Electrolysis: processes – compositions used therein – and methods – Electrolytic synthesis – Preparing inorganic compound
Reexamination Certificate
2001-02-20
2003-07-01
Ryan, Patrick (Department: 1745)
Electrolysis: processes, compositions used therein, and methods
Electrolytic synthesis
Preparing inorganic compound
C205S057000, C205S338000, C205S412000, C205S542000, C205S638000, C433S059000
Reexamination Certificate
active
06585881
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to manganese dioxide useful as cathode active material in electrochemical cells, particularly alkaline cells. The invention also relates to an electrolysis process for preparing manganese dioxide having lower manufacturing cost.
BACKGROUND
Conventional alkaline electrochemical cells are formed of a cylindrical casing. The casing is initially formed with an enlarged open end. After the cell contents are supplied, an end cap with insulating plug is inserted into the open end. The cell is closed by crimping the casing edge over an edge of the insulating plug and radially compressing the casing around the insulating plug to provide a tight seal. A portion of the cell casing forms the positive terminal.
The cell contents of a primary alkaline cell typically contain zinc anode active material, alkaline electrolyte, a manganese dioxide cathode active material, and an electrolyte permeable separator film, typically of cellulose. The anode active material comprises zinc particles admixed with conventional gelling agents, such as carboxymethylcellulose or acrylic acid copolymers, electrolyte and, optionally, some zinc oxide. The gelling agent holds the zinc particles in place and in contact with each other. A conductive metal nail, known as the anode current collector, is typically inserted into the anode active material. The alkaline electrolyte is typically an aqueous solution of potassium hydroxide, but other alkali solutions of sodium or lithium hydroxide may also be employed. The cathode material is typically of manganese dioxide and normally includes small amounts of carbon or graphite to increase conductivity. Conventional alkaline cells have solid cathodes comprising battery grade particulate manganese dioxide. Battery grade manganese dioxide as used herein refers to manganese dioxide generally having a purity of at least about 91 percent by weight (dry basis). Electrolytic MnO
2
(EMD) is the preferred form of manganese dioxide for alkaline cells because of its high density and since it is conveniently obtained at high purity by electrolytic methods.
EMD (electrolytic manganese dioxide) can be manufactured from the direct electrolysis of an aqueous bath of manganese sulfate and sulfuric acid. The EMD is a high purity, high density, gamma manganese dioxide, desirable as a cathode material for electrochemical cells particularly Zn/MnO
2
alkaline cells, Zn-carbon and lithium/MnO
2
cells. During the electrolysis process the gamma EMD is deposited directly on the anode which is typically made of titanium, lead, lead alloy, or graphite. The EMD is removed from the anode, crushed, ground, washed in water, neutralized by washing with dilute NaOH, Na2CO3, NH4OH or LiOH, and dried in a rotary dryer. The EMD product is generally heat treated to remove residual water before it is used in a lithium cell. Conventional electrolysis processes for the manufacture of EMD and a description of its properties appear in Batteries, edited by Karl V. Kordesch, Marcel Dekker, Inc. New York, Vol. 1 (1974), p.433-488. Conventional electrolysis processes for production of MnO
2
are normally carried out at temperature between about 80 and 980° C.
M. Mauthoor, A. W. Bryson, and F. K. Crudwell, Progress in Batteries & Battery Materials, Vol. 16 (1997), pp. 105-110 discloses an electrolysis method for manufacture of manganese dioxide. The electrolysis is performed at temperatures between 90 and 108° C. Although Mauthoor reports that discharge capacities of MnO
2
synthesized by electrolysis of an aqueous bath of MnSO
4
and H
2
SO
4
at between 95° C. to 108° C. was about 9% higher than that for MnO
2
material produced at 95°C., there was no substantial difference among the three MnO
2
products produced at electrolysis temperatures of 100° C., 105° C., and 108° C. In fact, as Mauthoor increased the electrolysis temperature from 105 to 108° C., the percent MnO
2
in the electrolysis product and the discharge capacity of the MnO
2
product both decreased slightly. Thus, electrolysis at temperatures higher than 108° C. were not attempted or contemplated.
