Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Electrode
Reexamination Certificate
2000-07-10
2002-06-11
Chaney, Carol (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Electrode
C423S599000
Reexamination Certificate
active
06403257
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a method for preparing a lithiated manganese dioxide having a stabilized &ggr;-MnO
2
-type crystal structure wherein a nominally dry mixture of a manganese dioxide powder and a lithium salt are mechanically activated prior to heat-treatment. The invention also relates to the application of said prepared lithiated manganese dioxide as an active cathode material in a primary lithium electrochemical cell.
BACKGROUND OF THE INVENTION
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. Typically, EMD is manufactured commercially by the direct electrolysis of a bath containing manganese sulfate dissolved in a sulfuric acid solution. Manganese dioxide produced by electrodeposition typically has high purity and high density. 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.
Typically, EMD is composed of a “gamma(&ggr;)-MnO
2
” phase having a complex crystal structure consisting of an irregular intergrowth of predominantly “ramsdellite”-type MnO
2
phase and a smaller portion of “pyrolusite” or beta(&bgr;)-MnO
2
phase as described by deWolfe (
Acta Crystallographica
, 12, 1959, pp.341-345) and 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), incorporated herein by reference. Disorder in the crystal lattice of the &ggr;-MnO
2
phase can include various non-coherent lattice defects, for example, stacking faults, micro-twinning, Mn
+4
cation vacancies, Mn
+3
cations from reduction of Mn
+4
cations, inserted protons (i.e., hydrogen ions), lattice distortions introduced by the presence of 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 Ruetschi and Giovanoli (
J. Electrochem. Soc
., 135(11), 1988, pp. 2663-9), incorporated herein by reference.
Electrochemical manganese dioxide is the preferred manganese dioxide for use in primary lithium cells. One consequence of the electrodeposition process is that EMD typically retains surface acidity from the sulfuric acid of the electrolysis bath. This residual surface acidity must be neutralized, for example, by treatment with a solution of aqueous base, before the EMD can be used as the active cathode material in primary lithium cells. Suitable aqueous bases include: sodium hydroxide, ammonium hydroxide (i.e., aqueous ammonia), calcium hydroxide, magnesium hydroxide, potassium hydroxide, lithium hydroxide, and combinations thereof. Typically, commercial EMD is neutralized with a strong base such as sodium hydroxide because it is highly effective and economical. However, before the neutralized EMD 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 water present in the crystal lattice in the form of protons. Heat-treatment of EMD prior to use in lithium cells is well known and has been described, for example, 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), incorporated herein by reference.
EMD suitable for use in primary lithium cells can be prepared by heat-treating commercial EMD at temperatures between about 200 and 350° C. as taught by Ikeda et al. in U.S. Pat. No. 4,133,856. This reference discloses that it is preferable to heat-treat the EMD in two steps. The first step can be performed at a temperature greater than about 100° C., but below about 250° C. in order to drive off surface and non-crystalline water. The second step is performed at between about 250 and 350° C. to remove the lattice water. This two-step heat-treatment is disclosed to improve discharge performance of primary lithium cells because surface, non-crystalline, and lattice water can be removed. An undesirable consequence of the heat-treatment is that the &ggr;-MnO
2
-type structure is gradually converted to a gamma/beta (&ggr;/&bgr;)-MnO
2
-type structure at temperatures >350° C. The term “gamma/beta-MnO
2
” as used in the art reflects the fact that a significant portion of &ggr;-MnO
2
(i.e., the ramsdellite-type MnO
2
phase) is converted to &bgr;-MnO
2
during heat-treatment. At least about 30 percent by weight and typically between about 60 and 90 percent of the ramsdellite-type MnO
2
phase is converted to &bgr;-MnO
2
during conventional heat treatment as taught, for example, in U.S. Pat. No. 4,921,689. The produced &ggr;/&bgr;-MnO
2
phase is less electrochemically active than EMD containing predominantly ramsdellite-type MnO
2
. For example, cathodes containing EMD enriched in &bgr;-MnO
2
are disclosed in U.S. Pat. No. 5,658,693 to provide less lithium uptake capacity during discharge in lithium cells.
A method for preparing an improved manganese dioxide from commercial lithium grade EMD having a &ggr;-MnO
2
structure that is not converted appreciably to the &ggr;/&bgr;-MnO
2
structure by a heat-treatment of the type described hereinabove is disclosed in co-pending commonly assigned U.S. application Ser. No. 09/496,233, filed on Feb. 1, 2000. In the method disclosed in this reference, commercial lithium grade EMD is treated with a liquid source of lithium cations by a process that promotes stepwise ion-exchange of mobile hydrogen ions in the &ggr;-MnO
2
crystal lattice and on the surface of the EMD particles with lithium cations followed by heat-treatment to eliminate residual water. After such heat-treatment, typically less than about 5 wt % of additional &bgr;-MnO
2
phase can be detected by x-ray powder diffraction analysis. In one aspect of the disclosed method, a suspension of commercial lithium grade EMD in water is treated with a basic lithium salt, such as lithium hydroxide, by a multi-step process in which the lithium salt is added incrementally with soaking periods between additions until a pH value between about 10 and 13 is obtained. During the soaking periods, hydrogen ions in the EMD crystal lattice can ion-exchange with lithium cations to form a lithium ion-exchanged intermediate product. This intermediate product is separated from the suspension, dried, and heat-treated. In another aspect of the disclosed method, an essentially dry mixture of commercial lithium grade EMD and a low melting point lithium salt, such as lithium nitrate, is heat-treated initially at a temperature above the melting point of the salt but less than about 300° C. and subsequently at a temperature greater than about 350° C., but less than about 420° C. The lithiated manganese dioxide products obtained by both processes were disclosed to have a stabilized &ggr;-MnO
2
-type crystal structure and to provide improved performance when used in primary lithium cells. Specifically, the average operating voltage of such cells at high discharge rates and low operating temperatures was disclosed to be substantially higher than that of primary lithium cells containing heat-treated manganese dioxide of prior art not treated stepwise with a liquid source of lithium cations prior to heat-treatment.
In addition to the prior art methods disclosed hereinabove, another method for preparing lithium manganese composite oxides having the formulas Li
0.33
MnO
2
and LiMn
2
O
4
for use in lithium ion rechargeable cells is disclosed by U.S. Pat. No. 5,911,920. The method disclosed in the r
Christian Paul A.
Mao Ou
Chaney Carol
Douglas Paul I.
Josephs Barry D.
Krivulka Thomas G.
The Gillette Company
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