS/ICP analysis was also performed on all samples synthe-
131
Figure 3: Placement in the (Pr , Mt) plane of samples presented
here. For reference, regions containing manganese dioxides
prepared by other methods have been added. The position of
Classical Electrolytic Manganese Dioxides (EMD’s), Classical
Chemical Manganese Dioxides (CMD’s), and Classical Heat
Treated Manganese Dioxides (HTMD’s) is taken from reference
[7]. ‘Other EMD’s’ were obtained at very low current densities (from [7] and [20]). The positions of ‘Unusual’ EMD’s
and ‘Unusual’ HTMD’s are taken from reference [21]. • = γMnO2 , N = γ-MnO2 + α-MnO2 , = γ-MnO2 + β-MnO2 , ◦
= heat treated γ-MnO2 , 4 = heat treated γ-MnO2 + α-MnO2 ,
= β-MnO2
sized in the presence of Li+ . All γ-MnO2 materials contained
little to no Li (<0.005 Li/Mn). The pure α-MnO2 materials obtained were found to contain only a small amount of lithium (≤
0.05 Li/Mn).
Especially noteworthy is the absence of large stabilizing cations
such as K+ or NH4 + in the α-MnO2 compounds prepared here
by electrochemical synthesis. Previous α-MnO2 compounds
prepared electrochemically contained K+ [16, 17]. In the synthesis performed in the absence of Li+ and increased current
density, α-MnO2 containing only H+ and H2 O in the channels
was formed. It is noted that for battery applications, the H2 O
within the channels can be removed by heating at 250◦ -300◦ C
in air, leaving only the small H+ or Li+ ions in the structure.
3.3
Lithium insertion study
The lithium insertion properties of these materials have been
studied in relation to their composition and structure. The first
three cycles were performed in potentiodynamic mode at
20mV/h, and the following cycles in galvanostatic mode at a
rate of C/10. Figure 4 presents the first two cycles in poten-
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L. I. Hill et al./ J. New Mat. Electrochem. Systems 5, 129-133 (2002)
tiodynamic mode for a heat-treated γ-MnO2 (Pr =26, Mt=11)
and an α-MnO2 prepared in the absence of Li+ . The γ-MnO2
shows a reversible capacity of 0.67 Li/Mn, comparable with
other γ-MnO2 compounds [8, 18], though a higher capacity
was expected due to the low defect concentration. The α-MnO2
shows a reversible capacity of 0.52 Li/Mn, comparable to other
H+ containing α-MnO2 compounds [4, 19]. This lower capacity may be associated with a larger polarization as seen in the
V-x curves. Figure 5 shows the cycled capacity of the same two
materials over 50 cycles. While the γ-MnO2 shows a slightly
higher initial capacity, it slowly decreases with increasing cycle number until the capacity is equal to that of the α-MnO2
compound after 50 cycles (~125 mAh/g). The α-MnO2 shows
remarkable stability in the cycled capacity.
Figure 5: Cycled capacity in potentiodynamic mode (20 mV/h),
cycles 1-3, and in galvanostatic mode (C/10 rate), following cycles; for (a) γ-MnO2 characterized by a Pr ≈ 26 and Mt ≈ 11,
and (b) α-MnO2 synthesized in the absence of Li+ .
4.
CONCLUSION
We have illustrated that the electrochemical-hydrothermal
method can be used to synthesize MnO2 materials of the α-,γ-,
or β-structure type, including mixtures of structure types. The
γ-MnO2 materials thus obtained are characterized by very low
concentrations of pyrolusite and microtwinning defects compared to traditionally prepared EMD’s. We have also shown
that electrochemically prepared α-MnO2 can be obtained without the presence of K+ .
Further studies are in progress to better characterize the defects
present in the γ-MnO2 structure as well as using these results
to better understand and improve the lithium insertion properties. Efforts are also underway to improve the capacity of the
α-MnO2 synthesized here by ion-exchange of H+ by Li+ , or
by the preparation of Li+ stabilized α-MnO2 in one single step.
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