In commercial EMD production, the electrolysis is normally carried out at temperatures between 94° C. and 97° C. and at current densities between 2 and 10 Amp/ft
2
, more typically between 4 and 10 Amp/ft
2
of anode surface area. A titanium anode and graphite or copper cathode are typically employed. Increasing current density tends to increase the MnO
2
specific surface area (SSA). When electrolysis is carried out at conventional temperatures and current density is increased beyond the normal bounds, there is a tendency for the specific surface area (SSA) of the MnO
2
product to increase to a level which is outside (greater than) the desired range of between 18-45 m
2
/g. Thus, at conventional temperatures it is very difficult to increase the current density and the deposition rate above a level of between about 10 to 11 Amp/ft
2
(108 to 119 Amp/m
2
) without adversely affecting the quality of the product.
In addition, under conventional conditions of temperature and electrolyte composition, at current densities greater than 10 Amp/ft
2
(108 Amp/m
2
) there is a tendency for passivation of the titanium anode to occur after a period of time, which may be shorter than the normal plating cycle of 1.5 to 3 weeks. The higher the current density, e.g. 12 Amp/ft
2
(130 Amp/m
2
) rather than 10 Amp/ft
2
(108 Amp/m
2
), the sooner such passivation is likely to occur. Passivation involves the formation of an insulating oxide film on the surface of the titanium, resulting in an increase in the operating Voltage of the anode. Once started the problem is self accelerating and soon results in a precipitous voltage rise which exceeds the capability of the power supply followed by a loss of current, ending in complete and irreversible shut-down of the plating process. Often a number of anodes will fail simultaneously due to passivation. When this occurs, the anodes must be withdrawn, deposited EMD removed and the anodes must be surface treated to remove the tenacious oxide film prior to being returned to service. This is a highly disruptive and expensive problem. In a commercial setting, great care is taken to avoid anode passivation and a margin of safety is preserved in setting the current density below that which borders on passivation, EMD quality considerations aside.
V. K. Nartey, L. Binder, and A. Huber, Journal of Power Sources, Vol. 87 (2000), p. 205-211 describes an electrolysis process for making MnO
2
wherein the electrolysis bath was doped with TiOSO
4
. The MnO
2
was used in an alkaline rechargeable battery. The reference states at page 210, col. 1 that the MnO
2
with TiOSO
4
doping (called M
2
, Table 7) performed poorly on the initial discharge cycle (i.e. similar to a primary, non-rechargeable cell) despite a high specific surface area. When the bath was doped with TiO
2
the MnO
2
product (called M
1
, Table 7) performed better on the initial discharge cycle, but still did not perform as well as the control MnO
2
(commercial grade EMD Tosoh GH-S). The electrolysis bath for the experiments described in Huber, et al. was maintained at conventional temperature of 98° C. and was performed at conventional current density of 6 milliAmp/cm
2
(5.57 Amp/ft
2
) based on anode surface area.
Conventional battery grade manganese dioxide does not have a true stoichiometric formula MnO
2
, but is better represented by the formula MnO
x
, wherein x is typically between about 1.92 to 1.96, corresponding to a manganese valence of between about 3.84 and 3.92. Conventional EMD may typically have a value for x of about 1.95 or 1.96, corresponding to a manganese valence of 3.90 and 3.92, respectively. In addition to manganese (Mn) and oxygen (O), conventional electrolytic manganese dioxide (EMD) also contains a certain quantity of SO
4
=
ions and of H
30
ions (protons) in the crystal lattice. When heated to temperatures above 110 deg. C., the lattice protons combine with oxygen and are liberated as H
2
O. Con
Davis Stuart M.
Miller Gary
Moses Peter R.
Douglas Paul I.
Josephs Barry D.
Krivulka Thomas G.
Parsons Thomas H.
Ryan Patrick
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