Lithiated manganese dioxide

Details Lithiated manganese dioxide Lithiated manganese dioxide

2000-02-01

2001-02-20

Weiner, Laura (Department: 1745)

Chemistry: electrical current producing apparatus, product, and

Current producing cell, elements, subcombinations and...

Electrode

C429S231950, C423S599000

Reexamination Certificate

active

06190800

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to a method for preparing an improved lithiated manganese dioxide having a stabilized gamma-MnO
2
-type structure. In particular, the invention relates to a lithiated manganese dioxide having a stabilized gamma-MnO
2
-type structure and its application as an active cathode material in a primary lithium electrochemical cell.
BACKGROUND OF THE INVENTION
Electrochemical cells commonly contain a negative electrode (anode) and a positive electrode (cathode), an ion-permeable separator therebetween and an electrolyte in contact with both of the electrodes. Typical electrolytes can be aqueous-based or non-aqueous organic solvent-based liquid electrolytes or polymeric electrolytes. There are two basic types of electrochemical cells, a primary and a secondary (rechargeable) electrochemical cell. A primary electrochemical cell is discharged to exhaustion only once. A secondary electrochemical cell, however, is rechargeable and thus can be discharged and recharged multiple times.
Primary lithium electrochemical cells typically employ an anode of lithium metal or lithium alloy, preferably a lithium-aluminum alloy; a cathode containing an electrochemically active material consisting of a transition metal oxide or chalcogenide, preferably manganese dioxide; and an electrolyte containing a chemically stable lithium salt dissolved in an organic solvent or a mixture of organic solvents.
The lithium anode is preferably formed from a sheet or foil of lithium metal or lithium alloy without any substrate. A lithium primary cell referenced hereinafter as having an anode comprising lithium shall be understood to mean an anode of lithium metal or a lithium alloy. If a lithium-aluminum alloy is employed, the aluminum is present in a very small amount, typically less than about 1 wt % of the alloy. The addition of aluminum primarily serves to improve the low temperature performance of the lithium anode in lithium primary cells.
Manganese dioxides suitable for use in lithium primary cells include both chemically produced manganese dioxide known as “chemical manganese dioxide” or “CMD” and electrochemically produced manganese dioxide known as “electrolytic manganese dioxide” or “EMD”. CMD can be produced economically and in high purity, for example, by the methods described by Welsh et al. in U.S. Pat. No. 2,956,860. However, CMD typically does not exhibit energy or power densities in lithium cells comparable to those of EMD. Typically, EMD is manufactured commercially by the direct electrolysis of a bath containing manganese sulfate dissolved in a sulfuric acid solution. Processes for the manufacture of EMD and representative properties are described in “Batteries”, edited by Karl V. Kordesch, Marcel Dekker, Inc., New York, Vol. 1, 1974, pp.433-488. Manganese dioxide produced by electrodeposition typically is a high purity, high density, “gamma(&ggr;)-MnO
2
” phase, which has a complex crystal structure containing irregular intergrowths of a “ramsdellite”-type MnO
2
phase and a smaller portion of a beta(&bgr;)- or “pyrolusite”-type MnO
2
phase as described by dewolfe (
Acta Crystallographica
, 12, 1959, pp.341-345). The gamma(&ggr;)-MnO
2
structure is discussed in more detail by Burns and Burns (e.g., in “Structural Relationships Between the Manganese (IV) Oxides”,
Manganese Dioxide Symposium
, 1, The Electrochemical Society, Cleveland, 1975, pp. 306-327), which is incorporated herein by reference.
The structural disorder present in the crystal lattice of gamma(&ggr;)-MnO
2
includes non-coherent lattice defects, such as stacking faults, micro-twinning, Mn
+4
cation vacancies, Mn
+3
cations from reduction of Mn
+4
cations, lattice distortion introduced by the Mn
+3
cations (i.e., Jahn-Teller effect), as well as compositional non-stoichiometry as described, for example, by Chabré and Pannetier (
Prog. Solid State Chem
., Vol. 23, 1995, pp. 1-130) and also by Ruetschi and Giovanoli (
J. Electrochem. Soc
., 135(11), 1988, pp. 2663-9), both incorporated herein by reference.
Ruetschi has proposed a chemical formula for &ggr;-MnO
2
which is based on a structural defect model (
J. Electrochem. Soc
., 131(12), 1984, pp. 2737-2744). In this model, the crystal lattice structure of &ggr;-MnO
2
can be described as comprising an anion sublattice consisting of a close-packed array of oxygen anions and a corresponding cation sublattice consisting of an array of predominantly Mn
+4
cations, some Mn
+3
cations, and occasional Mn
+4
cation vacancies. Further, in order to maintain overall electroneutrality of the &ggr;-MnO
2
crystal lattice, the positive charge deficiencies resulting from the presence of Mn
+3
cations as well as the Mn
+4
cation vacancies must be compensated. This can be accomplished by substitution of OH
−
(hydroxyl) ions for O
−2
ions in the anion sublattice, which is nominally equivalent to protonation of O
−2
anions by hydrogen ions. Thus, for each Mn
+3
cation present, one hydrogen ion must be introduced into the lattice to maintain charge compensation. Similarly, for each Mn
+4
cation vacancy, four hydrogen ions must be introduced to maintain the overall electroneutrality. The OH
−
anions formed are also referred to as “structural” or “lattice water”. Thus, the chemical formula for &ggr;-MnO
2
can be represented as:
Mn
+4
1−x−y
Mn
+3
y
x
O
2−4x−y
(OH)
4x+y
  (1)
wherein
stands for Mn
+4
vacancies; x is the fraction of Mn
+4
vacancies; and y is the fraction of Mn
+3
cations. Also, Reutschi has proposed that hydrogen ions associated with the Mn
+3
cations are mobile while hydrogen ions associated with the immobile Mn
+4
cation vacancies are localized.
It is theorized by the Applicants herein of the present Patent Application that such mobile hydrogen ions present in the &ggr;-MnO
2
lattice can be advantageously substituted by lithium cations by way of an ion-exchange process prior to the traditional heat-treatment without further reduction of Mn
+4
to Mn
+3
, in contrast to typical reductive lithium insertion processes of prior art. Although Reutschi has proposed that such hydrogen ions are mobile, neither a particular process for ion-exchanging the mobile hydrogen ions by lithium cations nor the desirability of such an ion-exchange process was disclosed.
Ruetschi further theorized that the number of mobile hydrogen ions depends on both the degree of oxidation of the manganese atoms and total lattice water content and can be determined experimentally. For example, it is theorized by the Applicants herein of the present Patent Application that according to Equation (1) hereinabove, about 20% of the lattice hydrogen ions of an EMD having a nominal chemical formula of MnO
1.96
.0.23 H
2
O, for example, can be ion-exchanged by lithium cations as shown in Equation (2):
Li
+
+Mn
+4
0.84
Mn
+3
0.73
0.087
O
1.579
(OH)
0.421
Li
0.08
MnO
2
.0.18 H
2
O  (2)
Electrochemical manganese dioxide (EMD) is the preferred manganese dioxide for use in primary lithium cells. However, before it can be used, it must be heat-treated to remove residual water. The term “residual water”, as used herein includes surface-adsorbed water, noncrystalline water (i.e., water physisorbed or occluded in pores), as well as lattice water. Heat-treatment of EMD prior to its use in lithium cells is well known and has been described by Ikeda et al. (e.g., in “Manganese Dioxide as Cathodes for Lithium Batteries”,
Manganese Dioxide Symposium
, Vol. 1, The Electrochemical Society, Cleveland, 1975, pp. 384-401) and is incorporated herein by reference.
EMD suitable for use in primary lithium cells can be heat-treated at temperatures between about 200 and 350° C. as taught by Ikeda et al. in U.S. Pat. No. 4,133,856. This reference also discloses that it is preferable to heat-treat the EMD in two steps. The first step is performed at temperatures up to about 250° C. in order to dri

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